Open file and return a stream. Raise IOError upon failure.

#打開文件並返回一個流？失敗則拋出IOError異常

mode：

========= ===============================================================

Character Meaning

--------- ---------------------------------------------------------------

'r' open for reading (default)

'w' open for writing, truncating the file first

'x' create a new file and open it for writing

'a' open for writing, appending to the end of the file if it exists

'b' binary mode

't' text mode (default)

'+' open a disk file for updating (reading and writing)

'U' universal newline mode (deprecated)

========= ===============================================================

mode不使用參數默認是'rt'，‘w’寫模式，會覆蓋原來全部的內容（會創建文件），‘x’創建一個新的文件，並寫入內容如果文件存在會‘FileExistsError’，‘a’在文件末尾追加內容，‘b’二進制模式，‘+’更新磁盤文件（讀寫），‘U’棄用

參數有a和w會創建不存在的文件

buffering：

buffering is an optional integer used to set the buffering policy.

Pass 0 to switch buffering off (only allowed in binary mode), 1 to select

line buffering (only usable in text mode), and an integer > 1 to indicate

the size of a fixed-size chunk buffer. When no buffering argument is

given, the default buffering policy works as follows:

* Binary files are buffered in fixed-size chunks; the size of the buffer

is chosen using a heuristic trying to determine the underlying device's

"block size" and falling back on `io.DEFAULT_BUFFER_SIZE`.

On many systems, the buffer will typically be 4096 or 8192 bytes long.

* "Interactive" text files (files for which isatty() returns True)

use line buffering. Other text files use the policy described above

for binary files.

0 只能用在二進制模式

1 行緩沖

>1 則使用給定的值做緩沖大小

*在沒有給出參數的情況下，二進制文件的大小有底層設備“block size”決定，可以通過‘io.DEFAULT_BUFFER_SIZE’獲取，在很多系統中這個值的大小為4096或者8192字節

*文本文件則采用行緩沖

encoding：

encoding is the name of the encoding used to decode or encode the

file. This should only be used in text mode. The default encoding is

platform dependent, but any encoding supported by Python can be

passed. See the codecs module for the list of supported encodings.

encoding是文件的解碼或者編碼方式，只能用於文本模式，默認的編碼方式依賴於平臺，任何python能夠支持編碼都可以在python中使用，可以查看編碼模塊

errors：

errors is an optional string that specifies how encoding errors are to

be handled---this argument should not be used in binary mode. Pass

'strict' to raise a ValueError exception if there is an encoding error

(the default of None has the same effect), or pass 'ignore' to ignore

errors. (Note that ignoring encoding errors can lead to data loss.)

See the documentation for codecs.register or run 'help(codecs.Codec)'

for a list of the permitted encoding error strings.

errors是一個可選的參數，並且不能用於二進制模式，如果出現編碼錯誤會排出ValueError錯誤，或者使用‘ignoe’忽略，可通過查看codecs.codec獲取錯誤編碼字符串

newline：

newline controls how universal newlines works (it only applies to text

mode). It can be None, '', '\n', '\r', and '\r\n'. It works as

follows:

* On input, if newline is None, universal newlines mode is

enabled. Lines in the input can end in '\n', '\r', or '\r\n', and

these are translated into '\n' before being returned to the

caller. If it is '', universal newline mode is enabled, but line

endings are returned to the caller untranslated. If it has any of

the other legal values, input lines are only terminated by the given

string, and the line ending is returned to the caller untranslated.

* On output, if newline is None, any '\n' characters written are

translated to the system default line separator, os.linesep. If

newline is '' or '\n', no translation takes place. If newline is any

of the other legal values, any '\n' characters written are translated

to the given string.

換行控制，參數可以用None, '', '\n', '\r', and '\r\n'（只能用於文本模式）

*輸入時，

如果參數為None，那麽換行符啟用，結尾可以是'\n', '\r', or '\r\n'，並且這些控制符都會編碼為'\n'。

如果是''換行符模式啟用，但是行位的換行符在返回調用時將不會被編碼。

如果給出其他有效參數，返回調用時將會使用指定的參數

*輸出時，

如果參數為None，任何‘\n’將會編碼成系統默認的分隔符

如果參數為‘’或者'\n'，將不會編碼

如果參數為其他有效值，'\n'將會編碼成給定的值

closefd：

If closefd is False, the underlying file descriptor will be kept open

when the file is closed. This does not work when a file name is given

and must be True in that case.

當文件關閉時，如果closefd為False，底層文件描述仍然是打開，設置為True底層文件描述同時也會關閉。

opener：

A custom opener can be used by passing a callable as *opener*. The

underlying file descriptor for the file object is then obtained by

calling *opener* with (*file*, *flags*). *opener* must return an open

file descriptor (passing os.open as *opener* results in functionality

similar to passing None).

可以通過調用*opener*來自定義opener，底層文件是通過調用*opener*, *file*, *flags*來獲取描述。*opener*必須返回一個打開的文件描述。os.open作為*opener*的返回結果類似於通過None。

open() returns a file object whose type depends on the mode, and

through which the standard file operations such as reading and writing

are performed. When open() is used to open a file in a text mode ('w',

'r', 'wt', 'rt', etc.), it returns a TextIOWrapper. When used to open

a file in a binary mode, the returned class varies: in read binary

mode, it returns a BufferedReader; in write binary and append binary

modes, it returns a BufferedWriter, and in read/write mode, it returns

a BufferedRandom.

It is also possible to use a string or bytearray as a file for both

reading and writing. For strings StringIO can be used like a file

opened in a text mode, and for bytes a BytesIO can be used like a file

opened in a binary mode.

:~/Code$ cat opentest
pythonis a open testthis is ab
abc
edf
dfc
dag
dagk
asgg
asdgag
aggfdn
sdnhsdfo
sdfigsodfnh
****

使用r+的結果

eg.

>>> f = open('opentest', 'r+')
>>> f.write('1111')
4
>>> f.write('2222')
4
>>> f.write('3333')
4
>>> f.close()

再次查看opentest內容

:~/Code$ cat opentest
111122223333pen testthis is ab
abc
edf
dfc
dag
dagk
asgg
asdgag
aggfdn
sdnhsdfo
sdfigsodfnh
****

使用r+，指針在開頭，會覆蓋掉原位置原有的內容

Python3中的open函數