Steven Barker added the comment:
Here's a patch against the default branch that fixes filecmp.cmp's behavior
when "shallow" is True, including an update to the module's docs (correcting
the previous ambiguity discussed in the 2011 python-dev thread mentioned by
Sandro Tosi) and a couple of new tests of files where shallow comparisons
should be expected get the answer wrong.
The changed behavior is very simple. The lines from Lib/filecmp.py in the
original code:
if shallow and s1 == s2:
return True
are changed to:
if shallow:
return s1 == s2
This presumes answers to the questions asked by Georg Brandl back in 2005 of
"No, shallow comparisons never read the file contents" and "No, non-shallow
comparisons don't care about mtimes (except for avoiding out of date cache
entries)".
If we only applied the test changes (not the behavior change) from my patch,
one of the new tests would fail, as the current filecmp.cmp code never gives
false negatives on comparisons in shallow mode (only false positives).
We probably don't want to commit the patch exactly as it stands, because the
changed behavior causes several of the previously existing tests to fail.
Almost all the dircmp tests fail for me, and the "test_matching" filecmp.cmp
test does so intermittently (due to my system sometimes being fast enough at
creating the files to beat my filesystem's mtime resolution).
The test failures are all because of shallow comparisons between files with the
same contents, but (slightly) different mtimes. The new shallow comparison
behavior is to say those files are unequal while the old code would fall back
on testing the file contents despite the shallow parameter, and so it would
return True. The failing tests can probably be updated to either explicitly set
the file mtimes with sys.utime (as I do with the files used in the new tests),
or we could rewrite some of the setup code to use something like shutil.copy2
to make copies of files while preserving their mtimes.
I'm not very familiar with the best practices when it comes to writing unit
tests, so I pretty much copied the style of the existing tests in
Lib/test/test_filecmp.py (though I've split mine up a bit more).
I understand from reading other filecmp bug reports that that test style is
considered to be pretty poor, so there's probably room to improve my code as
well. I thought I'd post this patch before making an effort at fixing the older
tests, in order to get some feedback. I'll be happy to incorporate any
suggested improvements!
This issue has been open for almost 9 years. Hopefully my patch can help get it
moving again towards being fixed!
----------
keywords: +patch
versions: +Python 3.5
Added file: http://bugs.python.org/file35159/filecmp_real_shallow.diff
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<http://bugs.python.org/issue1234674>
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