The behavior of os.path.abspath differs between Mac OS X and Windows. It seems like a bug that, on Windows, abspath strips trailing whitespace:
Windows:
>>> from os.path import abspath
>>> abspath('start  ') + 'END'
'c:\Users\jgray\code\startEND'
                         ^
Mac:
>>> from os.path import abspath
>>> abspath('start  ') + 'END'
'/Users/jgray/code/start  END'
                        ^^

I have only tested with python 2.7.3 (Enthought distribution), 32-bit, and only Mac 10.8.2 and Windows 7.

Maybe this a documentation / run-time warning issue, rather than bug. Trailing spaces in filenames are illegal in Win32; nonetheless they can occur, because there are instructions on how to remove such files: http://support.microsoft.com/kb/320081. So os.path.abspath() not only returns the absolute path of a (hypothetical) file, it is also ensuring that the file name is legal within the operating system. It does so silently. (I say hypothetical because there's no requirement that the file actually exists.)

This behavior from python was surprising since "explicit is better than implicit": I was not getting just an absolute path, but a changed filename (causing a file-not-found error later in my code), with no warning from python at any point that the filename was illegal and so was being "cleaned up" for me, behind the scenes. Maybe this is the Windows-like way to do things (longtime mac / linux here).

Playing around a little, I see "open('file  ', 'wb')" also silently changes the filename.

I'm going to close this as won't fix: the underlying implementation is simply calling GetFullPathName and removal of trailing spaces doesn't seem too surprising a result. It is possible to manipulate such files using Windows' special \\?\C:\... syntax.

While I admit this could catch you out if you were trying to nail an illegal-but-existent filename, we try not to put warnings in the docs for every cornercase lest they become cluttered.