Messages (10) msg190715 - (view) Author: Matej Fröbe (fbm) Date: 2013-06-06 14:07

Example:
  with open('file1', 'w') as f:
    f.write('a')
  with open('file2', 'w') as f:
    f.write('a')
  print filecmp.cmp('file1', 'file2', shallow=False) # true
  with open('file2', 'w') as f:
    f.write('b')
  print filecmp.cmp('file1', 'file2', shallow=False) # true
Because of the caching, both calls to filecmp.cmp() return true on my system.
When retrieving value from cache, the function filecmp.cmp() checks the signatures of the files:
  s1 = _sig(os.stat(f1))
  s2 = _sig(os.stat(f2))
  ...
  outcome = _cache.get((f1, f2, s1, s2))
But the signatures in cache are the same, if the file sizes and times of modification (os.stat().st_mtime) haven't changed from the last call, even if the content has changed.
The buffer is mentioned in the documentation, but there isn't any documented way to clear it. It also isn't nice IMO, that one has to worry about the file system's resolution of the file modification time when calling a simple file comparison.

msg190774 - (view) Author: Ned Deily (ned.deily) * Date: 2013-06-07 20:26

It seems like this would be a fairly rare situation and, as you note, dependent on the underlying file system.  But it would be easy to add a new function to the module to clear its cache in cases where it is known this might be a problem.  In fact, in Issue11802 a clear_cache function was proposed to solve the problem of the cache growing without bounds but that problem was solved by the simpler solution of discarding the cache when it gets above 100 entries.

msg190790 - (view) Author: Raymond Hettinger (rhettinger) * Date: 2013-06-08 01:56

+1 for a cache clearing function like the one in re.py

msg191026 - (view) Author: Mark Levitt (melevittfl) * Date: 2013-06-12 13:06

I've added a "clear_cache()" method to filecmp.py. Patch attached.
I had thought about implementing an optional parameter to only invalidate the cache of a specific file object, but figured I'd keep it simple for now.
First time submitting a patch, so apologies if I've done something the wrong way.

msg191048 - (view) Author: Ned Deily (ned.deily) * Date: 2013-06-12 21:03

Thanks for the patch, Mark. I've left some review comments via Rietveld (the review link next to the patch).  Also, if you haven't already, please fill out the contributor form as described in the Developer's Guide (http://docs.python.org/devguide/patch.html#licensing).

msg191051 - (view) Author: Mark Levitt (melevittfl) * Date: 2013-06-12 23:40

Ned,
Thanks for taking the time to review. I've updated the docs, added a unit test, signed the contributor form, and made the changes/corrections from your review.
Updated patch attached.

msg191060 - (view) Author: Ned Deily (ned.deily) * Date: 2013-06-13 06:29

Looks good to me, other than that the doc change should include a version added directive (which can be added by the committer):
 .. function:: clear_cache()
+   .. versionadded:: 3.4
+
    Clear the filecmp cache. This may be useful if a file is compared so quickly

msg191067 - (view) Author: Mark Levitt (melevittfl) * Date: 2013-06-13 08:32

Cool. I've gone ahead and generated a new patch with the version added directive included.

msg191162 - (view) Author: Roundup Robot (python-dev) Date: 2013-06-14 22:20

New changeset bfd53dcb02ff by Ned Deily in branch 'default':
Issue #18149: Add filecmp.clear_cache() to manually clear the filecmp cache.
http://hg.python.org/cpython/rev/bfd53dcb02ff

msg191163 - (view) Author: Ned Deily (ned.deily) * Date: 2013-06-14 22:22

Committed for release in 3.4.0.  Thanks, Mark.

History Date User Action Args 2022-04-11 14:57:46adminsetgithub: 62349 2013-06-14 22:22:19ned.deilysetstatus: open -> closed
resolution: fixed
messages: + msg191163

stage: commit review -> resolved

2013-06-14 22:20:23python-devsetnosy: + python-dev
messages: + msg191162
2013-06-13 08:32:28melevittflsetfiles: + 18149-3.patch

messages: + msg191067

2013-06-13 06:29:12ned.deilysetmessages: + msg191060
stage: needs patch -> commit review 2013-06-12 23:40:21melevittflsetfiles: + 18149-2.patch

messages: + msg191051

2013-06-12 21:03:40ned.deilysetmessages: + msg191048 2013-06-12 13:06:56melevittflsetfiles: + 18149.patch

nosy: + melevittfl
messages: + msg191026

keywords: + patch

2013-06-08 01:56:51rhettingersetmessages: + msg190790 2013-06-07 20:26:29ned.deilysettitle: filecmp.cmp() - cache invalidation fails when file modification times haven't changed -> filecmp.cmp() incorrect results when previously compared file is modified within modification time resolution

keywords: + easy
nosy: + rhettinger, ned.deily, nadeem.vawda
versions: + Python 3.4, - Python 2.7
messages: + msg190774
stage: needs patch

2013-06-06 14:07:35fbmcreate