一、概述

在flask應用程式中：

from flask import Flask


app = Flask(__name__)

@app.route('/index')
def helloWorld():
    return 'index'


if __name__ == '__main__':
    app.run()

可以看到通過app.run()來啟動程式，在這個函式中，通過：

“”“
...
run_simple(host, port, self, **options)
...
"""

這個函式實際上就是執行：

##
app.__call__
 
() #self是app物件，app()

1、__call__

    def __call__(self, environ, start_response):
        """The WSGI server calls the Flask application object as the
        WSGI application. This calls :meth:`wsgi_app` which can be
        wrapped to applying middleware."""
        return self.wsgi_app(environ, start_response)

__call__函式中接收了請求所有相關的資訊environ，最後返回的是wsgi_app函式。

2、wsgi_app

    def wsgi_app(self, environ, start_response):
        ctx = self.request_context(environ)  #將請求相關的資料封裝到了RequestContext物件中
        error = None
        try:
            try:
                ctx.push()  
                response = self.full_dispatch_request()
            
 
except Exception as e:
                error = e
                response = self.handle_exception(e)
            except:  # noqa: B001
                error = sys.exc_info()[1]
                raise
            return response(environ, start_response)
        finally:
            if self.should_ignore_error(error):
                error = None
            ctx.auto_pop(error)

• request_context(environ)

在上面的程式碼中首先建立了一個ctx物件，這個物件是通過request_context函式來建立的（通過RequestContext類建立，實際上就是對請求資料進行封裝）。

• ctx.push()

ctx是RequestContext類的物件，在這個類中push方法中：

def push(self):
    """
    ...
    _request_ctx_stack.push(self)
    ...
    """

實際上執行的是_request_ctx_stack物件（LocalStack類）中的push（_request_ctx_stack.push(self)）方法：

class LocalStack(object):
    """
    ...
        def push(self, obj):
        """Pushes a new item to the stack"""
        rv = getattr(self._local, "stack", None)
        if rv is None:
            self._local.stack = rv = []
        rv.append(obj)
        return rv
    ...
    """

這裡就是將obj（request物件）儲存在字典中的列表中，比如：

”“”
{
  11272：{‘stack’:[obj]}
}
“”“

3、full_dispatch_request()

在這個函式中，主要就是處理處理flask中的路由對映以及檢視函式然後將結果返回給使用者。

4、ctx.auto_pop(error)

這句程式碼是在finally語句中執行的，也就是最後是一定要執行的，在這句程式碼中。

#最後一定執行的是 
rv = _request_ctx_stack.pop()

實際上就是執行LocalStack類中pop方法，就是會將剛剛新增的request物件從那個大字典中移除掉。

class LocalStack:
    def pop(self):
        """Removes the topmost item from the stack, will return the
        old value or `None` if the stack was already empty.
        """
        stack = getattr(self._local, "stack", None)
        if stack is None:
            return None
        elif len(stack) == 1:
            release_local(self._local)
            return stack[-1]
        else:
            return stack.pop()

所以總結一下就是請求到達flask到最後請求結束總體會經過這麼三個階段：

• 請求到達，將所有請求相關的資料封裝到RequestContext物件，然後通過LocalStack物件將其封裝到Local物件中
• 處理請求，從Local物件中取出r對應的request物件，然後執行對應的路由和檢視函式，將結果進行返回
• 移除請求，將對應的請求request物件從Local物件中進行移除

二、原始碼分析

（一）Local類

class Local(object):
    __slots__ = ("__storage__", "__ident_func__")

    def __init__(self):
        object.__setattr__(self, "__storage__", {})
        object.__setattr__(self, "__ident_func__", get_ident)

    def __iter__(self):
        return iter(self.__storage__.items())

    def __call__(self, proxy):
        """Create a proxy for a name."""
        return LocalProxy(self, proxy)

    def __release_local__(self):
        self.__storage__.pop(self.__ident_func__(), None)

    def __getattr__(self, name):
        try:
            return self.__storage__[self.__ident_func__()][name]
        except KeyError:
            raise AttributeError(name)

    def __setattr__(self, name, value):
        ident = self.__ident_func__()
        storage = self.__storage__
        try:
            storage[ident][name] = value
        except KeyError:
            storage[ident] = {name: value}

    def __delattr__(self, name):
        try:
            del self.__storage__[self.__ident_func__()][name]
        except KeyError:
            raise AttributeError(name)

flask為了支援執行緒和協程，自定義一個Local類，可以儲存多個使用者requrst物件，不至於混亂。

class Local(object):
    __slots__ = ("__storage__", "__ident_func__")

    def __init__(self):  #初始化儲存字典、唯一執行緒（協程id）
        object.__setattr__(self, "__storage__", {})  
        object.__setattr__(self, "__ident_func__", get_ident)

    def __getattr__(self, name): #獲取值
        try:
            return self.__storage__[self.__ident_func__()][name]
        except KeyError:
            raise AttributeError(name)

    def __setattr__(self, name, value): #賦值操作
        ident = self.__ident_func__()
        storage = self.__storage__
        try:
            storage[ident][name] = value
        except KeyError:
            storage[ident] = {name: value}

（二）Flask類

在應用程式中，使用的就是這個類來建立的app物件。然後執行其run、__call__、wsgi_app方法。在wsgi_app方法中完成請求接收、處理請求以及最後的響應。

class Flask(_PackageBoundObject):
    """The flask object implements a WSGI application and acts as the central
    object.  It is passed the name of the module or package of the
    application.  Once it is created it will act as a central registry for
    the view functions, the URL rules, template configuration and much more.

    The name of the package is used to resolve resources from inside the
    package or the folder the module is contained in depending on if the
    package parameter resolves to an actual python package (a folder with
    an :file:`__init__.py` file inside) or a standard module (just a ``.py`` file).

    For more information about resource loading, see :func:`open_resource`.

    Usually you create a :class:`Flask` instance in your main module or
    in the :file:`__init__.py` file of your package like this::

        from flask import Flask
        app = Flask(__name__)

    .. admonition:: About the First Parameter

        The idea of the first parameter is to give Flask an idea of what
        belongs to your application.  This name is used to find resources
        on the filesystem, can be used by extensions to improve debugging
        information and a lot more.

        So it's important what you provide there.  If you are using a single
        module, `__name__` is always the correct value.  If you however are
        using a package, it's usually recommended to hardcode the name of
        your package there.

        For example if your application is defined in :file:`yourapplication/app.py`
        you should create it with one of the two versions below::

            app = Flask('yourapplication')
            app = Flask(__name__.split('.')[0])

        Why is that?  The application will work even with `__name__`, thanks
        to how resources are looked up.  However it will make debugging more
        painful.  Certain extensions can make assumptions based on the
        import name of your application.  For example the Flask-SQLAlchemy
        extension will look for the code in your application that triggered
        an SQL query in debug mode.  If the import name is not properly set
        up, that debugging information is lost.  (For example it would only
        pick up SQL queries in `yourapplication.app` and not
        `yourapplication.views.frontend`)

    .. versionadded:: 0.7
       The `static_url_path`, `static_folder`, and `template_folder`
       parameters were added.

    .. versionadded:: 0.8
       The `instance_path` and `instance_relative_config` parameters were
       added.

    .. versionadded:: 0.11
       The `root_path` parameter was added.

    .. versionadded:: 1.0
       The ``host_matching`` and ``static_host`` parameters were added.

    .. versionadded:: 1.0
       The ``subdomain_matching`` parameter was added. Subdomain
       matching needs to be enabled manually now. Setting
       :data:`SERVER_NAME` does not implicitly enable it.

    :param import_name: the name of the application package
    :param static_url_path: can be used to specify a different path for the
                            static files on the web.  Defaults to the name
                            of the `static_folder` folder.
    :param static_folder: The folder with static files that is served at
        ``static_url_path``. Relative to the application ``root_path``
        or an absolute path. Defaults to ``'static'``.
    :param static_host: the host to use when adding the static route.
        Defaults to None. Required when using ``host_matching=True``
        with a ``static_folder`` configured.
    :param host_matching: set ``url_map.host_matching`` attribute.
        Defaults to False.
    :param subdomain_matching: consider the subdomain relative to
        :data:`SERVER_NAME` when matching routes. Defaults to False.
    :param template_folder: the folder that contains the templates that should
                            be used by the application.  Defaults to
                            ``'templates'`` folder in the root path of the
                            application.
    :param instance_path: An alternative instance path for the application.
                          By default the folder ``'instance'`` next to the
                          package or module is assumed to be the instance
                          path.
    :param instance_relative_config: if set to ``True`` relative filenames
                                     for loading the config are assumed to
                                     be relative to the instance path instead
                                     of the application root.
    :param root_path: Flask by default will automatically calculate the path
                      to the root of the application.  In certain situations
                      this cannot be achieved (for instance if the package
                      is a Python 3 namespace package) and needs to be
                      manually defined.
    """

    #: The class that is used for request objects.  See :class:`~flask.Request`
    #: for more information.
    request_class = Request

    #: The class that is used for response objects.  See
    #: :class:`~flask.Response` for more information.
    response_class = Response

    #: The class that is used for the Jinja environment.
    #:
    #: .. versionadded:: 0.11
    jinja_environment = Environment

    #: The class that is used for the :data:`~flask.g` instance.
    #:
    #: Example use cases for a custom class:
    #:
    #: 1. Store arbitrary attributes on flask.g.
    #: 2. Add a property for lazy per-request database connectors.
    #: 3. Return None instead of AttributeError on unexpected attributes.
    #: 4. Raise exception if an unexpected attr is set, a "controlled" flask.g.
    #:
    #: In Flask 0.9 this property was called `request_globals_class` but it
    #: was changed in 0.10 to :attr:`app_ctx_globals_class` because the
    #: flask.g object is now application context scoped.
    #:
    #: .. versionadded:: 0.10
    app_ctx_globals_class = _AppCtxGlobals

    #: The class that is used for the ``config`` attribute of this app.
    #: Defaults to :class:`~flask.Config`.
    #:
    #: Example use cases for a custom class:
    #:
    #: 1. Default values for certain config options.
    #: 2. Access to config values through attributes in addition to keys.
    #:
    #: .. versionadded:: 0.11
    config_class = Config

    #: The testing flag.  Set this to ``True`` to enable the test mode of
    #: Flask extensions (and in the future probably also Flask itself).
    #: For example this might activate test helpers that have an
    #: additional runtime cost which should not be enabled by default.
    #:
    #: If this is enabled and PROPAGATE_EXCEPTIONS is not changed from the
    #: default it's implicitly enabled.
    #:
    #: This attribute can also be configured from the config with the
    #: ``TESTING`` configuration key.  Defaults to ``False``.
    testing = ConfigAttribute("TESTING")

    #: If a secret key is set, cryptographic components can use this to
    #: sign cookies and other things. Set this to a complex random value
    #: when you want to use the secure cookie for instance.
    #:
    #: This attribute can also be configured from the config with the
    #: :data:`SECRET_KEY` configuration key. Defaults to ``None``.
    secret_key = ConfigAttribute("SECRET_KEY")

    #: The secure cookie uses this for the name of the session cookie.
    #:
    #: This attribute can also be configured from the config with the
    #: ``SESSION_COOKIE_NAME`` configuration key.  Defaults to ``'session'``
    session_cookie_name = ConfigAttribute("SESSION_COOKIE_NAME")

    #: A :class:`~datetime.timedelta` which is used to set the expiration
    #: date of a permanent session.  The default is 31 days which makes a
    #: permanent session survive for roughly one month.
    #:
    #: This attribute can also be configured from the config with the
    #: ``PERMANENT_SESSION_LIFETIME`` configuration key.  Defaults to
    #: ``timedelta(days=31)``
    permanent_session_lifetime = ConfigAttribute(
        "PERMANENT_SESSION_LIFETIME", get_converter=_make_timedelta
    )

    #: A :class:`~datetime.timedelta` which is used as default cache_timeout
    #: for the :func:`send_file` functions. The default is 12 hours.
    #:
    #: This attribute can also be configured from the config with the
    #: ``SEND_FILE_MAX_AGE_DEFAULT`` configuration key. This configuration
    #: variable can also be set with an integer value used as seconds.
    #: Defaults to ``timedelta(hours=12)``
    send_file_max_age_default = ConfigAttribute(
        "SEND_FILE_MAX_AGE_DEFAULT", get_converter=_make_timedelta
    )

    #: Enable this if you want to use the X-Sendfile feature.  Keep in
    #: mind that the server has to support this.  This only affects files
    #: sent with the :func:`send_file` method.
    #:
    #: .. versionadded:: 0.2
    #:
    #: This attribute can also be configured from the config with the
    #: ``USE_X_SENDFILE`` configuration key.  Defaults to ``False``.
    use_x_sendfile = ConfigAttribute("USE_X_SENDFILE")

    #: The JSON encoder class to use.  Defaults to :class:`~flask.json.JSONEncoder`.
    #:
    #: .. versionadded:: 0.10
    json_encoder = json.JSONEncoder

    #: The JSON decoder class to use.  Defaults to :class:`~flask.json.JSONDecoder`.
    #:
    #: .. versionadded:: 0.10
    json_decoder = json.JSONDecoder

    #: Options that are passed to the Jinja environment in
    #: :meth:`create_jinja_environment`. Changing these options after
    #: the environment is created (accessing :attr:`jinja_env`) will
    #: have no effect.
    #:
    #: .. versionchanged:: 1.1.0
    #:     This is a ``dict`` instead of an ``ImmutableDict`` to allow
    #:     easier configuration.
    #:
    jinja_options = {"extensions": ["jinja2.ext.autoescape", "jinja2.ext.with_"]}

    #: Default configuration parameters.
    default_config = ImmutableDict(
        {
            "ENV": None,
            "DEBUG": None,
            "TESTING": False,
            "PROPAGATE_EXCEPTIONS": None,
            "PRESERVE_CONTEXT_ON_EXCEPTION": None,
            "SECRET_KEY": None,
            "PERMANENT_SESSION_LIFETIME": timedelta(days=31),
            "USE_X_SENDFILE": False,
            "SERVER_NAME": None,
            "APPLICATION_ROOT": "/",
            "SESSION_COOKIE_NAME": "session",
            "SESSION_COOKIE_DOMAIN": None,
            "SESSION_COOKIE_PATH": None,
            "SESSION_COOKIE_HTTPONLY": True,
            "SESSION_COOKIE_SECURE": False,
            "SESSION_COOKIE_SAMESITE": None,
            "SESSION_REFRESH_EACH_REQUEST": True,
            "MAX_CONTENT_LENGTH": None,
            "SEND_FILE_MAX_AGE_DEFAULT": timedelta(hours=12),
            "TRAP_BAD_REQUEST_ERRORS": None,
            "TRAP_HTTP_EXCEPTIONS": False,
            "EXPLAIN_TEMPLATE_LOADING": False,
            "PREFERRED_URL_SCHEME": "http",
            "JSON_AS_ASCII": True,
            "JSON_SORT_KEYS": True,
            "JSONIFY_PRETTYPRINT_REGULAR": False,
            "JSONIFY_MIMETYPE": "application/json",
            "TEMPLATES_AUTO_RELOAD": None,
            "MAX_COOKIE_SIZE": 4093,
        }
    )

    #: The rule object to use for URL rules created.  This is used by
    #: :meth:`add_url_rule`.  Defaults to :class:`werkzeug.routing.Rule`.
    #:
    #: .. versionadded:: 0.7
    url_rule_class = Rule

    #: The map object to use for storing the URL rules and routing
    #: configuration parameters. Defaults to :class:`werkzeug.routing.Map`.
    #:
    #: .. versionadded:: 1.1.0
    url_map_class = Map

    #: the test client that is used with when `test_client` is used.
    #:
    #: .. versionadded:: 0.7
    test_client_class = None

    #: The :class:`~click.testing.CliRunner` subclass, by default
    #: :class:`~flask.testing.FlaskCliRunner` that is used by
    #: :meth:`test_cli_runner`. Its ``__init__`` method should take a
    #: Flask app object as the first argument.
    #:
    #: .. versionadded:: 1.0
    test_cli_runner_class = None

    #: the session interface to use.  By default an instance of
    #: :class:`~flask.sessions.SecureCookieSessionInterface` is used here.
    #:
    #: .. versionadded:: 0.8
    session_interface = SecureCookieSessionInterface()

    # TODO remove the next three attrs when Sphinx :inherited-members: works
    # https://github.com/sphinx-doc/sphinx/issues/741

    #: The name of the package or module that this app belongs to. Do not
    #: change this once it is set by the constructor.
    import_name = None

    #: Location of the template files to be added to the template lookup.
    #: ``None`` if templates should not be added.
    template_folder = None

    #: Absolute path to the package on the filesystem. Used to look up
    #: resources contained in the package.
    root_path = None

    def __init__(
        self,
        import_name,
        static_url_path=None,
        static_folder="static",
        static_host=None,
        host_matching=False,
        subdomain_matching=False,
        template_folder="templates",
        instance_path=None,
        instance_relative_config=False,
        root_path=None,
    ):
        _PackageBoundObject.__init__(
            self, import_name, template_folder=template_folder, root_path=root_path
        )

        self.static_url_path = static_url_path
        self.static_folder = static_folder

        if instance_path is None:
            instance_path = self.auto_find_instance_path()
        elif not os.path.isabs(instance_path):
            raise ValueError(
                "If an instance path is provided it must be absolute."
                " A relative path was given instead."
            )

        #: Holds the path to the instance folder.
        #:
        #: .. versionadded:: 0.8
        self.instance_path = instance_path

        #: The configuration dictionary as :class:`Config`.  This behaves
        #: exactly like a regular dictionary but supports additional methods
        #: to load a config from files.
        self.config = self.make_config(instance_relative_config)

        #: A dictionary of all view functions registered.  The keys will
        #: be function names which are also used to generate URLs and
        #: the values are the function objects themselves.
        #: To register a view function, use the :meth:`route` decorator.
        self.view_functions = {}

        #: A dictionary of all registered error handlers.  The key is ``None``
        #: for error handlers active on the application, otherwise the key is
        #: the name of the blueprint.  Each key points to another dictionary
        #: where the key is the status code of the http exception.  The
        #: special key ``None`` points to a list of tuples where the first item
        #: is the class for the instance check and the second the error handler
        #: function.
        #:
        #: To register an error handler, use the :meth:`errorhandler`
        #: decorator.
        self.error_handler_spec = {}

        #: A list of functions that are called when :meth:`url_for` raises a
        #: :exc:`~werkzeug.routing.BuildError`.  Each function registered here
        #: is called with `error`, `endpoint` and `values`.  If a function
        #: returns ``None`` or raises a :exc:`BuildError` the next function is
        #: tried.
        #:
        #: .. versionadded:: 0.9
        self.url_build_error_handlers = []

        #: A dictionary with lists of functions that will be called at the
        #: beginning of each request. The key of the dictionary is the name of
        #: the blueprint this function is active for, or ``None`` for all
        #: requests. To register a function, use the :meth:`before_request`
        #: decorator.
        self.before_request_funcs = {}

        #: A list of functions that will be called at the beginning of the
        #: first request to this instance. To register a function, use the
        #: :meth:`before_first_request` decorator.
        #:
        #: .. versionadded:: 0.8
        self.before_first_request_funcs = []

        #: A dictionary with lists of functions that should be called after
        #: each request.  The key of the dictionary is the name of the blueprint
        #: this function is active for, ``None`` for all requests.  This can for
        #: example be used to close database connections. To register a function
        #: here, use the :meth:`after_request` decorator.
        self.after_request_funcs = {}

        #: A dictionary with lists of functions that are called after
        #: each request, even if an exception has occurred. The key of the
        #: dictionary is the name of the blueprint this function is active for,
        #: ``None`` for all requests. These functions are not allowed to modify
        #: the request, and their return values are ignored. If an exception
        #: occurred while processing the request, it gets passed to each
        #: teardown_request function. To register a function here, use the
        #: :meth:`teardown_request` decorator.
        #:
        #: .. versionadded:: 0.7
        self.teardown_request_funcs = {}

        #: A list of functions that are called when the application context
        #: is destroyed.  Since the application context is also torn down
        #: if the request ends this is the place to store code that disconnects
        #: from databases.
        #:
        #: .. versionadded:: 0.9
        self.teardown_appcontext_funcs = []

        #: A dictionary with lists of functions that are called before the
        #: :attr:`before_request_funcs` functions. The key of the dictionary is
        #: the name of the blueprint this function is active for, or ``None``
        #: for all requests. To register a function, use
        #: :meth:`url_value_preprocessor`.
        #:
        #: .. versionadded:: 0.7
        self.url_value_preprocessors = {}

        #: A dictionary with lists of functions that can be used as URL value
        #: preprocessors.  The key ``None`` here is used for application wide
        #: callbacks, otherwise the key is the name of the blueprint.
        #: Each of these functions has the chance to modify the dictionary
        #: of URL values before they are used as the keyword arguments of the
        #: view function.  For each function registered this one should also
        #: provide a :meth:`url_defaults` function that adds the parameters
        #: automatically again that were removed that way.
        #:
        #: .. versionadded:: 0.7
        self.url_default_functions = {}

        #: A dictionary with list of functions that are called without argument
        #: to populate the template context.  The key of the dictionary is the
        #: name of the blueprint this function is active for, ``None`` for all
        #: requests.  Each returns a dictionary that the template context is
        #: updated with.  To register a function here, use the
        #: :meth:`context_processor` decorator.
        self.template_context_processors = {None: [_default_template_ctx_processor]}

        #: A list of shell context processor functions that should be run
        #: when a shell context is created.
        #:
        #: .. versionadded:: 0.11
        self.shell_context_processors = []

        #: all the attached blueprints in a dictionary by name.  Blueprints
        #: can be attached multiple times so this dictionary does not tell
        #: you how often they got attached.
        #:
        #: .. versionadded:: 0.7
        self.blueprints = {}
        self._blueprint_order = []

        #: a place where extensions can store application specific state.  For
        #: example this is where an extension could store database engines and
        #: similar things.  For backwards compatibility extensions should register
        #: themselves like this::
        #:
        #:      if not hasattr(app, 'extensions'):
        #:          app.extensions = {}
        #:      app.extensions['extensionname'] = SomeObject()
        #:
        #: The key must match the name of the extension module. For example in
        #: case of a "Flask-Foo" extension in `flask_foo`, the key would be
        #: ``'foo'``.
        #:
        #: .. versionadded:: 0.7
        self.extensions = {}

        #: The :class:`~werkzeug.routing.Map` for this instance.  You can use
        #: this to change the routing converters after the class was created
        #: but before any routes are connected.  Example::
        #:
        #:    from werkzeug.routing import BaseConverter
        #:
        #:    class ListConverter(BaseConverter):
        #:        def to_python(self, value):
        #:            return value.split(',')
        #:        def to_url(self, values):
        #:            return ','.join(super(ListConverter, self).to_url(value)
        #:                            for value in values)
        #:
        #:    app = Flask(__name__)
        #:    app.url_map.converters['list'] = ListConverter
        self.url_map = self.url_map_class()

        self.url_map.host_matching = host_matching
        self.subdomain_matching = subdomain_matching

        # tracks internally if the application already handled at least one
        # request.
        self._got_first_request = False
        self._before_request_lock = Lock()

        # Add a static route using the provided static_url_path, static_host,
        # and static_folder if there is a configured static_folder.
        # Note we do this without checking if static_folder exists.
        # For one, it might be created while the server is running (e.g. during
        # development). Also, Google App Engine stores static files somewhere
        if self.has_static_folder:
            assert (
                bool(static_host) == host_matching
            ), "Invalid static_host/host_matching combination"
            self.add_url_rule(
                self.static_url_path + "/<path:filename>",
                endpoint="static",
                host=static_host,
                view_func=self.send_static_file,
            )

        # Set the name of the Click group in case someone wants to add
        # the app's commands to another CLI tool.
        self.cli.name = self.name

    @locked_cached_property
    def name(self):
        """The name of the application.  This is usually the import name
        with the difference that it's guessed from the run file if the
        import name is main.  This name is used as a display name when
        Flask needs the name of the application.  It can be set and overridden
        to change the value.

        .. versionadded:: 0.8
        """
        if self.import_name == "__main__":
            fn = getattr(sys.modules["__main__"], "__file__", None)
            if fn is None:
                return "__main__"
            return os.path.splitext(os.path.basename(fn))[0]
        return self.import_name

    @property
    def propagate_exceptions(self):
        """Returns the value of the ``PROPAGATE_EXCEPTIONS`` configuration
        value in case it's set, otherwise a sensible default is returned.

        .. versionadded:: 0.7
        """
        rv = self.config["PROPAGATE_EXCEPTIONS"]
        if rv is not None:
            return rv
        return self.testing or self.debug

    @property
    def preserve_context_on_exception(self):
        """Returns the value of the ``PRESERVE_CONTEXT_ON_EXCEPTION``
        configuration value in case it's set, otherwise a sensible default
        is returned.

        .. versionadded:: 0.7
        """
        rv = self.config["PRESERVE_CONTEXT_ON_EXCEPTION"]
        if rv is not None:
            return rv
        return self.debug

    @locked_cached_property
    def logger(self):
        """A standard Python :class:`~logging.Logger` for the app, with
        the same name as :attr:`name`.

        In debug mode, the logger's :attr:`~logging.Logger.level` will
        be set to :data:`~logging.DEBUG`.

        If there are no handlers configured, a default handler will be
        added. See :doc:`/logging` for more information.

        .. versionchanged:: 1.1.0
            The logger takes the same name as :attr:`name` rather than
            hard-coding ``"flask.app"``.

        .. versionchanged:: 1.0.0
            Behavior was simplified. The logger is always named
            ``"flask.app"``. The level is only set during configuration,
            it doesn't check ``app.debug`` each time. Only one format is
            used, not different ones depending on ``app.debug``. No
            handlers are removed, and a handler is only added if no
            handlers are already configured.

        .. versionadded:: 0.3
        """
        return create_logger(self)

    @locked_cached_property
    def jinja_env(self):
        """The Jinja environment used to load templates.

        The environment is created the first time this property is
        accessed. Changing :attr:`jinja_options` after that will have no
        effect.
        """
        return self.create_jinja_environment()

    @property
    def got_first_request(self):
        """This attribute is set to ``True`` if the application started
        handling the first request.

        .. versionadded:: 0.8
        """
        return self._got_first_request

    def make_config(self, instance_relative=False):
        """Used to create the config attribute by the Flask constructor.
        The `instance_relative` parameter is passed in from the constructor
        of Flask (there named `instance_relative_config`) and indicates if
        the config should be relative to the instance path or the root path
        of the application.

        .. versionadded:: 0.8
        """
        root_path = self.root_path
        if instance_relative:
            root_path = self.instance_path
        defaults = dict(self.default_config)
        defaults["ENV"] = get_env()
        defaults["DEBUG"] = get_debug_flag()
        return self.config_class(root_path, defaults)

    def auto_find_instance_path(self):
        """Tries to locate the instance path if it was not provided to the
        constructor of the application class.  It will basically calculate
        the path to a folder named ``instance`` next to your main file or
        the package.

        .. versionadded:: 0.8
        """
        prefix, package_path = find_package(self.import_name)
        if prefix is None:
            return os.path.join(package_path, "instance")
        return os.path.join(prefix, "var", self.name + "-instance")

    def open_instance_resource(self, resource, mode="rb"):
        """Opens a resource from the application's instance folder
        (:attr:`instance_path`).  Otherwise works like
        :meth:`open_resource`.  Instance resources can also be opened for
        writing.

        :param resource: the name of the resource.  To access resources within
                         subfolders use forward slashes as separator.
        :param mode: resource file opening mode, default is 'rb'.
        """
        return open(os.path.join(self.instance_path, resource), mode)

    @property
    def templates_auto_reload(self):
        """Reload templates when they are changed. Used by
        :meth:`create_jinja_environment`.

        This attribute can be configured with :data:`TEMPLATES_AUTO_RELOAD`. If
        not set, it will be enabled in debug mode.

        .. versionadded:: 1.0
            This property was added but the underlying config and behavior
            already existed.
        """
        rv = self.config["TEMPLATES_AUTO_RELOAD"]
        return rv if rv is not None else self.debug

    @templates_auto_reload.setter
    def templates_auto_reload(self, value):
        self.config["TEMPLATES_AUTO_RELOAD"] = value

    def create_jinja_environment(self):
        """Create the Jinja environment based on :attr:`jinja_options`
        and the various Jinja-related methods of the app. Changing
        :attr:`jinja_options` after this will have no effect. Also adds
        Flask-related globals and filters to the environment.

        .. versionchanged:: 0.11
           ``Environment.auto_reload`` set in accordance with
           ``TEMPLATES_AUTO_RELOAD`` configuration option.

        .. versionadded:: 0.5
        """
        options = dict(self.jinja_options)

        if "autoescape" not in options:
            options["autoescape"] = self.select_jinja_autoescape

        if "auto_reload" not in options:
            options["auto_reload"] = self.templates_auto_reload

        rv = self.jinja_environment(self, **options)
        rv.globals.update(
            url_for=url_for,
            get_flashed_messages=get_flashed_messages,
            config=self.config,
            # request, session and g are normally added with the
            # context processor for efficiency reasons but for imported
            # templates we also want the proxies in there.
            request=request,
            session=session,
            g=g,
        )
        rv.filters["tojson"] = json.tojson_filter
        return rv

    def create_global_jinja_loader(self):
        """Creates the loader for the Jinja2 environment.  Can be used to
        override just the loader and keeping the rest unchanged.  It's
        discouraged to override this function.  Instead one should override
        the :meth:`jinja_loader` function instead.

        The global loader dispatches between the loaders of the application
        and the individual blueprints.

        .. versionadded:: 0.7
        """
        return DispatchingJinjaLoader(self)

    def select_jinja_autoescape(self, filename):
        """Returns ``True`` if autoescaping should be active for the given
        template name. If no template name is given, returns `True`.

        .. versionadded:: 0.5
        """
        if filename is None:
            return True
        return filename.endswith((".html", ".htm", ".xml", ".xhtml"))

    def update_template_context(self, context):
        """Update the template context with some commonly used variables.
        This injects request, session, config and g into the template
        context as well as everything template context processors want
        to inject.  Note that the as of Flask 0.6, the original values
        in the context will not be overridden if a context processor
        decides to return a value with the same key.

        :param context: the context as a dictionary that is updated in place
                        to add extra variables.
        """
        funcs = self.template_context_processors[None]
        reqctx = _request_ctx_stack.top
        if reqctx is not None:
            bp = reqctx.request.blueprint
            if bp is not None and bp in self.template_context_processors:
                funcs = chain(funcs, self.template_context_processors[bp])
        orig_ctx = context.copy()
        for func in funcs:
            context.update(func())
        # make sure the original values win.  This makes it possible to
        # easier add new variables in context processors without breaking
        # existing views.
        context.update(orig_ctx)

    def make_shell_context(self):
        """Returns the shell context for an interactive shell for this
        application.  This runs all the registered shell context
        processors.

        .. versionadded:: 0.11
        """
        rv = {"app": self, "g": g}
        for processor in self.shell_context_processors:
            rv.update(processor())
        return rv

    #: What environment the app is running in. Flask and extensions may
    #: enable behaviors based on the environment, such as enabling debug
    #: mode. This maps to the :data:`ENV` config key. This is set by the
    #: :envvar:`FLASK_ENV` environment variable and may not behave as
    #: expected if set in code.
    #:
    #: **Do not enable development when deploying in production.**
    #:
    #: Default: ``'production'``
    env = ConfigAttribute("ENV")

    @property
    def debug(self):
        """Whether debug mode is enabled. When using ``flask run`` to start
        the development server, an interactive debugger will be shown for
        unhandled exceptions, and the server will be reloaded when code
        changes. This maps to the :data:`DEBUG` config key. This is
        enabled when :attr:`env` is ``'development'`` and is overridden
        by the ``FLASK_DEBUG`` environment variable. It may not behave as
        expected if set in code.

        **Do not enable debug mode when deploying in production.**

        Default: ``True`` if :attr:`env` is ``'development'``, or
        ``False`` otherwise.
        """
        return self.config["DEBUG"]

    @debug.setter
    def debug(self, value):
        self.config["DEBUG"] = value
        self.jinja_env.auto_reload = self.templates_auto_reload

    def run(self, host=None, port=None, debug=None, load_dotenv=True, **options):
        """Runs the application on a local development server.

        Do not use ``run()`` in a production setting. It is not intended to
        meet security and performance requirements for a production server.
        Instead, see :ref:`deployment` for WSGI server recommendations.

        If the :attr:`debug` flag is set the server will automatically reload
        for code changes and show a debugger in case an exception happened.

        If you want to run the application in debug mode, but disable the
        code execution on the interactive debugger, you can pass
        ``use_evalex=False`` as parameter.  This will keep the debugger's
        traceback screen active, but disable code execution.

        It is not recommended to use this function for development with
        automatic reloading as this is badly supported.  Instead you should
        be using the :command:`flask` command line script's ``run`` support.

        .. admonition:: Keep in Mind

           Flask will suppress any server error with a generic error page
           unless it is in debug mode.  As such to enable just the
           interactive debugger without the code reloading, you have to
           invoke :meth:`run` with ``debug=True`` and ``use_reloader=False``.
           Setting ``use_debugger`` to ``True`` without being in debug mode
           won't catch any exceptions because there won't be any to
           catch.

        :param host: the hostname to listen on. Set this to ``'0.0.0.0'`` to
            have the server available externally as well. Defaults to
            ``'127.0.0.1'`` or the host in the ``SERVER_NAME`` config variable
            if present.
        :param port: the port of the webserver. Defaults to ``5000`` or the
            port defined in the ``SERVER_NAME`` config variable if present.
        :param debug: if given, enable or disable debug mode. See
            :attr:`debug`.
        :param load_dotenv: Load the nearest :file:`.env` and :file:`.flaskenv`
            files to set environment variables. Will also change the working
            directory to the directory containing the first file found.
        :param options: the options to be forwarded to the underlying Werkzeug
            server. See :func:`werkzeug.serving.run_simple` for more
            information.

        .. versionchanged:: 1.0
            If installed, python-dotenv will be used to load environment
            variables from :file:`.env` and :file:`.flaskenv` files.

            If set, the :envvar:`FLASK_ENV` and :envvar:`FLASK_DEBUG`
            environment variables will override :attr:`env` and
            :attr:`debug`.

            Threaded mode is enabled by default.

        .. versionchanged:: 0.10
            The default port is now picked from the ``SERVER_NAME``
            variable.
        """
        # Change this into a no-op if the server is invoked from the
        # command line. Have a look at cli.py for more information.
        if os.environ.get("FLASK_RUN_FROM_CLI") == "true":
            from .debughelpers import explain_ignored_app_run

            explain_ignored_app_run()
            return

        if get_load_dotenv(load_dotenv):
            cli.load_dotenv()

            # if set, let env vars override previous values
            if "FLASK_ENV" in os.environ:
                self.env = get_env()
                self.debug = get_debug_flag()
            elif "FLASK_DEBUG" in os.environ:
                self.debug = get_debug_flag()

        # debug passed to method overrides all other sources
        if debug is not None:
            self.debug = bool(debug)

        _host = "127.0.0.1"
        _port = 5000
        server_name = self.config.get("SERVER_NAME")
        sn_host, sn_port = None, None

        if server_name:
            sn_host, _, sn_port = server_name.partition(":")

        host = host or sn_host or _host
        # pick the first value that's not None (0 is allowed)
        port = int(next((p for p in (port, sn_port) if p is not None), _port))

        options.setdefault("use_reloader", self.debug)
        options.setdefault("use_debugger", self.debug)
        options.setdefault("threaded", True)

        cli.show_server_banner(self.env, self.debug, self.name, False)

        from werkzeug.serving import run_simple

        try:
            run_simple(host, port, self, **options)
        finally:
            # reset the first request information if the development server
            # reset normally.  This makes it possible to restart the server
            # without reloader and that stuff from an interactive shell.
            self._got_first_request = False

    def test_client(self, use_cookies=True, **kwargs):
        """Creates a test client for this application.  For information
        about unit testing head over to :ref:`testing`.

        Note that if you are testing for assertions or exceptions in your
        application code, you must set ``app.testing = True`` in order for the
        exceptions to propagate to the test client.  Otherwise, the exception
        will be handled by the application (not visible to the test client) and
        the only indication of an AssertionError or other exception will be a
        500 status code response to the test client.  See the :attr:`testing`
        attribute.  For example::

            app.testing = True
            client = app.test_client()

        The test client can be used in a ``with`` block to defer the closing down
        of the context until the end of the ``with`` block.  This is useful if
        you want to access the context locals for testing::

            with app.test_client() as c:
                rv = c.get('/?vodka=42')
                assert request.args['vodka'] == '42'

        Additionally, you may pass optional keyword arguments that will then
        be passed to the application's :attr:`test_client_class` constructor.
        For example::

            from flask.testing import FlaskClient

            class CustomClient(FlaskClient):
                def __init__(self, *args, **kwargs):
                    self._authentication = kwargs.pop("authentication")
                    super(CustomClient,self).__init__( *args, **kwargs)

            app.test_client_class = CustomClient
            client = app.test_client(authentication='Basic ....')

        See :class:`~flask.testing.FlaskClient` for more information.

        .. versionchanged:: 0.4
           added support for ``with`` block usage for the client.

        .. versionadded:: 0.7
           The `use_cookies` parameter was added as well as the ability
           to override the client to be used by setting the
           :attr:`test_client_class` attribute.

        .. versionchanged:: 0.11
           Added `**kwargs` to support passing additional keyword arguments to
           the constructor of :attr:`test_client_class`.
        """
        cls = self.test_client_class
        if cls is None:
            from .testing import FlaskClient as cls
        return cls(self, self.response_class, use_cookies=use_cookies, **kwargs)

    def test_cli_runner(self, **kwargs):
        """Create a CLI runner for testing CLI commands.
        See :ref:`testing-cli`.

        Returns an instance of :attr:`test_cli_runner_class`, by default
        :class:`~flask.testing.FlaskCliRunner`. The Flask app object is
        passed as the first argument.

        .. versionadded:: 1.0
        """
        cls = self.test_cli_runner_class

        if cls is None:
            from .testing import FlaskCliRunner as cls

        return cls(self, **kwargs)

    def open_session(self, request):
        """Creates or opens a new session.  Default implementation stores all
        session data in a signed cookie.  This requires that the
        :attr:`secret_key` is set.  Instead of overriding this method
        we recommend replacing the :class:`session_interface`.

        .. deprecated: 1.0
            Will be removed in 2.0. Use
            ``session_interface.open_session`` instead.

        :param request: an instance of :attr:`request_class`.
        """

        warnings.warn(
            DeprecationWarning(
                '"open_session" is deprecated and will be removed in'
                ' 2.0. Use "session_interface.open_session" instead.'
            )
        )
        return self.session_interface.open_session(self, request)

    def save_session(self, session, response):
        """Saves the session if it needs updates.  For the default
        implementation, check :meth:`open_session`.  Instead of overriding this
        method we recommend replacing the :class:`session_interface`.

        .. deprecated: 1.0
            Will be removed in 2.0. Use
            ``session_interface.save_session`` instead.

        :param session: the session to be saved (a
                        :class:`~werkzeug.contrib.securecookie.SecureCookie`
                        object)
        :param response: an instance of :attr:`response_class`
        """

        warnings.warn(
            DeprecationWarning(
                '"save_session" is deprecated and will be removed in'
                ' 2.0. Use "session_interface.save_session" instead.'
            )
        )
        return self.session_interface.save_session(self, session, response)

    def make_null_session(self):
        """Creates a new instance of a missing session.  Instead of overriding
        this method we recommend replacing the :class:`session_interface`.

        .. deprecated: 1.0
            Will be removed in 2.0. Use
            ``session_interface.make_null_session`` instead.

        .. versionadded:: 0.7
        """

        warnings.warn(
            DeprecationWarning(
                '"make_null_session" is deprecated and will be removed'
                ' in 2.0. Use "session_interface.make_null_session"'
                " instead."
            )
        )
        return self.session_interface.make_null_session(self)

    @setupmethod
    def register_blueprint(self, blueprint, **options):
        """Register a :class:`~flask.Blueprint` on the application. Keyword
        arguments passed to this method will override the defaults set on the
        blueprint.

        Calls the blueprint's :meth:`~flask.Blueprint.register` method after
        recording the blueprint in the application's :attr:`blueprints`.

        :param blueprint: The blueprint to register.
        :param url_prefix: Blueprint routes will be prefixed with this.
        :param subdomain: Blueprint routes will match on this subdomain.
        :param url_defaults: Blueprint routes will use these default values for
            view arguments.
        :param options: Additional keyword arguments are passed to
            :class:`~flask.blueprints.BlueprintSetupState`. They can be
            accessed in :meth:`~flask.Blueprint.record` callbacks.

        .. versionadded:: 0.7
        """
        first_registration = False

        if blueprint.name in self.blueprints:
            assert self.blueprints[blueprint.name] is blueprint, (
                "A name collision occurred between blueprints %r and %r. Both"
                ' share the same name "%s". Blueprints that are created on the'
                " fly need unique names."
                % (blueprint, self.blueprints[blueprint.name], blueprint.name)
            )
        else:
            self.blueprints[blueprint.name] = blueprint
            self._blueprint_order.append(blueprint)
            first_registration = True

        blueprint.register(self, options, first_registration)

    def iter_blueprints(self):
        """Iterates over all blueprints by the order they were registered.

        .. versionadded:: 0.11
        """
        return iter(self._blueprint_order)

    @setupmethod
    def add_url_rule(
        self,
        rule,
        endpoint=None,
        view_func=None,
        provide_automatic_options=None,
        **options
    ):
        """Connects a URL rule.  Works exactly like the :meth:`route`
        decorator.  If a view_func is provided it will be registered with the
        endpoint.

        Basically this example::

            @app.route('/')
            def index():
                pass

        Is equivalent to the following::

            def index():
                pass
            app.add_url_rule('/', 'index', index)

        If the view_func is not provided you will need to connect the endpoint
        to a view function like so::

            app.view_functions['index'] = index

        Internally :meth:`route` invokes :meth:`add_url_rule` so if you want
        to customize the behavior via subclassing you only need to change
        this method.

        For more information refer to :ref:`url-route-registrations`.

        .. versionchanged:: 0.2
           `view_func` parameter added.

        .. versionchanged:: 0.6
           ``OPTIONS`` is added automatically as method.

        :param rule: the URL rule as string
        :param endpoint: the endpoint for the registered URL rule.  Flask
                         itself assumes the name of the view function as
                         endpoint
        :param view_func: the function to call when serving a request to the
                          provided endpoint
        :param provide_automatic_options: controls whether the ``OPTIONS``
            method should be added automatically. This can also be controlled
            by setting the ``view_func.provide_automatic_options = False``
            before adding the rule.
        :param options: the options to be forwarded to the underlying
                        :class:`~werkzeug.routing.Rule` object.  A change
                        to Werkzeug is handling of method options.  methods
                        is a list of methods this rule should be limited
                        to (``GET``, ``POST`` etc.).  By default a rule
                        just listens for ``GET`` (and implicitly ``HEAD``).
                        Starting with Flask 0.6, ``OPTIONS`` is implicitly
                        added and handled by the standard request handling.
        """
        if endpoint is None:
            endpoint = _endpoint_from_view_func(view_func)
        options["endpoint"] = endpoint
        methods = options.pop("methods", None)

        # if the methods are not given and the view_func object knows its
        # methods we can use that instead.  If neither exists, we go with
        # a tuple of only ``GET`` as default.
        if methods is None:
            methods = getattr(view_func, "methods", None) or ("GET",)
        if isinstance(methods, string_types):
            raise TypeError(
                "Allowed methods have to be iterables of strings, "
                'for example: @app.route(..., methods=["POST"])'
            )
        methods = set(item.upper() for item in methods)

        # Methods that should always be added
        required_methods = set(getattr(view_func, "required_methods", ()))

        # starting with Flask 0.8 the view_func object can disable and
        # force-enable the automatic options handling.
        if provide_automatic_options is None:
            provide_automatic_options = getattr(
                view_func, "provide_automatic_options", None
            )

        if provide_automatic_options is None:
            if "OPTIONS" not in methods:
                provide_automatic_options = True
                required_methods.add("OPTIONS")
            else:
                provide_automatic_options = False

        # Add the required methods now.
        methods |= required_methods

        rule = self.url_rule_class(rule, methods=methods, **options)
        rule.provide_automatic_options = provide_automatic_options

        self.url_map.add(rule)
        if view_func is not None:
            old_func = self.view_functions.get(endpoint)
            if old_func is not None and old_func != view_func:
                raise AssertionError(
                    "View function mapping is overwriting an "
                    "existing endpoint function: %s" % endpoint
                )
            self.view_functions[endpoint] = view_func

    def route(self, rule, **options):
        """A decorator that is used to register a view function for a
        given URL rule.  This does the same thing as :meth:`add_url_rule`
        but is intended for decorator usage::

            @app.route('/')
            def index():
                return 'Hello World'

        For more information refer to :ref:`url-route-registrations`.

        :param rule: the URL rule as string
        :param endpoint: the endpoint for the registered URL rule.  Flask
                         itself assumes the name of the view function as
                         endpoint
        :param options: the options to be forwarded to the underlying
                        :class:`~werkzeug.routing.Rule` object.  A change
                        to Werkzeug is handling of method options.  methods
                        is a list of methods this rule should be limited
                        to (``GET``, ``POST`` etc.).  By default a rule
                        just listens for ``GET`` (and implicitly ``HEAD``).
                        Starting with Flask 0.6, ``OPTIONS`` is implicitly
                        added and handled by the standard request handling.
        """

        def decorator(f):
            endpoint = options.pop("endpoint", None)
            self.add_url_rule(rule, endpoint, f, **options)
            return f

        return decorator

    @setupmethod
    def endpoint(self, endpoint):
        """A decorator to register a function as an endpoint.
        Example::

            @app.endpoint('example.endpoint')
            def example():
                return "example"

        :param endpoint: the name of the endpoint
        """

        def decorator(f):
            self.view_functions[endpoint] = f
            return f

        return decorator

    @staticmethod
    def _get_exc_class_and_code(exc_class_or_code):
        """Get the exception class being handled. For HTTP status codes
        or ``HTTPException`` subclasses, return both the exception and
        status code.

        :param exc_class_or_code: Any exception class, or an HTTP status
            code as an integer.
        """
        if isinstance(exc_class_or_code, integer_types):
            exc_class = default_exceptions[exc_class_or_code]
        else:
            exc_class = exc_class_or_code

        assert issubclass(exc_class, Exception)

        if issubclass(exc_class, HTTPException):
            return exc_class, exc_class.code
        else:
            return exc_class, None

    @setupmethod
    def errorhandler(self, code_or_exception):
        """Register a function to handle errors by code or exception class.

        A decorator that is used to register a function given an
        error code.  Example::

            @app.errorhandler(404)
            def page_not_found(error):
                return 'This page does not exist', 404

        You can also register handlers for arbitrary exceptions::

            @app.errorhandler(DatabaseError)
            def special_exception_handler(error):
                return 'Database connection failed', 500

        .. versionadded:: 0.7
            Use :meth:`register_error_handler` instead of modifying
            :attr:`error_handler_spec` directly, for application wide error
            handlers.

        .. versionadded:: 0.7
           One can now additionally also register custom exception types
           that do not necessarily have to be a subclass of the
           :class:`~werkzeug.exceptions.HTTPException` class.

        :param code_or_exception: the code as integer for the handler, or
                                  an arbitrary exception
        """

        def decorator(f):
            self._register_error_handler(None, code_or_exception, f)
            return f

        return decorator

    @setupmethod
    def register_error_handler(self, code_or_exception, f):
        """Alternative error attach function to the :meth:`errorhandler`
        decorator that is more straightforward to use for non decorator
        usage.

        .. versionadded:: 0.7
        """
        self._register_error_handler(None, code_or_exception, f)

    @setupmethod
    def _register_error_handler(self, key, code_or_exception, f):
        """
        :type key: None|str
        :type code_or_exception: int|T<=Exception
        :type f: callable
        """
        if isinstance(code_or_exception, HTTPException):  # old broken behavior
            raise ValueError(
                "Tried to register a handler for an exception instance {0!r}."
                " Handlers can only be registered for exception classes or"
                " HTTP error codes.".format(code_or_exception)
            )

        try:
            exc_class, code = self._get_exc_class_and_code(code_or_exception)
        except KeyError:
            raise KeyError(
                "'{0}' is not a recognized HTTP error code. Use a subclass of"
                " HTTPException with that code instead.".format(code_or_exception)
            )

        handlers = self.error_handler_spec.setdefault(key, {}).setdefault(code, {})
        handlers[exc_class] = f

    @setupmethod
    def template_filter(self, name=None):
        """A decorator that is used to register custom template filter.
        You can specify a name for the filter, otherwise the function
        name will be used. Example::

          @app.template_filter()
          def reverse(s):
              return s[::-1]

        :param name: the optional name of the filter, otherwise the
                     function name will be used.
        """

        def decorator(f):
            self.add_template_filter(f, name=name)
            return f

        return decorator

    @setupmethod
    def add_template_filter(self, f, name=None):
        """Register a custom template filter.  Works exactly like the
        :meth:`template_filter` decorator.

        :param name: the optional name of the filter, otherwise the
                     function name will be used.
        """
        self.jinja_env.filters[name or f.__name__] = f

    @setupmethod
    def template_test(self, name=None):
        """A decorator that is used to register custom template test.
        You can specify a name for the test, otherwise the function
        name will be used. Example::

          @app.template_test()
          def is_prime(n):
              if n == 2:
                  return True
              for i in range(2, int(math.ceil(math.sqrt(n))) + 1):
                  if n % i == 0:
                      return False
              return True

        .. versionadded:: 0.10

        :param name: the optional name of the test, otherwise the
                     function name will be used.
        """

        def decorator(f):
            self.add_template_test(f, name=name)
            return f

        return decorator

    @setupmethod
    def add_template_test(self, f, name=None):
        """Register a custom template test.  Works exactly like the
        :meth:`template_test` decorator.

        .. versionadded:: 0.10

        :param name: the optional name of the test, otherwise the
                     function name will be used.
        """
        self.jinja_env.tests[name or f.__name__] = f

    @setupmethod
    def template_global(self, name=None):
        """A decorator that is used to register a custom template global function.
        You can specify a name for the global function, otherwise the function
        name will be used. Example::

            @app.template_global()
            def double(n):
                return 2 * n

        .. versionadded:: 0.10

        :param name: the optional name of the global function, otherwise the
                     function name will be used.
        """

        def decorator(f):
            self.add_template_global(f, name=name)
            return f

        return decorator

    @setupmethod
    def add_template_global(self, f, name=None):
        """Register a custom template global function. Works exactly like the
        :meth:`template_global` decorator.

        .. versionadded:: 0.10

        :param name: the optional name of the global function, otherwise the
                     function name will be used.
        """
        self.jinja_env.globals[name or f.__name__] = f

    @setupmethod
    def before_request(self, f):
        """Registers a function to run before each request.

        For example, this can be used to open a database connection, or to load
        the logged in user from the session.

        The function will be called without any arguments. If it returns a
        non-None value, the value is handled as if it was the return value from
        the view, and further request handling is stopped.
        """
        self.before_request_funcs.setdefault(None, []).append(f)
        return f

    @setupmethod
    def before_first_request(self, f):
        """Registers a function to be run before the first request to this
        instance of the application.

        The function will be called without any arguments and its return
        value is ignored.

        .. versionadded:: 0.8
        """
        self.before_first_request_funcs.append(f)
        return f

    @setupmethod
    def after_request(self, f):
        """Register a function to be run after each request.

        Your function must take one parameter, an instance of
        :attr:`response_class` and return a new response object or the
        same (see :meth:`process_response`).

        As of Flask 0.7 this function might not be executed at the end of the
        request in case an unhandled exception occurred.
        """
        self.after_request_funcs.setdefault(None, []).append(f)
        return f

    @setupmethod
    def teardown_request(self, f):
        """Register a function to be run at the end of each request,
        regardless of whether there was an exception or not.  These functions
        are executed when the request context is popped, even if not an
        actual request was performed.

        Example::

            ctx = app.test_request_context()
            ctx.push()
            ...
            ctx.pop()

        When ``ctx.pop()`` is executed in the above example, the teardown
        functions are called just before the request context moves from the
        stack of active contexts.  This becomes relevant if you are using
        such constructs in tests.

        Generally teardown functions must take every necessary step to avoid
        that they will fail.  If they do execute code that might fail they
        will have to surround the execution of these code by try/except
        statements and log occurring errors.

        When a teardown function was called because of an exception it will
        be passed an error object.

        The return values of teardown functions are ignored.

        .. admonition:: Debug Note

           In debug mode Flask will not tear down a request on an exception
           immediately.  Instead it will keep it alive so that the interactive
           debugger can still access it.  This behavior can be controlled
           by the ``PRESERVE_CONTEXT_ON_EXCEPTION`` configuration variable.
        """
        self.teardown_request_funcs.setdefault(None, []).append(f)
        return f

    @setupmethod
    def teardown_appcontext(self, f):
        """Registers a function to be called when the application context
        ends.  These functions are typically also called when the request
        context is popped.

        Example::

            ctx = app.app_context()
            ctx.push()
            ...
            ctx.pop()

        When ``ctx.pop()`` is executed in the above example, the teardown
        functions are called just before the app context moves from the
        stack of active contexts.  This becomes relevant if you are using
        such constructs in tests.

        Since a request context typically also manages an application
        context it would also be called when you pop a request context.

        When a teardown function was called because of an unhandled exception
        it will be passed an error object. If an :meth:`errorhandler` is
        registered, it will handle the exception and the teardown will not
        receive it.

        The return values of teardown functions are ignored.

        .. versionadded:: 0.9
        """
        self.teardown_appcontext_funcs.append(f)
        return f

    @setupmethod
    def context_processor(self, f):
        """Registers a template context processor function."""
        self.template_context_processors[None].append(f)
        return f

    @setupmethod
    def shell_context_processor(self, f):
        """Registers a shell context processor function.

        .. versionadded:: 0.11
        """
        self.shell_context_processors.append(f)
        return f

    @setupmethod
    def url_value_preprocessor(self, f):
        """Register a URL value preprocessor function for all view
        functions in the application. These functions will be called before the
        :meth:`before_request` functions.

        The function can modify the values captured from the matched url before
        they are passed to the view. For example, this can be used to pop a
        common language code value and place it in ``g`` rather than pass it to
        every view.

        The function is passed the endpoint name and values dict. The return
        value is ignored.
        """
        self.url_value_preprocessors.setdefault(None, []).append(f)
        return f

    @setupmethod
    def url_defaults(self, f):
        """Callback function for URL defaults for all view functions of the
        application.  It's called with the endpoint and values and should
        update the values passed in place.
        """
        self.url_default_functions.setdefault(None, []).append(f)
        return f

    def _find_error_handler(self, e):
        """Return a registered error handler for an exception in this order:
        blueprint handler for a specific code, app handler for a specific code,
        blueprint handler for an exception class, app handler for an exception
        class, or ``None`` if a suitable handler is not found.
        """
        exc_class, code = self._get_exc_class_and_code(type(e))

        for name, c in (
            (request.blueprint, code),
            (None, code),
            (request.blueprint, None),
            (None, None),
        ):
            handler_map = self.error_handler_spec.setdefault(name, {}).get(c)

            if not handler_map:
                continue

            for cls in exc_class.__mro__:
                handler = handler_map.get(cls)

                if handler is not None:
                    return handler

    def handle_http_exception(self, e):
        """Handles an HTTP exception.  By default this will invoke the
        registered error handlers and fall back to returning the
        exception as response.

        .. versionchanged:: 1.0.3
            ``RoutingException``, used internally for actions such as
             slash redirects during routing, is not passed to error
             handlers.

        .. versionchanged:: 1.0
            Exceptions are looked up by code *and* by MRO, so
            ``HTTPExcpetion`` subclasses can be handled with a catch-all
            handler for the base ``HTTPException``.

        .. versionadded:: 0.3
        """
        # Proxy exceptions don't have error codes.  We want to always return
        # those unchanged as errors
        if e.code is None:
            return e

        # RoutingExceptions are used internally to trigger routing
        # actions, such as slash redirects raising RequestRedirect. They
        # are not raised or handled in user code.
        if isinstance(e, RoutingException):
            return e

        handler = self._find_error_handler(e)
        if handler is None:
            return e
        return handler(e)

    def trap_http_exception(self, e):
        """Checks if an HTTP exception should be trapped or not.  By default
        this will return ``False`` for all exceptions except for a bad request
        key error if ``TRAP_BAD_REQUEST_ERRORS`` is set to ``True``.  It
        also returns ``True`` if ``TRAP_HTTP_EXCEPTIONS`` is set to ``True``.

        This is called for all HTTP exceptions raised by a view function.
        If it returns ``True`` for any exception the error handler for this
        exception is not called and it shows up as regular exception in the
        traceback.  This is helpful for debugging implicitly raised HTTP
        exceptions.

        .. versionchanged:: 1.0
            Bad request errors are not trapped by default in debug mode.

        .. versionadded:: 0.8
        """
        if self.config["TRAP_HTTP_EXCEPTIONS"]:
            return True

        trap_bad_request = self.config["TRAP_BAD_REQUEST_ERRORS"]

        # if unset, trap key errors in debug mode
        if (
            trap_bad_request is None
            and self.debug
            and isinstance(e, BadRequestKeyError)
        ):
            return True

        if trap_bad_request:
            return isinstance(e, BadRequest)

        return False

    def handle_user_exception(self, e):
        """This method is called whenever an exception occurs that
        should be handled. A special case is :class:`~werkzeug
        .exceptions.HTTPException` which is forwarded to the
        :meth:`handle_http_exception` method. This function will either
        return a response value or reraise the exception with the same
        traceback.

        .. versionchanged:: 1.0
            Key errors raised from request data like ``form`` show the
            bad key in debug mode rather than a generic bad request
            message.

        .. versionadded:: 0.7
        """
        exc_type, exc_value, tb = sys.exc_info()
        assert exc_value is e
        # ensure not to trash sys.exc_info() at that point in case someone
        # wants the traceback preserved in handle_http_exception.  Of course
        # we cannot prevent users from trashing it themselves in a custom
        # trap_http_exception method so that's their fault then.

        if isinstance(e, BadRequestKeyError):
            if self.debug or self.config["TRAP_BAD_REQUEST_ERRORS"]:
                e.show_exception = True

                # Werkzeug < 0.15 doesn't add the KeyError to the 400
                # message, add it in manually.
                # TODO: clean up once Werkzeug >= 0.15.5 is required
                if e.args[0] not in e.get_description():
                    e.description = "KeyError: '{}'".format(*e.args)
            elif not hasattr(BadRequestKeyError, "show_exception"):
                e.args = ()

        if isinstance(e, HTTPException) and not self.trap_http_exception(e):
            return self.handle_http_exception(e)

        handler = self._find_error_handler(e)

        if handler is None:
            reraise(exc_type, exc_value, tb)
        return handler(e)

    def handle_exception(self, e):
        """Handle an exception that did not have an error handler
        associated with it, or that was raised from an error handler.
        This always causes a 500 ``InternalServerError``.

        Always sends the :data:`got_request_exception` signal.

        If :attr:`propagate_exceptions` is ``True``, such as in debug
        mode, the error will be re-raised so that the debugger can
        display it. Otherwise, the original exception is logged, and
        an :exc:`~werkzeug.exceptions.InternalServerError` is returned.

        If an error handler is registered for ``InternalServerError`` or
        ``500``, it will be used. For consistency, the handler will
        always receive the ``InternalServerError``. The original
        unhandled exception is available as ``e.original_exception``.

        .. note::
            Prior to Werkzeug 1.0.0, ``InternalServerError`` will not
            always have an ``original_exception`` attribute. Use
            ``getattr(e, "original_exception", None)`` to simulate the
            behavior for compatibility.

        .. versionchanged:: 1.1.0
            Always passes the ``InternalServerError`` instance to the
            handler, setting ``original_exception`` to the unhandled
            error.

        .. versionchanged:: 1.1.0
            ``after_request`` functions and other finalization is done
            even for the default 500 response when there is no handler.

        .. versionadded:: 0.3
        """
        exc_type, exc_value, tb = sys.exc_info()
        got_request_exception.send(self, exception=e)

        if self.propagate_exceptions:
            # if we want to repropagate the exception, we can attempt to
            # raise it with the whole traceback in case we can do that
            # (the function was actually called from the except part)
            # otherwise, we just raise the error again
            if exc_value is e:
                reraise(exc_type, exc_value, tb)
            else:
                raise e

        self.log_exception((exc_type, exc_value, tb))
        server_error = InternalServerError()
        # TODO: pass as param when Werkzeug>=1.0.0 is required
        # TODO: also remove note about this from docstring and docs
        server_error.original_exception = e
        handler = self._find_error_handler(server_error)

        if handler is not None:
            server_error = handler(server_error)

        return self.finalize_request(server_error, from_error_handler=True)

    def log_exception(self, exc_info):
        """Logs an exception.  This is called by :meth:`handle_exception`
        if debugging is disabled and right before the handler is called.
        The default implementation logs the exception as error on the
        :attr:`logger`.

        .. versionadded:: 0.8
        """
        self.logger.error(
            "Exception on %s [%s]" % (request.path, request.method), exc_info=exc_info
        )

    def raise_routing_exception(self, request):
        """Exceptions that are recording during routing are reraised with
        this method.  During debug we are not reraising redirect requests
        for non ``GET``, ``HEAD``, or ``OPTIONS`` requests and we're raising
        a different error instead to help debug situations.

        :internal:
        """
        if (
            not self.debug
            or not isinstance(request.routing_exception, RequestRedirect)
            or request.method in ("GET", "HEAD", "OPTIONS")
        ):
            raise request.routing_exception

        from .debughelpers import FormDataRoutingRedirect

        raise FormDataRoutingRedirect(request)

    def dispatch_request(self):
        """Does the request dispatching.  Matches the URL and returns the
        return value of the view or error handler.  This does not have to
        be a response object.  In order to convert the return value to a
        proper response object, call :func:`make_response`.

        .. versionchanged:: 0.7
           This no longer does the exception handling, this code was
           moved to the new :meth:`full_dispatch_request`.
        """
        req = _request_ctx_stack.top.request
        if req.routing_exception is not None:
            self.raise_routing_exception(req)
        rule = req.url_rule
        # if we provide automatic options for this URL and the
        # request came with the OPTIONS method, reply automatically
        if (
            getattr(rule, "provide_automatic_options", False)
            and req.method == "OPTIONS"
        ):
            return self.make_default_options_response()
        # otherwise dispatch to the handler for that endpoint
        return self.view_functions[rule.endpoint](**req.view_args)

    def full_dispatch_request(self):
        """Dispatches the request and on top of that performs request
        pre and postprocessing as well as HTTP exception catching and
        error handling.

        .. versionadded:: 0.7
        """
        self.try_trigger_before_first_request_functions()
        try:
            request_started.send(self)
            rv = self.preprocess_request()
            if rv is None:
                rv = self.dispatch_request()
        except Exception as e:
            rv = self.handle_user_exception(e)
        return self.finalize_request(rv)

    def finalize_request(self, rv, from_error_handler=False):
        """Given the return value from a view function this finalizes
        the request by converting it into a response and invoking the
        postprocessing functions.  This is invoked for both normal
        request dispatching as well as error handlers.

        Because this means that it might be called as a result of a
        failure a special safe mode is available which can be enabled
        with the `from_error_handler` flag.  If enabled, failures in
        response processing will be logged and otherwise ignored.

        :internal:
        """
        response = self.make_response(rv)
        try:
            response = self.process_response(response)
            request_finished.send(self, response=response)
        except Exception:
            if not from_error_handler:
                raise
            self.logger.exception(
                "Request finalizing failed with an error while handling an error"
            )
        return response

    def try_trigger_before_first_request_functions(self):
        """Called before each request and will ensure that it triggers
        the :attr:`before_first_request_funcs` and only exactly once per
        application instance (which means process usually).

        :internal:
        """
        if self._got_first_request:
            return
        with self._before_request_lock:
            if self._got_first_request:
                return
            for func in self.before_first_request_funcs:
                func()
            self._got_first_request = True

    def make_default_options_response(self):
        """This method is called to create the default ``OPTIONS`` response.
        This can be changed through subclassing to change the default
        behavior of ``OPTIONS`` responses.

        .. versionadded:: 0.7
        """
        adapter = _request_ctx_stack.top.url_adapter
        if hasattr(adapter, "allowed_methods"):
            methods = adapter.allowed_methods()
        else:
            # fallback for Werkzeug < 0.7
            methods = []
            try:
                adapter.match(method="--")
            except MethodNotAllowed as e:
                methods = e.valid_methods
            except HTTPException:
                pass
        rv = self.response_class()
        rv.allow.update(methods)
        return rv

    def should_ignore_error(self, error):
        """This is called to figure out if an error should be ignored
        or not as far as the teardown system is concerned.  If this
        function returns ``True`` then the teardown handlers will not be
        passed the error.

        .. versionadded:: 0.10
        """
        return False

    def make_response(self, rv):
        """Convert the return value from a view function to an instance of
        :attr:`response_class`.

        :param rv: the return value from the view function. The view function
            must return a response. Returning ``None``, or the view ending
            without returning, is not allowed. The following types are allowed
            for ``view_rv``:

            ``str`` (``unicode`` in Python 2)
                A response object is created with the string encoded to UTF-8
                as the body.

            ``bytes`` (``str`` in Python 2)
                A response object is created with the bytes as the body.

            ``dict``
                A dictionary that will be jsonify'd before being returned.

            ``tuple``
                Either ``(body, status, headers)``, ``(body, status)``, or
                ``(body, headers)``, where ``body`` is any of the other types
                allowed here, ``status`` is a string or an integer, and
                ``headers`` is a dictionary or a list of ``(key, value)``
                tuples. If ``body`` is a :attr:`response_class` instance,
                ``status`` overwrites the exiting value and ``headers`` are
                extended.

            :attr:`response_class`
                The object is returned unchanged.

            other :class:`~werkzeug.wrappers.Response` class
                The object is coerced to :attr:`response_class`.

            :func:`callable`
                The function is called as a WSGI application. The result is
                used to create a response object.

        .. versionchanged:: 0.9
           Previously a tuple was interpreted as the arguments for the
           response object.
        """

        status = headers = None

        # unpack tuple returns
        if isinstance(rv, tuple):
            len_rv = len(rv)

            # a 3-tuple is unpacked directly
            if len_rv == 3:
                rv, status, headers = rv
            # decide if a 2-tuple has status or headers
            elif len_rv == 2:
                if isinstance(rv[1], (Headers, dict, tuple, list)):
                    rv, headers = rv
                else:
                    rv, status = rv
            # other sized tuples are not allowed
            else:
                raise TypeError(
                    "The view function did not return a valid response tuple."
                    " The tuple must have the form (body, status, headers),"
                    " (body, status), or (body, headers)."
                )

        # the body must not be None
        if rv is None:
            raise TypeError(
                "The view function did not return a valid response. The"
                " function either returned None or ended without a return"
                " statement."
            )

        # make sure the body is an instance of the response class
        if not isinstance(rv, self.response_class):
            if isinstance(rv, (text_type, bytes, bytearray)):
                # let the response class set the status and headers instead of
                # waiting to do it manually, so that the class can handle any
                # special logic
                rv = self.response_class(rv, status=status, headers=headers)
                status = headers = None
            elif isinstance(rv, dict):
                rv = jsonify(rv)
            elif isinstance(rv, BaseResponse) or callable(rv):
                # evaluate a WSGI callable, or coerce a different response
                # class to the correct type
                try:
                    rv = self.response_class.force_type(rv, request.environ)
                except TypeError as e:
                    new_error = TypeError(
                        "{e}\nThe view function did not return a valid"
                        " response. The return type must be a string, dict, tuple,"
                        " Response instance, or WSGI callable, but it was a"
                        " {rv.__class__.__name__}.".format(e=e, rv=rv)
                    )
                    reraise(TypeError, new_error, sys.exc_info()[2])
            else:
                raise TypeError(
                    "The view function did not return a valid"
                    " response. The return type must be a string, dict, tuple,"
                    " Response instance, or WSGI callable, but it was a"
                    " {rv.__class__.__name__}.".format(rv=rv)
                )

        # prefer the status if it was provided
        if status is not None:
            if isinstance(status, (text_type, bytes, bytearray)):
                rv.status = status
            else:
                rv.status_code = status

        # extend existing headers with provided headers
        if headers:
            rv.headers.extend(headers)

        return rv

    def create_url_adapter(self, request):
        """Creates a URL adapter for the given request. The URL adapter
        is created at a point where the request context is not yet set
        up so the request is passed explicitly.

        .. versionadded:: 0.6

        .. versionchanged:: 0.9
           This can now also be called without a request object when the
           URL adapter is created for the application context.

        .. versionchanged:: 1.0
            :data:`SERVER_NAME` no longer implicitly enables subdomain
            matching. Use :attr:`subdomain_matching` instead.
        """
        if request is not None:
            # If subdomain matching is disabled (the default), use the
            # default subdomain in all cases. This should be the default
            # in Werkzeug but it currently does not have that feature.
            subdomain = (
                (self.url_map.default_subdomain or None)
                if not self.subdomain_matching
                else None
            )
            return self.url_map.bind_to_environ(
                request.environ,
                server_name=self.config["SERVER_NAME"],
                subdomain=subdomain,
            )
        # We need at the very least the server name to be set for this
        # to work.
        if self.config["SERVER_NAME"] is not None:
            return self.url_map.bind(
                self.config["SERVER_NAME"],
                script_name=self.config["APPLICATION_ROOT"],
                url_scheme=self.config["PREFERRED_URL_SCHEME"],
            )

    def inject_url_defaults(self, endpoint, values):
        """Injects the URL defaults for the given endpoint directly into
        the values dictionary passed.  This is used internally and
        automatically called on URL building.

        .. versionadded:: 0.7
        """
        funcs = self.url_default_functions.get(None, ())
        if "." in endpoint:
            bp = endpoint.rsplit(".", 1)[0]
            funcs = chain(funcs, self.url_default_functions.get(bp, ()))
        for func in funcs:
            func(endpoint, values)

    def handle_url_build_error(self, error, endpoint, values):
        """Handle :class:`~werkzeug.routing.BuildError` on :meth:`url_for`.
        """
        exc_type, exc_value, tb = sys.exc_info()
        for handler in self.url_build_error_handlers:
            try:
                rv = handler(error, endpoint, values)
                if rv is not None:
                    return rv
            except BuildError as e:
                # make error available outside except block (py3)
                error = e

        # At this point we want to reraise the exception.  If the error is
        # still the same one we can reraise it with the original traceback,
        # otherwise we raise it from here.
        if error is exc_value:
            reraise(exc_type, exc_value, tb)
        raise error

    def preprocess_request(self):
        """Called before the request is dispatched. Calls
        :attr:`url_value_preprocessors` registered with the app and the
        current blueprint (if any). Then calls :attr:`before_request_funcs`
        registered with the app and the blueprint.

        If any :meth:`before_request` handler returns a non-None value, the
        value is handled as if it was the return value from the view, and
        further request handling is stopped.
        """

        bp = _request_ctx_stack.top.request.blueprint

        funcs = self.url_value_preprocessors.get(None, ())
        if bp is not None and bp in self.url_value_preprocessors:
            funcs = chain(funcs, self.url_value_preprocessors[bp])
        for func in funcs:
            func(request.endpoint, request.view_args)

        funcs = self.before_request_funcs.get(None, ())
        if bp is not None and bp in self.before_request_funcs:
            funcs = chain(funcs, self.before_request_funcs[bp])
        for func in funcs:
            rv = func()
            if rv is not None:
                return rv

    def process_response(self, response):
        """Can be overridden in order to modify the response object
        before it's sent to the WSGI server.  By default this will
        call all the :meth:`after_request` decorated functions.

        .. versionchanged:: 0.5
           As of Flask 0.5 the functions registered for after request
           execution are called in reverse order of registration.

        :param response: a :attr:`response_class` object.
        :return: a new response object or the same, has to be an
                 instance of :attr:`response_class`.
        """
        ctx = _request_ctx_stack.top
        bp = ctx.request.blueprint
        funcs = ctx._after_request_functions
        if bp is not None and bp in self.after_request_funcs:
            funcs = chain(funcs, reversed(self.after_request_funcs[bp]))
        if None in self.after_request_funcs:
            funcs = chain(funcs, reversed(self.after_request_funcs[None]))
        for handler in funcs:
            response = handler(response)
        if not self.session_interface.is_null_session(ctx.session):
            self.session_interface.save_session(self, ctx.session, response)
        return response

    def do_teardown_request(self, exc=_sentinel):
        """Called after the request is dispatched and the response is
        returned, right before the request context is popped.

        This calls all functions decorated with
        :meth:`teardown_request`, and :meth:`Blueprint.teardown_request`
        if a blueprint handled the request. Finally, the
        :data:`request_tearing_down` signal is sent.

        This is called by
        :meth:`RequestContext.pop() <flask.ctx.RequestContext.pop>`,
        which may be delayed during testing to maintain access to
        resources.

        :param exc: An unhandled exception raised while dispatching the
            request. Detected from the current exception information if
            not passed. Passed to each teardown function.

        .. versionchanged:: 0.9
            Added the ``exc`` argument.
        """
        if exc is _sentinel:
            exc = sys.exc_info()[1]
        funcs = reversed(self.teardown_request_funcs.get(None, ()))
        bp = _request_ctx_stack.top.request.blueprint
        if bp is not None and bp in self.teardown_request_funcs:
            funcs = chain(funcs, reversed(self.teardown_request_funcs[bp]))
        for func in funcs:
            func(exc)
        request_tearing_down.send(self, exc=exc)

    def do_teardown_appcontext(self, exc=_sentinel):
        """Called right before the application context is popped.

        When handling a request, the application context is popped
        after the request context. See :meth:`do_teardown_request`.

        This calls all functions decorated with
        :meth:`teardown_appcontext`. Then the
        :data:`appcontext_tearing_down` signal is sent.

        This is called by
        :meth:`AppContext.pop() <flask.ctx.AppContext.pop>`.

        .. versionadded:: 0.9
        """
        if exc is _sentinel:
            exc = sys.exc_info()[1]
        for func in reversed(self.teardown_appcontext_funcs):
            func(exc)
        appcontext_tearing_down.send(self, exc=exc)

    def app_context(self):
        """Create an :class:`~flask.ctx.AppContext`. Use as a ``with``
        block to push the context, which will make :data:`current_app`
        point at this application.

        An application context is automatically pushed by
        :meth:`RequestContext.push() <flask.ctx.RequestContext.push>`
        when handling a request, and when running a CLI command. Use
        this to manually create a context outside of these situations.

        ::

            with app.app_context():
                init_db()

        See :doc:`/appcontext`.

        .. versionadded:: 0.9
        """
        return AppContext(self)

    def request_context(self, environ):
        """Create a :class:`~flask.ctx.RequestContext` representing a
        WSGI environment. Use a ``with`` block to push the context,
        which will make :data:`request` point at this request.

        See :doc:`/reqcontext`.

        Typically you should not call this from your own code. A request
        context is automatically pushed by the :meth:`wsgi_app` when
        handling a request. Use :meth:`test_request_context` to create
        an environment and context instead of this method.

        :param environ: a WSGI environment
        """
        return RequestContext(self, environ)

    def test_request_context(self, *args, **kwargs):
        """Create a :class:`~flask.ctx.RequestContext` for a WSGI
        environment created from the given values. This is mostly useful
        during testing, where you may want to run a function that uses
        request data without dispatching a full request.

        See :doc:`/reqcontext`.

        Use a ``with`` block to push the context, which will make
        :data:`request` point at the request for the created
        environment. ::

            with test_request_context(...):
                generate_report()

        When using the shell, it may be easier to push and pop the
        context manually to avoid indentation. ::

            ctx = app.test_request_context(...)
            ctx.push()
            ...
            ctx.pop()

        Takes the same arguments as Werkzeug's
        :class:`~werkzeug.test.EnvironBuilder`, with some defaults from
        the application. See the linked Werkzeug docs for most of the
        available arguments. Flask-specific behavior is listed here.

        :param path: URL path being requested.
        :param base_url: Base URL where the app is being served, which
            ``path`` is relative to. If not given, built from
            :data:`PREFERRED_URL_SCHEME`, ``subdomain``,
            :data:`SERVER_NAME`, and :data:`APPLICATION_ROOT`.
        :param subdomain: Subdomain name to append to
            :data:`SERVER_NAME`.
        :param url_scheme: Scheme to use instead of
            :data:`PREFERRED_URL_SCHEME`.
        :param data: The request body, either as a string or a dict of
            form keys and values.
        :param json: If given, this is serialized as JSON and passed as
            ``data``. Also defaults ``content_type`` to
            ``application/json``.
        :param args: other positional arguments passed to
            :class:`~werkzeug.test.EnvironBuilder`.
        :param kwargs: other keyword arguments passed to
            :class:`~werkzeug.test.EnvironBuilder`.
        """
        from .testing import EnvironBuilder

        builder = EnvironBuilder(self, *args, **kwargs)

        try:
            return self.request_context(builder.get_environ())
        finally:
            builder.close()

    def wsgi_app(self, environ, start_response):
        """The actual WSGI application. This is not implemented in
        :meth:`__call__` so that middlewares can be applied without
        losing a reference to the app object. Instead of doing this::

            app = MyMiddleware(app)

        It's a better idea to do this instead::

            app.wsgi_app = MyMiddleware(app.wsgi_app)

        Then you still have the original application object around and
        can continue to call methods on it.

        .. versionchanged:: 0.7
            Teardown events for the request and app contexts are called
            even if an unhandled error occurs. Other events may not be
            called depending on when an error occurs during dispatch.
            See :ref:`callbacks-and-errors`.

        :param environ: A WSGI environment.
        :param start_response: A callable accepting a status code,
            a list of headers, and an optional exception context to
            start the response.
        """
        ctx = self.request_context(environ)
        error = None
        try:
            try:
                ctx.push()
                response = self.full_dispatch_request()
            except Exception as e:
                error = e
                response = self.handle_exception(e)
            except:  # noqa: B001
                error = sys.exc_info()[1]
                raise
            return response(environ, start_response)
        finally:
            if self.should_ignore_error(error):
                error = None
            ctx.auto_pop(error)

    def __call__(self, environ, start_response):
        """The WSGI server calls the Flask application object as the
        WSGI application. This calls :meth:`wsgi_app` which can be
        wrapped to applying middleware."""
        return self.wsgi_app(environ, start_response)

    def __repr__(self):
        return "<%s %r>" % (self.__class__.__name__, self.name)

    def wsgi_app(self, environ, start_response):
        """The actual WSGI application. This is not implemented in
        :meth:`__call__` so that middlewares can be applied without
        losing a reference to the app object. Instead of doing this::

            app = MyMiddleware(app)

        It's a better idea to do this instead::

            app.wsgi_app = MyMiddleware(app.wsgi_app)

        Then you still have the original application object around and
        can continue to call methods on it.

        .. versionchanged:: 0.7
            Teardown events for the request and app contexts are called
            even if an unhandled error occurs. Other events may not be
            called depending on when an error occurs during dispatch.
            See :ref:`callbacks-and-errors`.

        :param environ: A WSGI environment.
        :param start_response: A callable accepting a status code,
            a list of headers, and an optional exception context to
            start the response.
        """
        ctx = self.request_context(environ)
        error = None
        try:
            try:
                ctx.push()
                response = self.full_dispatch_request()
            except Exception as e:
                error = e
                response = self.handle_exception(e)
            except:  # noqa: B001
                error = sys.exc_info()[1]
                raise
            return response(environ, start_response)
        finally:
            if self.should_ignore_error(error):
                error = None
            ctx.auto_pop(error)

（三）RequestContext類

class RequestContext(object):
    """The request context contains all request relevant information.  It is
    created at the beginning of the request and pushed to the
    `_request_ctx_stack` and removed at the end of it.  It will create the
    URL adapter and request object for the WSGI environment provided.

    Do not attempt to use this class directly, instead use
    :meth:`~flask.Flask.test_request_context` and
    :meth:`~flask.Flask.request_context` to create this object.

    When the request context is popped, it will evaluate all the
    functions registered on the application for teardown execution
    (:meth:`~flask.Flask.teardown_request`).

    The request context is automatically popped at the end of the request
    for you.  In debug mode the request context is kept around if
    exceptions happen so that interactive debuggers have a chance to
    introspect the data.  With 0.4 this can also be forced for requests
    that did not fail and outside of ``DEBUG`` mode.  By setting
    ``'flask._preserve_context'`` to ``True`` on the WSGI environment the
    context will not pop itself at the end of the request.  This is used by
    the :meth:`~flask.Flask.test_client` for example to implement the
    deferred cleanup functionality.

    You might find this helpful for unittests where you need the
    information from the context local around for a little longer.  Make
    sure to properly :meth:`~werkzeug.LocalStack.pop` the stack yourself in
    that situation, otherwise your unittests will leak memory.
    """

    def __init__(self, app, environ, request=None, session=None):
        self.app = app
        if request is None:
            request = app.request_class(environ)
        self.request = request
        self.url_adapter = None
        try:
            self.url_adapter = app.create_url_adapter(self.request)
        except HTTPException as e:
            self.request.routing_exception = e
        self.flashes = None
        self.session = session

        # Request contexts can be pushed multiple times and interleaved with
        # other request contexts.  Now only if the last level is popped we
        # get rid of them.  Additionally if an application context is missing
        # one is created implicitly so for each level we add this information
        self._implicit_app_ctx_stack = []

        # indicator if the context was preserved.  Next time another context
        # is pushed the preserved context is popped.
        self.preserved = False

        # remembers the exception for pop if there is one in case the context
        # preservation kicks in.
        self._preserved_exc = None

        # Functions that should be executed after the request on the response
        # object.  These will be called before the regular "after_request"
        # functions.
        self._after_request_functions = []

    @property
    def g(self):
        return _app_ctx_stack.top.g

    @g.setter
    def g(self, value):
        _app_ctx_stack.top.g = value

    def copy(self):
        """Creates a copy of this request context with the same request object.
        This can be used to move a request context to a different greenlet.
        Because the actual request object is the same this cannot be used to
        move a request context to a different thread unless access to the
        request object is locked.

        .. versionadded:: 0.10

        .. versionchanged:: 1.1
           The current session object is used instead of reloading the original
           data. This prevents `flask.session` pointing to an out-of-date object.
        """
        return self.__class__(
            self.app,
            environ=self.request.environ,
            request=self.request,
            session=self.session,
        )

    def match_request(self):
        """Can be overridden by a subclass to hook into the matching
        of the request.
        """
        try:
            result = self.url_adapter.match(return_rule=True)
            self.request.url_rule, self.request.view_args = result
        except HTTPException as e:
            self.request.routing_exception = e

    def push(self):
        """Binds the request context to the current context."""
        # If an exception occurs in debug mode or if context preservation is
        # activated under exception situations exactly one context stays
        # on the stack.  The rationale is that you want to access that
        # information under debug situations.  However if someone forgets to
        # pop that context again we want to make sure that on the next push
        # it's invalidated, otherwise we run at risk that something leaks
        # memory.  This is usually only a problem in test suite since this
        # functionality is not active in production environments.
        top = _request_ctx_stack.top
        if top is not None and top.preserved:
            top.pop(top._preserved_exc)

        # Before we push the request context we have to ensure that there
        # is an application context.
        app_ctx = _app_ctx_stack.top
        if app_ctx is None or app_ctx.app != self.app:
            app_ctx = self.app.app_context()
            app_ctx.push()
            self._implicit_app_ctx_stack.append(app_ctx)
        else:
            self._implicit_app_ctx_stack.append(None)

        if hasattr(sys, "exc_clear"):
            sys.exc_clear()

        _request_ctx_stack.push(self)

        # Open the session at the moment that the request context is available.
        # This allows a custom open_session method to use the request context.
        # Only open a new session if this is the first time the request was
        # pushed, otherwise stream_with_context loses the session.
        if self.session is None:
            session_interface = self.app.session_interface
            self.session = session_interface.open_session(self.app, self.request)

            if self.session is None:
                self.session = session_interface.make_null_session(self.app)

        if self.url_adapter is not None:
            self.match_request()

    def pop(self, exc=_sentinel):
        """Pops the request context and unbinds it by doing that.  This will
        also trigger the execution of functions registered by the
        :meth:`~flask.Flask.teardown_request` decorator.

        .. versionchanged:: 0.9
           Added the `exc` argument.
        """
        app_ctx = self._implicit_app_ctx_stack.pop()

        try:
            clear_request = False
            if not self._implicit_app_ctx_stack:
                self.preserved = False
                self._preserved_exc = None
                if exc is _sentinel:
                    exc = sys.exc_info()[1]
                self.app.do_teardown_request(exc)

                # If this interpreter supports clearing the exception information
                # we do that now.  This will only go into effect on Python 2.x,
                # on 3.x it disappears automatically at the end of the exception
                # stack.
                if hasattr(sys, "exc_clear"):
                    sys.exc_clear()

                request_close = getattr(self.request, "close", None)
                if request_close is not None:
                    request_close()
                clear_request = True
        finally:
            rv = _request_ctx_stack.pop()

            # get rid of circular dependencies at the end of the request
            # so that we don't require the GC to be active.
            if clear_request:
                rv.request.environ["werkzeug.request"] = None

            # Get rid of the app as well if necessary.
            if app_ctx is not None:
                app_ctx.pop(exc)

            assert rv is self, "Popped wrong request context. (%r instead of %r)" % (
                rv,
                self,
            )

    def auto_pop(self, exc):
        if self.request.environ.get("flask._preserve_context") or (
            exc is not None and self.app.preserve_context_on_exception
        ):
            self.preserved = True
            self._preserved_exc = exc
        else:
            self.pop(exc)

    def __enter__(self):
        self.push()
        return self

    def __exit__(self, exc_type, exc_value, tb):
        # do not pop the request stack if we are in debug mode and an
        # exception happened.  This will allow the debugger to still
        # access the request object in the interactive shell.  Furthermore
        # the context can be force kept alive for the test client.
        # See flask.testing for how this works.
        self.auto_pop(exc_value)

        if BROKEN_PYPY_CTXMGR_EXIT and exc_type is not None:
            reraise(exc_type, exc_value, tb)

    def __repr__(self):
        return "<%s '%s' [%s] of %s>" % (
            self.__class__.__name__,
            self.request.url,
            self.request.method,
            self.app.name,
        )

這個接收的引數其中有app物件（Flask類）、environ（請求相關資訊）等。

    def __init__(self, app, environ, request=None, session=None):
        self.app = app
        if request is None: #剛開始request是None
            request = app.request_class(environ)
        self.request = request
        self.url_adapter = None
        try:
            self.url_adapter = app.create_url_adapter(self.request)
        except HTTPException as e:
            self.request.routing_exception = e
        self.flashes = None
        self.session = session

請求剛進來，request引數是None，所以執行app.request_class：

class Flask(_PackageBoundObject):

    #: The class that is used for request objects.  See :class:`~flask.Request`
    #: for more information.
    request_class = Request  #from .wrappers import Request

對請求的字串資訊進行處理，返回Request物件，並且將Request物件封裝到RequestContext類中。

（四）Request類

class Request(RequestBase, JSONMixin):
    """The request object used by default in Flask.  Remembers the
    matched endpoint and view arguments.

    It is what ends up as :class:`~flask.request`.  If you want to replace
    the request object used you can subclass this and set
    :attr:`~flask.Flask.request_class` to your subclass.

    The request object is a :class:`~werkzeug.wrappers.Request` subclass and
    provides all of the attributes Werkzeug defines plus a few Flask
    specific ones.
    """

    #: The internal URL rule that matched the request.  This can be
    #: useful to inspect which methods are allowed for the URL from
    #: a before/after handler (``request.url_rule.methods``) etc.
    #: Though if the request's method was invalid for the URL rule,
    #: the valid list is available in ``routing_exception.valid_methods``
    #: instead (an attribute of the Werkzeug exception
    #: :exc:`~werkzeug.exceptions.MethodNotAllowed`)
    #: because the request was never internally bound.
    #:
    #: .. versionadded:: 0.6
    url_rule = None

    #: A dict of view arguments that matched the request.  If an exception
    #: happened when matching, this will be ``None``.
    view_args = None

    #: If matching the URL failed, this is the exception that will be
    #: raised / was raised as part of the request handling.  This is
    #: usually a :exc:`~werkzeug.exceptions.NotFound` exception or
    #: something similar.
    routing_exception = None

    @property
    def max_content_length(self):
        """Read-only view of the ``MAX_CONTENT_LENGTH`` config key."""
        if current_app:
            return current_app.config["MAX_CONTENT_LENGTH"]

    @property
    def endpoint(self):
        """The endpoint that matched the request.  This in combination with
        :attr:`view_args` can be used to reconstruct the same or a
        modified URL.  If an exception happened when matching, this will
        be ``None``.
        """
        if self.url_rule is not None:
            return self.url_rule.endpoint

    @property
    def blueprint(self):
        """The name of the current blueprint"""
        if self.url_rule and "." in self.url_rule.endpoint:
            return self.url_rule.endpoint.rsplit(".", 1)[0]

    def _load_form_data(self):
        RequestBase._load_form_data(self)

        # In debug mode we're replacing the files multidict with an ad-hoc
        # subclass that raises a different error for key errors.
        if (
            current_app
            and current_app.debug
            and self.mimetype != "multipart/form-data"
            and not self.files
        ):
            from .debughelpers import attach_enctype_error_multidict

            attach_enctype_error_multidict(self)

RequestContext封裝就是這個類的物件，這個類的基類BaseRequest初始化時接收了environ，並且對請求字串進行處理。

class BaseRequest(object):
    """Very basic request object.  This does not implement advanced stuff like
    entity tag parsing or cache controls.  The request object is created with
    the WSGI environment as first argument and will add itself to the WSGI
    environment as ``'werkzeug.request'`` unless it's created with
    `populate_request` set to False.

    There are a couple of mixins available that add additional functionality
    to the request object, there is also a class called `Request` which
    subclasses `BaseRequest` and all the important mixins.

    It's a good idea to create a custom subclass of the :class:`BaseRequest`
    and add missing functionality either via mixins or direct implementation.
    Here an example for such subclasses::

        from werkzeug.wrappers import BaseRequest, ETagRequestMixin

        class Request(BaseRequest, ETagRequestMixin):
            pass

    Request objects are **read only**.  As of 0.5 modifications are not
    allowed in any place.  Unlike the lower level parsing functions the
    request object will use immutable objects everywhere possible.

    Per default the request object will assume all the text data is `utf-8`
    encoded.  Please refer to :doc:`the unicode chapter </unicode>` for more
    details about customizing the behavior.

    Per default the request object will be added to the WSGI
    environment as `werkzeug.request` to support the debugging system.
    If you don't want that, set `populate_request` to `False`.

    If `shallow` is `True` the environment is initialized as shallow
    object around the environ.  Every operation that would modify the
    environ in any way (such as consuming form data) raises an exception
    unless the `shallow` attribute is explicitly set to `False`.  This
    is useful for middlewares where you don't want to consume the form
    data by accident.  A shallow request is not populated to the WSGI
    environment.

    .. versionchanged:: 0.5
       read-only mode was enforced by using immutables classes for all
       data.
    """

    #: the charset for the request, defaults to utf-8
    charset = "utf-8"

    #: the error handling procedure for errors, defaults to 'replace'
    encoding_errors = "replace"

    #: the maximum content length.  This is forwarded to the form data
    #: parsing function (:func:`parse_form_data`).  When set and the
    #: :attr:`form` or :attr:`files` attribute is accessed and the
    #: parsing fails because more than the specified value is transmitted
    #: a :exc:`~werkzeug.exceptions.RequestEntityTooLarge` exception is raised.
    #:
    #: Have a look at :ref:`dealing-with-request-data` for more details.
    #:
    #: .. versionadded:: 0.5
    max_content_length = None

    #: the maximum form field size.  This is forwarded to the form data
    #: parsing function (:func:`parse_form_data`).  When set and the
    #: :attr:`form` or :attr:`files` attribute is accessed and the
    #: data in memory for post data is longer than the specified value a
    #: :exc:`~werkzeug.exceptions.RequestEntityTooLarge` exception is raised.
    #:
    #: Have a look at :ref:`dealing-with-request-data` for more details.
    #:
    #: .. versionadded:: 0.5
    max_form_memory_size = None

    #: the class to use for `args` and `form`.  The default is an
    #: :class:`~werkzeug.datastructures.ImmutableMultiDict` which supports
    #: multiple values per key.  alternatively it makes sense to use an
    #: :class:`~werkzeug.datastructures.ImmutableOrderedMultiDict` which
    #: preserves order or a :class:`~werkzeug.datastructures.ImmutableDict`
    #: which is the fastest but only remembers the last key.  It is also
    #: possible to use mutable structures, but this is not recommended.
    #:
    #: .. versionadded:: 0.6
    parameter_storage_class = ImmutableMultiDict

    #: the type to be used for list values from the incoming WSGI environment.
    #: By default an :class:`~werkzeug.datastructures.ImmutableList` is used
    #: (for example for :attr:`access_list`).
    #:
    #: .. versionadded:: 0.6
    list_storage_class = ImmutableList

    #: The type to be used for dict values from the incoming WSGI
    #: environment. (For example for :attr:`cookies`.) By default an
    #: :class:`~werkzeug.datastructures.ImmutableMultiDict` is used.
    #:
    #: .. versionchanged:: 1.0.0
    #:     Changed to ``ImmutableMultiDict`` to support multiple values.
    #:
    #: .. versionadded:: 0.6
    dict_storage_class = ImmutableMultiDict

    #: The form data parser that shoud be used.  Can be replaced to customize
    #: the form date parsing.
    form_data_parser_class = FormDataParser

    #: Optionally a list of hosts that is trusted by this request.  By default
    #: all hosts are trusted which means that whatever the client sends the
    #: host is will be accepted.
    #:
    #: Because `Host` and `X-Forwarded-Host` headers can be set to any value by
    #: a malicious client, it is recommended to either set this property or
    #: implement similar validation in the proxy (if application is being run
    #: behind one).
    #:
    #: .. versionadded:: 0.9
    trusted_hosts = None

    #: Indicates whether the data descriptor should be allowed to read and
    #: buffer up the input stream.  By default it's enabled.
    #:
    #: .. versionadded:: 0.9
    disable_data_descriptor = False

    def __init__(self, environ, populate_request=True, shallow=False):
        self.environ = environ
        if populate_request and not shallow:
            self.environ["werkzeug.request"] = self
        self.shallow = shallow

    def __repr__(self):
        # make sure the __repr__ even works if the request was created
        # from an invalid WSGI environment.  If we display the request
        # in a debug session we don't want the repr to blow up.
        args = []
        try:
            args.append("'%s'" % to_native(self.url, self.url_charset))
            args.append("[%s]" % self.method)
        except Exception:
            args.append("(invalid WSGI environ)")

        return "<%s %s>" % (self.__class__.__name__, " ".join(args))

    @property
    def url_charset(self):
        """The charset that is assumed for URLs.  Defaults to the value
        of :attr:`charset`.

        .. versionadded:: 0.6
        """
        return self.charset

    @classmethod
    def from_values(cls, *args, **kwargs):
        """Create a new request object based on the values provided.  If
        environ is given missing values are filled from there.  This method is
        useful for small scripts when you need to simulate a request from an URL.
        Do not use this method for unittesting, there is a full featured client
        object (:class:`Client`) that allows to create multipart requests,
        support for cookies etc.

        This accepts the same options as the
        :class:`~werkzeug.test.EnvironBuilder`.

        .. versionchanged:: 0.5
           This method now accepts the same arguments as
           :class:`~werkzeug.test.EnvironBuilder`.  Because of this the
           `environ` parameter is now called `environ_overrides`.

        :return: request object
        """
        from ..test import EnvironBuilder

        charset = kwargs.pop("charset", cls.charset)
        kwargs["charset"] = charset
        builder = EnvironBuilder(*args, **kwargs)
        try:
            return builder.get_request(cls)
        finally:
            builder.close()

    @classmethod
    def application(cls, f):
        """Decorate a function as responder that accepts the request as
        the last argument.  This works like the :func:`responder`
        decorator but the function is passed the request object as the
        last argument and the request object will be closed
        automatically::

            @Request.application
            def my_wsgi_app(request):
                return Response('Hello World!')

        As of Werkzeug 0.14 HTTP exceptions are automatically caught and
        converted to responses instead of failing.

        :param f: the WSGI callable to decorate
        :return: a new WSGI callable
        """
        #: return a callable that wraps the -2nd argument with the request
        #: and calls the function with all the arguments up to that one and
        #: the request.  The return value is then called with the latest
        #: two arguments.  This makes it possible to use this decorator for
        #: both standalone WSGI functions as well as bound methods and
        #: partially applied functions.
        from ..exceptions import HTTPException

        def application(*args):
            request = cls(args[-2])
            with request:
                try:
                    resp = f(*args[:-2] + (request,))
                except HTTPException as e:
                    resp = e.get_response(args[-2])
                return resp(*args[-2:])

        return update_wrapper(application, f)

    def _get_file_stream(
        self, total_content_length, content_type, filename=None, content_length=None
    ):
        """Called to get a stream for the file upload.

        This must provide a file-like class with `read()`, `readline()`
        and `seek()` methods that is both writeable and readable.

        The default implementation returns a temporary file if the total
        content length is higher than 500KB.  Because many browsers do not
        provide a content length for the files only the total content
        length matters.

        :param total_content_length: the total content length of all the
                                     data in the request combined.  This value
                                     is guaranteed to be there.
        :param content_type: the mimetype of the uploaded file.
        :param filename: the filename of the uploaded file.  May be `None`.
        :param content_length: the length of this file.  This value is usually
                               not provided because webbrowsers do not provide
                               this value.
        """
        return default_stream_factory(
            total_content_length=total_content_length,
            filename=filename,
            content_type=content_type,
            content_length=content_length,
        )

    @property
    def want_form_data_parsed(self):
        """Returns True if the request method carries content.  As of
        Werkzeug 0.9 this will be the case if a content type is transmitted.

        .. versionadded:: 0.8
        """
        return bool(self.environ.get("CONTENT_TYPE"))

    def make_form_data_parser(self):
        """Creates the form data parser. Instantiates the
        :attr:`form_data_parser_class` with some parameters.

        .. versionadded:: 0.8
        """
        return self.form_data_parser_class(
            self._get_file_stream,
            self.charset,
            self.encoding_errors,
            self.max_form_memory_size,
            self.max_content_length,
            self.parameter_storage_class,
        )

    def _load_form_data(self):
        """Method used internally to retrieve submitted data.  After calling
        this sets `form` and `files` on the request object to multi dicts
        filled with the incoming form data.  As a matter of fact the input
        stream will be empty afterwards.  You can also call this method to
        force the parsing of the form data.

        .. versionadded:: 0.8
        """
        # abort early if we have already consumed the stream
        if "form" in self.__dict__:
            return

        _assert_not_shallow(self)

        if self.want_form_data_parsed:
            content_type = self.environ.get("CONTENT_TYPE", "")
            content_length = get_content_length(self.environ)
            mimetype, options = parse_options_header(content_type)
            parser = self.make_form_data_parser()
            data = parser.parse(
                self._get_stream_for_parsing(), mimetype, content_length, options
            )
        else:
            data = (
                self.stream,
                self.parameter_storage_class(),
                self.parameter_storage_class(),
            )

        # inject the values into the instance dict so that we bypass
        # our cached_property non-data descriptor.
        d = self.__dict__
        d["stream"], d["form"], d["files"] = data

    def _get_stream_for_parsing(self):
        """This is the same as accessing :attr:`stream` with the difference
        that if it finds cached data from calling :meth:`get_data` first it
        will create a new stream out of the cached data.

        .. versionadded:: 0.9.3
        """
        cached_data = getattr(self, "_cached_data", None)
        if cached_data is not None:
            return BytesIO(cached_data)
        return self.stream

    def close(self):
        """Closes associated resources of this request object.  This
        closes all file handles explicitly.  You can also use the request
        object in a with statement which will automatically close it.

        .. versionadded:: 0.9
        """
        files = self.__dict__.get("files")
        for _key, value in iter_multi_items(files or ()):
            value.close()

    def __enter__(self):
        return self

    def __exit__(self, exc_type, exc_value, tb):
        self.close()

    @cached_property
    def stream(self):
        """
        If the incoming form data was not encoded with a known mimetype
        the data is stored unmodified in this stream for consumption.  Most
        of the time it is a better idea to use :attr:`data` which will give
        you that data as a string.  The stream only returns the data once.

        Unlike :attr:`input_stream` this stream is properly guarded that you
        can't accidentally read past the length of the input.  Werkzeug will
        internally always refer to this stream to read data which makes it
        possible to wrap this object with a stream that does filtering.

        .. versionchanged:: 0.9
           This stream is now always available but might be consumed by the
           form parser later on.  Previously the stream was only set if no
           parsing happened.
        """
        _assert_not_shallow(self)
        return get_input_stream(self.environ)

    input_stream = environ_property(
        "wsgi.input",
        """The WSGI input stream.

        In general it's a bad idea to use this one because you can
        easily read past the boundary.  Use the :attr:`stream`
        instead.""",
    )

    @cached_property
    def args(self):
        """The parsed URL parameters (the part in the URL after the question
        mark).

        By default an
        :class:`~werkzeug.datastructures.ImmutableMultiDict`
        is returned from this function.  This can be changed by setting
        :attr:`parameter_storage_class` to a different type.  This might
        be necessary if the order of the form data is important.
        """
        return url_decode(
            wsgi_get_bytes(self.environ.get("QUERY_STRING", "")),
            self.url_charset,
            errors=self.encoding_errors,
            cls=self.parameter_storage_class,
        )

    @cached_property
    def data(self):
        """
        Contains the incoming request data as string in case it came with
        a mimetype Werkzeug does not handle.
        """

        if self.disable_data_descriptor:
            raise AttributeError("data descriptor is disabled")
        # XXX: this should eventually be deprecated.

        # We trigger form data parsing first which means that the descriptor
        # will not cache the data that would otherwise be .form or .files
        # data.  This restores the behavior that was there in Werkzeug
        # before 0.9.  New code should use :meth:`get_data` explicitly as
        # this will make behavior explicit.
        return self.get_data(parse_form_data=True)

    def get_data(self, cache=True, as_text=False, parse_form_data=False):
        """This reads the buffered incoming data from the client into one
        bytestring.  By default this is cached but that behavior can be
        changed by setting `cache` to `False`.

        Usually it's a bad idea to call this method without checking the
        content length first as a client could send dozens of megabytes or more
        to cause memory problems on the server.

        Note that if the form data was already parsed this method will not
        return anything as form data parsing does not cache the data like
        this method does.  To implicitly invoke form data parsing function
        set `parse_form_data` to `True`.  When this is done the return value
        of this method will be an empty string if the form parser handles
        the data.  This generally is not necessary as if the whole data is
        cached (which is the default) the form parser will used the cached
        data to parse the form data.  Please be generally aware of checking
        the content length first in any case before calling this method
        to avoid exhausting server memory.

        If `as_text` is set to `True` the return value will be a decoded
        unicode string.

        .. versionadded:: 0.9
        """
        rv = getattr(self, "_cached_data", None)
        if rv is None:
            if parse_form_data:
                self._load_form_data()
            rv = self.stream.read()
            if cache:
                self._cached_data = rv
        if as_text:
            rv = rv.decode(self.charset, self.encoding_errors)
        return rv

    @cached_property
    def form(self):
        """The form parameters.  By default an
        :class:`~werkzeug.datastructures.ImmutableMultiDict`
        is returned from this function.  This can be changed by setting
        :attr:`parameter_storage_class` to a different type.  This might
        be necessary if the order of the form data is important.

        Please keep in mind that file uploads will not end up here, but instead
        in the :attr:`files` attribute.

        .. versionchanged:: 0.9

            Previous to Werkzeug 0.9 this would only contain form data for POST
            and PUT requests.
        """
        self._load_form_data()
        return self.form

    @cached_property
    def values(self):
        """A :class:`werkzeug.datastructures.CombinedMultiDict` that combines
        :attr:`args` and :attr:`form`."""
        args = []
        for d in self.args, self.form:
            if not isinstance(d, MultiDict):
                d = MultiDict(d)
            args.append(d)
        return CombinedMultiDict(args)

    @cached_property
    def files(self):
        """:class:`~werkzeug.datastructures.MultiDict` object containing
        all uploaded files.  Each key in :attr:`files` is the name from the
        ``<input type="file" name="">``.  Each value in :attr:`files` is a
        Werkzeug :class:`~werkzeug.datastructures.FileStorage` object.

        It basically behaves like a standard file object you know from Python,
        with the difference that it also has a
        :meth:`~werkzeug.datastructures.FileStorage.save` function that can
        store the file on the filesystem.

        Note that :attr:`files` will only contain data if the request method was
        POST, PUT or PATCH and the ``<form>`` that posted to the request had
        ``enctype="multipart/form-data"``.  It will be empty otherwise.

        See the :class:`~werkzeug.datastructures.MultiDict` /
        :class:`~werkzeug.datastructures.FileStorage` documentation for
        more details about the used data structure.
        """
        self._load_form_data()
        return self.files

    @cached_property
    def cookies(self):
        """A :class:`dict` with the contents of all cookies transmitted with
        the request."""
        return parse_cookie(
            self.environ,
            self.charset,
            self.encoding_errors,
            cls=self.dict_storage_class,
        )

    @cached_property
    def headers(self):
        """The headers from the WSGI environ as immutable
        :class:`~werkzeug.datastructures.EnvironHeaders`.
        """
        return EnvironHeaders(self.environ)

    @cached_property
    def path(self):
        """Requested path as unicode.  This works a bit like the regular path
        info in the WSGI environment but will always include a leading slash,
        even if the URL root is accessed.
        """
        raw_path = wsgi_decoding_dance(
            self.environ.get("PATH_INFO") or "", self.charset, self.encoding_errors
        )
        return "/" + raw_path.lstrip("/")

    @cached_property
    def full_path(self):
        """Requested path as unicode, including the query string."""
        return self.path + u"?" + to_unicode(self.query_string, self.url_charset)

    @cached_property
    def script_root(self):
        """The root path of the script without the trailing slash."""
        raw_path = wsgi_decoding_dance(
            self.environ.get("SCRIPT_NAME") or "", self.charset, self.encoding_errors
        )
        return raw_path.rstrip("/")

    @cached_property
    def url(self):
        """The reconstructed current URL as IRI.
        See also: :attr:`trusted_hosts`.
        """
        return get_current_url(self.environ, trusted_hosts=self.trusted_hosts)

    @cached_property
    def base_url(self):
        """Like :attr:`url` but without the querystring
        See also: :attr:`trusted_hosts`.
        """
        return get_current_url(
            self.environ, strip_querystring=True, trusted_hosts=self.trusted_hosts
        )

    @cached_property
    def url_root(self):
        """The full URL root (with hostname), this is the application
        root as IRI.
        See also: :attr:`trusted_hosts`.
        """
        return get_current_url(self.environ, True, trusted_hosts=self.trusted_hosts)

    @cached_property
    def host_url(self):
        """Just the host with scheme as IRI.
        See also: :attr:`trusted_hosts`.
        """
        return get_current_url(
            self.environ, host_only=True, trusted_hosts=self.trusted_hosts
        )

    @cached_property
    def host(self):
        """Just the host including the port if available.
        See also: :attr:`trusted_hosts`.
        """
        return get_host(self.environ, trusted_hosts=self.trusted_hosts)

    query_string = environ_property(
        "QUERY_STRING",
        "",
        read_only=True,
        load_func=wsgi_get_bytes,
        doc="The URL parameters as raw bytestring.",
    )
    method = environ_property(
        "REQUEST_METHOD",
        "GET",
        read_only=True,
        load_func=lambda x: x.upper(),
        doc="The request method. (For example ``'GET'`` or ``'POST'``).",
    )

    @cached_property
    def access_route(self):
        """If a forwarded header exists this is a list of all ip addresses
        from the client ip to the last proxy server.
        """
        if "HTTP_X_FORWARDED_FOR" in self.environ:
            return self.list_storage_class(
                parse_list_header(self.environ["HTTP_X_FORWARDED_FOR"])
            )
        elif "REMOTE_ADDR" in self.environ:
            return self.list_storage_class([self.environ["REMOTE_ADDR"]])
        return self.list_storage_class()

    @property
    def remote_addr(self):
        """The remote address of the client."""
        return self.environ.get("REMOTE_ADDR")

    remote_user = environ_property(
        "REMOTE_USER",
        doc="""If the server supports user authentication, and the
        script is protected, this attribute contains the username the
        user has authenticated as.""",
    )
    scheme = environ_property(
        "wsgi.url_scheme",
        doc="""
        URL scheme (http or https).

        .. versionadded:: 0.7""",
    )
    is_secure = property(
        lambda self: self.environ["wsgi.url_scheme"] == "https",
        doc="`True` if the request is secure.",
    )
    is_multithread = environ_property(
        "wsgi.multithread",
        doc="""boolean that is `True` if the application is served by a
        multithreaded WSGI server.""",
    )
    is_multiprocess = environ_property(
        "wsgi.multiprocess",
        doc="""boolean that is `True` if the application is served by a
        WSGI server that spawns multiple processes.""",
    )
    is_run_once = environ_property(
        "wsgi.run_once",
        doc="""boolean that is `True` if the application will be
        executed only once in a process lifetime.  This is the case for
        CGI for example, but it's not guaranteed that the execution only
        happens one time.""",
    )

（五）LocalStack類

這個類是做什麼的呢？上面已經將封裝請求RequestContext物件得到，這個類就是將請求儲存在Local物件中。

class LocalStack(object):
    """This class works similar to a :class:`Local` but keeps a stack
    of objects instead.  This is best explained with an example::

        >>> ls = LocalStack()
        >>> ls.push(42)
        >>> ls.top
        42
        >>> ls.push(23)
        >>> ls.top
        23
        >>> ls.pop()
        23
        >>> ls.top
        42

    They can be force released by using a :class:`LocalManager` or with
    the :func:`release_local` function but the correct way is to pop the
    item from the stack after using.  When the stack is empty it will
    no longer be bound to the current context (and as such released).

    By calling the stack without arguments it returns a proxy that resolves to
    the topmost item on the stack.

    .. versionadded:: 0.6.1
    """

    def __init__(self):
        self._local = Local()

    def __release_local__(self):
        self._local.__release_local__()

    @property
    def __ident_func__(self):
        return self._local.__ident_func__

    @__ident_func__.setter
    def __ident_func__(self, value):
        object.__setattr__(self._local, "__ident_func__", value)

    def __call__(self):
        def _lookup():
            rv = self.top
            if rv is None:
                raise RuntimeError("object unbound")
            return rv

        return LocalProxy(_lookup)

    def push(self, obj):
        """Pushes a new item to the stack"""
        rv = getattr(self._local, "stack", None)
        if rv is None:
            self._local.stack = rv = []
        rv.append(obj)
        return rv

    def pop(self):
        """Removes the topmost item from the stack, will return the
        old value or `None` if the stack was already empty.
        """
        stack = getattr(self._local, "stack", None)
        if stack is None:
            return None
        elif len(stack) == 1:
            release_local(self._local)
            return stack[-1]
        else:
            return stack.pop()

    @property
    def top(self):
        """The topmost item on the stack.  If the stack is empty,
        `None` is returned.
        """
        try:
            return self._local.stack[-1]
        except (AttributeError, IndexError):
            return None

class LocalStack(object):
    def push(self, obj):
        """Pushes a new item to the stack"""
        rv = getattr(self._local, "stack", None)
        if rv is None:
            self._local.stack = rv = []
        rv.append(obj)
        return rv

其中obj就是RequestContext，self._local 就是Local物件。最後Local物件得到的就是類似這樣的東西：

”“”
{
    2651:{'stack':[obj]}    
    ...
}

“”“

最後移除這個物件通過這個類中pop來實現。

（六）LocalProxy類

@implements_bool
class LocalProxy(object):
    """Acts as a proxy for a werkzeug local.  Forwards all operations to
    a proxied object.  The only operations not supported for forwarding
    are right handed operands and any kind of assignment.

    Example usage::

        from werkzeug.local import Local
        l = Local()

        # these are proxies
        request = l('request')
        user = l('user')


        from werkzeug.local import LocalStack
        _response_local = LocalStack()

        # this is a proxy
        response = _response_local()

    Whenever something is bound to l.user / l.request the proxy objects
    will forward all operations.  If no object is bound a :exc:`RuntimeError`
    will be raised.

    To create proxies to :class:`Local` or :class:`LocalStack` objects,
    call the object as shown above.  If you want to have a proxy to an
    object looked up by a function, you can (as of Werkzeug 0.6.1) pass
    a function to the :class:`LocalProxy` constructor::

        session = LocalProxy(lambda: get_current_request().session)

    .. versionchanged:: 0.6.1
       The class can be instantiated with a callable as well now.
    """

    __slots__ = ("__local", "__dict__", "__name__", "__wrapped__")

    def __init__(self, local, name=None):
        object.__setattr__(self, "_LocalProxy__local", local)
        object.__setattr__(self, "__name__", name)
        if callable(local) and not hasattr(local, "__release_local__"):
            # "local" is a callable that is not an instance of Local or
            # LocalManager: mark it as a wrapped function.
            object.__setattr__(self, "__wrapped__", local)

    def _get_current_object(self):
        """Return the current object.  This is useful if you want the real
        object behind the proxy at a time for performance reasons or because
        you want to pass the object into a different context.
        """
        if not hasattr(self.__local, "__release_local__"):
            return self.__local()
        try:
            return getattr(self.__local, self.__name__)
        except AttributeError:
            raise RuntimeError("no object bound to %s" % self.__name__)

    @property
    def __dict__(self):
        try:
            return self._get_current_object().__dict__
        except RuntimeError:
            raise AttributeError("__dict__")

    def __repr__(self):
        try:
            obj = self._get_current_object()
        except RuntimeError:
            return "<%s unbound>" % self.__class__.__name__
        return repr(obj)

    def __bool__(self):
        try:
            return bool(self._get_current_object())
        except RuntimeError:
            return False

    def __unicode__(self):
        try:
            return unicode(self._get_current_object())  # noqa
        except RuntimeError:
            return repr(self)

    def __dir__(self):
        try:
            return dir(self._get_current_object())
        except RuntimeError:
            return []

    def __getattr__(self, name):
        if name == "__members__":
            return dir(self._get_current_object())
        return getattr(self._get_current_object(), name)

    def __setitem__(self, key, value):
        self._get_current_object()[key] = value

    def __delitem__(self, key):
        del self._get_current_object()[key]

    if PY2:
        __getslice__ = lambda x, i, j: x._get_current_object()[i:j]

        def __setslice__(self, i, j, seq):
            self._get_current_object()[i:j] = seq

        def __delslice__(self, i, j):
            del self._get_current_object()[i:j]

    __setattr__ = lambda x, n, v: setattr(x._get_current_object(), n, v)
    __delattr__ = lambda x, n: delattr(x._get_current_object(), n)
    __str__ = lambda x: str(x._get_current_object())
    __lt__ = lambda x, o: x._get_current_object() < o
    __le__ = lambda x, o: x._get_current_object() <= o
    __eq__ = lambda x, o: x._get_current_object() == o
    __ne__ = lambda x, o: x._get_current_object() != o
    __gt__ = lambda x, o: x._get_current_object() > o
    __ge__ = lambda x, o: x._get_current_object() >= o
    __cmp__ = lambda x, o: cmp(x._get_current_object(), o)  # noqa
    __hash__ = lambda x: hash(x._get_current_object())
    __call__ = lambda x, *a, **kw: x._get_current_object()(*a, **kw)
    __len__ = lambda x: len(x._get_current_object())
    __getitem__ = lambda x, i: x._get_current_object()[i]
    __iter__ = lambda x: iter(x._get_current_object())
    __contains__ = lambda x, i: i in x._get_current_object()
    __add__ = lambda x, o: x._get_current_object() + o
    __sub__ = lambda x, o: x._get_current_object() - o
    __mul__ = lambda x, o: x._get_current_object() * o
    __floordiv__ = lambda x, o: x._get_current_object() // o
    __mod__ = lambda x, o: x._get_current_object() % o
    __divmod__ = lambda x, o: x._get_current_object().__divmod__(o)
    __pow__ = lambda x, o: x._get_current_object() ** o
    __lshift__ = lambda x, o: x._get_current_object() << o
    __rshift__ = lambda x, o: x._get_current_object() >> o
    __and__ = lambda x, o: x._get_current_object() & o
    __xor__ = lambda x, o: x._get_current_object() ^ o
    __or__ = lambda x, o: x._get_current_object() | o
    __div__ = lambda x, o: x._get_current_object().__div__(o)
    __truediv__ = lambda x, o: x._get_current_object().__truediv__(o)
    __neg__ = lambda x: -(x._get_current_object())
    __pos__ = lambda x: +(x._get_current_object())
    __abs__ = lambda x: abs(x._get_current_object())
    __invert__ = lambda x: ~(x._get_current_object())
    __complex__ = lambda x: complex(x._get_current_object())
    __int__ = lambda x: int(x._get_current_object())
    __long__ = lambda x: long(x._get_current_object())  # noqa
    __float__ = lambda x: float(x._get_current_object())
    __oct__ = lambda x: oct(x._get_current_object())
    __hex__ = lambda x: hex(x._get_current_object())
    __index__ = lambda x: x._get_current_object().__index__()
    __coerce__ = lambda x, o: x._get_current_object().__coerce__(x, o)
    __enter__ = lambda x: x._get_current_object().__enter__()
    __exit__ = lambda x, *a, **kw: x._get_current_object().__exit__(*a, **kw)
    __radd__ = lambda x, o: o + x._get_current_object()
    __rsub__ = lambda x, o: o - x._get_current_object()
    __rmul__ = lambda x, o: o * x._get_current_object()
    __rdiv__ = lambda x, o: o / x._get_current_object()
    if PY2:
        __rtruediv__ = lambda x, o: x._get_current_object().__rtruediv__(o)
    else:
        __rtruediv__ = __rdiv__
    __rfloordiv__ = lambda x, o: o // x._get_current_object()
    __rmod__ = lambda x, o: o % x._get_current_object()
    __rdivmod__ = lambda x, o: x._get_current_object().__rdivmod__(o)
    __copy__ = lambda x: copy.copy(x._get_current_object())
    __deepcopy__ = lambda x, memo: copy.deepcopy(x._get_current_object(), memo)

我們在flask中匯入request物件，實際上就是LocalProxy類的物件：

request = LocalProxy(partial(_lookup_req_object, "request"))

顯request操作方面就是和這個類有關，比如：在檢視函式中進行request.method獲取當前請求的方法，那麼就會呼叫LocalProxy這個類中的__getattr__方法：

    def __getattr__(self, name):
        if name == "__members__":
            return dir(self._get_current_object())
        return getattr(self._get_current_object(), name)  

返回的是getattr(self._get_current_object(), name) ，那麼_get_current_object()和name分別是是什麼呢？

    def _get_current_object(self):
        """Return the current object.  This is useful if you want the real
        object behind the proxy at a time for performance reasons or because
        you want to pass the object into a different context.
        """
        if not hasattr(self.__local, "__release_local__"):
            return self.__local()
        try:
            return getattr(self.__local, self.__name__)
        except AttributeError:
            raise RuntimeError("no object bound to %s" % self.__name__)

在進行這個之前，先執行傳入LocalProxy的偏函式：

def _lookup_req_object(name):
    top = _request_ctx_stack.top  #獲取Local物件中新增的RequestContext物件
    if top is None:
        raise RuntimeError(_request_ctx_err_msg)
    return getattr(top, name)

所以getattr(self.__local, self.__name__)中self.__local是RequestContext物件，__name__就是‘request'字串。這樣就獲取到了Request物件。從而得到request.method的值。

三、類之間的關係

那麼這六個類之間又是如何進行關聯呢？

可以通過一個簡單的圖將之間的關係表示出來。