打开模式与C标准库函数fopen（）的打开模式完全相同。

BSD fopen手册页对它们的定义如下：

 The argument mode points to a string beginning with one of the following
 sequences (Additional characters may follow these sequences.):

 ``r''   Open text file for reading.  The stream is positioned at the
         beginning of the file.

 ``r+''  Open for reading and writing.  The stream is positioned at the
         beginning of the file.

 ``w''   Truncate file to zero length or create text file for writing.
         The stream is positioned at the beginning of the file.

 ``w+''  Open for reading and writing.  The file is created if it does not
         exist, otherwise it is truncated.  The stream is positioned at
         the beginning of the file.

 ``a''   Open for writing.  The file is created if it does not exist.  The
         stream is positioned at the end of the file.  Subsequent writes
         to the file will always end up at the then current end of file,
         irrespective of any intervening fseek(3) or similar.

 ``a+''  Open for reading and writing.  The file is created if it does not
         exist.  The stream is positioned at the end of the file.  Subse-
         quent writes to the file will always end up at the then current
         end of file, irrespective of any intervening fseek(3) or similar.

Mode

如果未选择任何模式；使用文本模式（t）。因此，r与rt相同。

选项与C标准库中fopen函数的选项相同：

w截断文件，覆盖已经存在的内容

a附加到文件，添加到已经存在的文件中

w+打开用于读取和写入，截断文件，但也允许您读取已写入文件的内容

a+打开用于附加和读取，允许您附加到文件并读取其内容

许多人特别疑惑的是“r+和w+模式之间有什么区别？”？

r+帮助您在不截断的情况下将数据读写到已经存在的文件上（如果没有这样的文件，则为错误）。

另一方面，w+模式也允许读取和写入，但它会截断文件（如果不存在这样的文件，则会创建一个新文件）。如果您想知道如何从截断的文件中读取，可以使用读取方法来读取新写入的文件（或空文件）。

干杯

打开模式与C标准库函数fopen（）的打开模式完全相同。

BSD fopen手册页对它们的定义如下：

 The argument mode points to a string beginning with one of the following
 sequences (Additional characters may follow these sequences.):

 ``r''   Open text file for reading.  The stream is positioned at the
         beginning of the file.

 ``r+''  Open for reading and writing.  The stream is positioned at the
         beginning of the file.

 ``w''   Truncate file to zero length or create text file for writing.
         The stream is positioned at the beginning of the file.

 ``w+''  Open for reading and writing.  The file is created if it does not
         exist, otherwise it is truncated.  The stream is positioned at
         the beginning of the file.

 ``a''   Open for writing.  The file is created if it does not exist.  The
         stream is positioned at the end of the file.  Subsequent writes
         to the file will always end up at the then current end of file,
         irrespective of any intervening fseek(3) or similar.

 ``a+''  Open for reading and writing.  The file is created if it does not
         exist.  The stream is positioned at the end of the file.  Subse-
         quent writes to the file will always end up at the then current
         end of file, irrespective of any intervening fseek(3) or similar.

我认为这对于跨平台执行（即作为CYA）来说非常重要。：）

在Windows上，附加到模式的“b”以二进制模式打开文件，因此也有类似“rb”、“wb”和“r+b”的模式。Windows上的Python区分了文本文件和二进制文件；当读取或写入数据时，文本文件中的行尾字符会自动轻微更改。这种对文件数据的幕后修改对于ASCII文本文件来说很好，但它会破坏二进制数据，就像JPEG或EXE文件中的二进制数据一样。在读取和写入此类文件时，请非常小心地使用二进制模式。在Unix上，在模式中附加一个“b”并不有害，因此您可以对所有二进制文件独立地使用它。

这直接引用自Python Software Foundation 2.7.x。