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+?

Answer 1

Same info, just in table form

                  | r   r+   w   w+   a   a+
------------------|--------------------------
read              | +   +        +        +
write             |     +    +   +    +   +
write after seek  |     +    +   +
create            |          +   +    +   +
truncate          |          +   +
position at start | +   +    +   +
position at end   |                   +   +

where meanings are: (just to avoid any misinterpretation)

• read - reading from file is allowed

• write - writing to file is allowed

• create - file is created if it does not exist yet

• trunctate - during opening of the file it is made empty (all content of the file is erased)

• position at start - after file is opened, initial position is set to the start of the file

• position at end - after file is opened, initial position is set to the end of the file

Note: a and a+ always append to the end of file - ignores any seek movements.
BTW. interesting behavior at least on my win7 / python2.7, for new file opened in a+ mode:
write('aa'); seek(0, 0); read(1); write('b') - second write is ignored
write('aa'); seek(0, 0); read(2); write('b') - second write raises IOError