前言

Flask是目前為止我最喜歡的一個Python Web框架了，為了更好的掌握其內部實現機制，這兩天準備學習下Flask的原始碼，將由淺入深跟大家分享下，其中Flask版本為1.1.1。

Flask系列文章：

1. Flask開發初探

正文

本文將結合原始碼跟蹤看下Flask是如何啟動並執行一個服務的。

首先，繼續貼上最簡單的應用：

from flask import Flask
app = Flask(__name__)

@app.route('/')
def hello_world():
    return 'Hello Flask!'

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

我們看到，這段程式碼先初始化了Flask類並被app所指向，然後執行run()來啟動程式的。

檢視run方法：

def run(self, host=None, port=None, debug=None, load_dotenv=True, **options):
        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
 

首先入參：

該方法的處理流程是：對入參進行配置處理之後，執行werkzeug的run_simple()方法，

run_simple將啟動一個WSGI服務。

關於WSGI協議：

1. 它是關於HTTP伺服器和Web應用的橋樑，定義了標準介面以提升Web應用之間的可移植性，是一套介面互動規範。
2. 它的功能是監聽指定埠服務，將來自HTTP伺服器的請求解析為WSGI格式，呼叫Flask app處理請求。

run_simple中的inner方法是核心，inner呼叫make_server().serve_forever()啟動服務。關於make_server:

def make_server(host=None, port=None, app=None, threaded=False, processes=1,
                request_handler=None, passthrough_errors=False,
                ssl_context=None, fd=None):
    if threaded and processes > 1:
        raise ValueError("cannot have a multithreaded and "
                         "multi process server.")
    elif threaded:
        return ThreadedWSGIServer(host, port, app, request_handler,
                                  passthrough_errors, ssl_context, fd=fd)
    elif processes > 1:
        return ForkingWSGIServer(host, port, app, processes, request_handler,
                                 passthrough_errors, ssl_context, fd=fd)
    else:
        return BaseWSGIServer(host, port, app, request_handler,
                              passthrough_errors, ssl_context, fd=fd)

make_server會根據執行緒或者程序數返回相應的WSGI伺服器，預設情況下返回BaseWSGIServer，ThreadedWSGIServer和ForkingWSGIServer均集成了BaserWSGIServer，接下來我們看下serve_forever()方法：

def serve_forever(self):
    self.shutdown_signal = False
    try:
        HTTPServer.serve_forever(self)
    except KeyboardInterrupt:
        pass
    finally:
        self.server_close()

最終呼叫了Python標準類庫介面HTTPServer的serve_forever()方法，而HTTPServer又是socketserver.TCPServer的子類，通過server_bind來監聽服務：

class HTTPServer(socketserver.TCPServer):

    allow_reuse_address = 1    # Seems to make sense in testing environment

    def server_bind(self):
        """Override server_bind to store the server name."""
        socketserver.TCPServer.server_bind(self)
        host, port = self.server_address[:2]
        self.server_name = socket.getfqdn(host)
        self.server_port = port

以上，就是Flask服務啟動的流程