Difference between modes a, a+, w, w+, and r+ in built-in open function?

Question

In the python built-in open function, what is the exact difference between the modes w, a, w+, a+, and r+?

In particular, the documentation implies that all of these will allow writing to the file, and says that it opens the files for "appending", "writing", and "updating" specifically, but does not define what these terms mean.

Answer 1

The opening modes are exactly the same as those for the C standard library function fopen().

The BSD fopen manpage defines them as follows:

 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.