This probably only applies to Windows. The redundant 'b' modes still work on Linux.

These worked on Windows in 2.6.4:

open('test', 'rbb')
open('test', 'wbb')
open('test', 'abb')

and possibly others.

In 2.7a2, they throw ValueErrors like this:

>>> open('test', 'wbb')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ValueError: Invalid mode ('wbb')

It would be nice if the old behavior were preserved for backwards compatibility. Some programs append a 'b' indiscriminately.

With 2.7 I can reproduce the issue on Windows 7 (i.e. I get a ValueError).
However I'm not sure how common is to add a 'b' indiscriminately:
1) most of the times the modes are written as strings, and not generated automatically;
2) even if generated, checking if there's a 'b' is as easy as doing "if 'b' not in mode: mode = mode + 'b'".

For consistency both Windows and Linux should have the same behavior, and if the extra 'b' is not so common I think it's better if both raise an error.

For Python 2.7 we can no longer do anything about it. Any chance might break 3rd party software. Python 3.x and Python 2.7's io module don't allow redundant information.

I suggest that you follow Ezio's advice and check for redundancy in your application.