Python3中的open函数
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open(file, mode='r', buffering=-1, encoding=None, errors=None, newline=None, closefd=True, opener=None)
Open file and return a stream. Raise IOError upon failure.
#打开文件并返回一个流？失败则抛出IOError异常
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不使用参数默认是'rt'，‘w’写模式，会覆盖原来全部的内容（会创建文件），‘x’创建一个新的文件，并写入内容如果文件存在会‘FileExistsError’，‘a’在文件末尾追加内容，‘b’二进制模式，‘+’更新磁盘文件（读写），‘U’弃用
参数有a和w会创建不存在的文件
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 只能用在二进制模式
1 行缓冲
>1 则使用给定的值做缓冲大小
*在没有给出参数的情况下，二进制文件的大小有底层设备“block size”决定，可以通过‘io.DEFAULT_BUFFER_SIZE’获取，在很多系统中这个值的大小为4096或者8192字节
*文本文件则采用行缓冲
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是文件的解码或者编码方式，只能用于文本模式，默认的编码方式依赖于平台，任何python能够支持编码都可以在python中使用，可以查看编码模块
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是一个可选的参数，并且不能用于二进制模式，如果出现编码错误会排出ValueError错误，或者使用‘ignoe’忽略，可通过查看codecs.codec获取错误编码字符串
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.
换行控制，参数可以用None, '', '\n', '\r', and '\r\n'（只能用于文本模式）
*输入时，
如果参数为None，那么换行符启用，结尾可以是'\n', '\r', or '\r\n'，并且这些控制符都会编码为'\n'。
如果是''换行符模式启用，但是行位的换行符在返回调用时将不会被编码。
如果给出其他有效参数，返回调用时将会使用指定的参数
*输出时，
如果参数为None，任何‘\n’将会编码成系统默认的分隔符
如果参数为‘’或者'\n'，将不会编码
如果参数为其他有效值，'\n'将会编码成给定的值
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.
当文件关闭时，如果closefd为False，底层文件描述仍然是打开，设置为True底层文件描述同时也会关闭。
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).
可以通过调用*opener*来自定义opener，底层文件是通过调用*opener*, *file*, *flags*来获取描述。*opener*必须返回一个打开的文件描述。os.open作为*opener*的返回结果类似于通过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+的结果
eg.
>>> f = open('opentest', 'r+')
>>> f.write('1111')
4
>>> f.write('2222')
4
>>> f.write('3333')
4
>>> f.close()再次查看opentest内容
:~/Code$ cat opentest 111122223333pen testthis is ab abc edf dfc dag dagk asgg asdgag aggfdn sdnhsdfo sdfigsodfnh ****
使用r+，指针在开头，会覆盖掉原位置原有的内容
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