源码：
def open(file, mode='r', buffering=None, encoding=None, errors=None, newline=None, closefd=True): # known special case of open """ Open file and return a stream. Raise IOError upon failure. file is either a text or byte string giving the name (and the path if the file isn't in the current working directory) of the file to be opened or an integer file descriptor of the file to be wrapped. (If a file descriptor is given, it is closed when the returned I/O object is closed, unless closefd is set to False.) mode is an optional string that specifies the mode in which the file is opened. It defaults to 'r' which means open for reading in text mode. Other common values are 'w' for writing (truncating the file if it already exists), 'x' for creating and writing to a new file, and 'a' for appending (which on some Unix systems, means that all writes append to the end of the file regardless of the current seek position). In text mode, if encoding is not specified the encoding used is platform dependent: locale.getpreferredencoding(False) is called to get the current locale encoding. (For reading and writing raw bytes use binary mode and leave encoding unspecified.) The available modes are: ========= =============================================================== 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) ========= =============================================================== The default mode is 'rt' (open for reading text). For binary random access, the mode 'w+b' opens and truncates the file to 0 bytes, while 'r+b' opens the file without truncation. The 'x' mode implies 'w' and raises an `FileExistsError` if the file already exists. Python distinguishes between files opened in binary and text modes, even when the underlying operating system doesn't. Files opened in binary mode (appending 'b' to the mode argument) return contents as bytes objects without any decoding. In text mode (the default, or when 't' is appended to the mode argument), the contents of the file are returned as strings, the bytes having been first decoded using a platform-dependent encoding or using the specified encoding if given. 'U' mode is deprecated and will raise an exception in future versions of Python. It has no effect in Python 3. Use newline to control universal newlines mode. 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. 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. 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. 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. 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. 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). 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. """def open（file，mode ='r'，buffering = None，encoding = None，errors = None，newline = None，closefd = True）：＃已知的特殊情况open
“””
打开文件并返回一个流。失败时引发IOError。
文件是一个文本或字节字符串给出的名称（和路径
如果该文件不在当前工作目录中）
被打开或文件的整数文件描述符为
包裹。 （如果给出了一个文件描述符，那么它将在关闭时关闭
返回的I / O对象关闭，除非closefd设置为False。）
mode是一个可选字符串，用于指定文件的模式
被打开。它默认为'r'，这意味着可以在文本中阅读
模式。其他常见的值是'w'用于写入（如果是，则截断该文件
它已经存在），用于创建和写入新文件的'x'，以及
'a'用于追加（在某些Unix系统上，这意味着所有的写入
追加到文件的末尾，而不管当前的搜索位置）。
在文本模式下，如果未指定编码，则使用的编码是平台
依赖于：locale.getpreferredencoding（False）被调用来获取
当前的区域设置编码。 （用于读取和写入原始字节使用二进制
模式并且未指定编码。）可用的模式是：
========= ========================================= ======================
字符意义
--------- ----------------------------------------- ----------------------
'r'打开阅读（默认）
'w'开始写入，首先截断文件
'x'创建一个新文件并打开它进行写入
'a'开放写入，如果存在则附加到文件末尾
'b'二进制模式
't'文本模式（默认）
'+'打开磁盘文件进行更新（读取和写入）
'U'通用换行符模式（已弃用）
========= ========================================= ======================
默认模式是'rt'（打开阅读文本）。对于二进制随机
访问模式'w + b'打开并截断文件到0字节，而
'r + b'打开文件而不截断。 'x'模式意味着'w'和
如果文件已经存在，则引发一个`FileExistsError`。
Python区分以二进制和文本模式打开的文件，
即使底层操作系统没有。打开文件
二进制模式（向模式参数附加'b'）返回内容为
字节对象没有任何解码。在文本模式下（默认或当
't'被附加到模式参数中），文件的内容是
作为字符串返回，字节先用a解码
依赖于平台的编码或者在给定的情况下使用指定编码。
'U'模式已被弃用，并将在未来的版本中引发异常
的Python。它在Python 3中没有效果。使用换行符来控制
通用换行模式。
缓冲是用于设置缓冲策略的可选整数。
通过0切换缓冲关闭（仅在二进制模式下允许），1选择
行缓冲（只能在文本模式下使用）和一个> 1的整数来表示
一个固定大小的块缓冲区的大小。没有缓冲参数时
给定，默认缓冲策略的工作原理如下：
*二进制文件被缓冲在固定大小的块中;缓冲区的大小
选择使用试图确定底层设备的启发式
“块大小”并回落到“io.DEFAULT_BUFFER_SIZE”。
在很多系统上，缓冲区的长度通常为4096或8192字节。
*“交互式”文本文件（isatty（）返回True的文件）
使用行缓冲。其他文本文件使用上述策略
二进制文件。
编码是用于解码或编码的编码的名称
文件。这只能用于文本模式。默认编码是
依赖于平台，但Python支持的任何编码都可以
通过。请参阅编解码器模块以获取支持的编码列表。
错误是指定编码错误的可选字符串
被处理---这个参数不应该在二进制模式下使用。通过
如果存在编码错误，'strict'会引发ValueError异常
（默认的None有相同的效果），或者通过'ignore'忽略
错误。 （请注意，忽略编码错误可能会导致数据丢失。）
请参阅codecs.register的文档或运行'help（codecs.Codec）'
获取允许的编码错误字符串的列表。
换行符控制通用换行符的工作方式（仅适用于文本
模式）。它可以是None，''，'\ n'，'\ r'和'\ r \ n'。它的工作原理是
如下：
*在输入时，如果换行符是None，通用换行符模式就是
启用。输入中的行可以以'\ n'，'\ r'或'\ r \ n'结尾，而
这些被转换成'\ n'之后被返回到
呼叫者。如果是“'，则启用通用换行模式