下面是python中builtins文件里对open函数的定义，我将英文按照我的理解翻译成中文，方便以后查看。

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 OSError upon failure.　　　打开文件返回的是一个文件流，我觉得应该是字节流。当打开失败了就造成操作系统错误

    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.)　　 文件应该是一个文本或字节字符串（    如果该文件不在当前工作目录中就应该给出路径）    提供被打开的文件名或是被包装的文件的文件描述符     （如果给出了一个文件描述符，那么它返回的io对象如果关闭了，它就将被关闭    ，除非closefd设置为False。）    

　　 mode is an optional（可选的） string that specifies（指定） the mode in which the file    is opened. 　　 It defaults to 'r' which means open for reading in text    mode. 　　 'w' for writing (truncating（截断） the file if    it already exists)（文件存在就会覆盖）, 　　'x' for creating and writing to a new file    'a' for appending (which on some Unix systems, means that all writes    append to the end of the file regardless of （不管，不顾）the current seek position).    In text mode, if encoding is not specified （指定）the encoding used is platform（平台）    dependent: locale.getpreferredencoding(False) is called to get the    current locale（场所） encoding. (For reading and writing raw（原始的） 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(磁盘) file for updating (reading and writing)    'U'       universal（一般的） newline mode (deprecated)    ========= ===============================================================

    The default mode is 'rt' (open for reading text). For binary random    access（进入）, the mode 'w+b' opens and truncates（截断） the file to 0 bytes, while    'r+b' opens the file without truncation（切断）. The 'x' mode implies（指的是） 'w' and    raises an `FileExistsError` if the file already exists.

    Python distinguishes（辨别） between files opened in binary and text modes,    even when the underlying（底层的） operating system doesn't. Files opened in    binary mode (appending 'b' to the mode argument) return contents（内容） as    bytes objects without any decoding（解码）. 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（依赖平台的） encoding（编码） or using the specified（指定的） encoding if given.　　　这段理解：文件通过二进制读进来不会进行解码，而如果文件通过文本字符串方式读进来需要解码，而解码方式要么是平它默认的编码方式，　　　　　　　　要么就是我们通过encoding参数指定的。  

    'U' mode is deprecated（不赞成） 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（策略）.    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（底层的） file descriptor for the file object is then obtained（获得） 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（标准） file operations such as reading and writing    are performed（执行）. 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.    """    pass

第一次打开一个文件准备写文件操作时，如果文件已经存在就会覆盖，而在一次打开文件的操作中多次执行write操作，是不会覆盖之前write进去的内容