Hi,

Here are some small comments to your otherwise good to have patch:

-- assertEquals has been deprecated in favor of assertEqual, and usually it's great to be consistent across the test suite
-- likewise maxDiff should be max_diff mainly because of consistency and because of pep8's naming styles
-- self.temp_dir is never cleaned up (on tearDown maybe?) and gettempdir() doesn't really look like a good option; maybe you wanted to create a new temporary directory rather than pointing to the filesystem's tmp dir ?
-- the patch doesn't apply cleanly, at least for me - the ACKS hunk is failing for me