在python内置的open http://docs.python.org/library/functions.html#open功能，模式之间的确切区别是什么w, a, w+, a+, and r+?
特别是，文档暗示所有这些都将允许写入文件，并表示它打开文件专门用于“追加”、“写入”和“更新”，但没有定义这些术语的含义。
与C标准库函数的打开方式完全相同fopen().
The BSD fopen manpage http://www.manpagez.com/man/3/fopen/将它们定义如下：
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.
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