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gh-145131: difflib.Differ._fancy_replace performance (re-use ratio) - #145133

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tim-one merged 1 commit into
python:mainfrom
dg-pb:difflib_opt
Feb 23, 2026
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gh-145131: difflib.Differ._fancy_replace performance (re-use ratio)#145133
tim-one merged 1 commit into
python:mainfrom
dg-pb:difflib_opt

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@dg-pb

@dg-pb dg-pb commented Feb 23, 2026 •
edited by bedevere-app Bot
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@bedevere-app bedevere-app Bot mentioned this pull request Feb 23, 2026
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tim-one self-requested a review February 23, 2026 01:51
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tim-one approved these changes Feb 23, 2026
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I'll merge it, but reluctantly. It damages readability a little for very little gain. The matching blocks were already computed and cached for reuse on the first call to cr(), so it saves very little time.- just saves summing the lengths of the already-computed matching blocks between two lines. Why bother? It's not worth it to me, but damages readability so little I won't argue about it. So, thanks - I guess 😉.

@tim-one
tim-one merged commit 79f6caf into python:main Feb 23, 2026
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@dg-pb
dg-pb deleted the difflib_opt branch February 23, 2026 02:08
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bedevere-bot commented Feb 23, 2026

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⚠️⚠️⚠️ Buildbot failure ⚠️⚠️⚠️

Hi! The buildbot AMD64 Ubuntu Shared 3.x (tier-1) has failed when building commit 79f6caf.

What do you need to do:

  1. Don't panic.
  2. Check the buildbot page in the devguide if you don't know what the buildbots are or how they work.
  3. Go to the page of the buildbot that failed (https://buildbot.python.org/#/builders/506/builds/12404) and take a look at the build logs.
  4. Check if the failure is related to this commit (79f6caf) or if it is a false positive.
  5. If the failure is related to this commit, please, reflect that on the issue and make a new Pull Request with a fix.

You can take a look at the buildbot page here:

https://buildbot.python.org/#/builders/506/builds/12404

Failed tests:

  • test_pyrepl

Failed subtests:

  • test_repl_eio - test.test_pyrepl.test_unix_console.TestUnixConsoleEIOHandling.test_repl_eio

Summary of the results of the build (if available):

==

Click to see traceback logs 
Traceback (most recent call last):
  File "/srv/buildbot/buildarea/3.x.bolen-ubuntu/build/Lib/subprocess.py", line 1191, in __del__
    _warn("subprocess %s is still running" % self.pid,
    ~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
          ResourceWarning, source=self)
          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
ResourceWarning: subprocess 1078638 is still running
Warning -- Unraisable exception
Exception ignored while finalizing file <_io.FileIO name=10 mode='rb' closefd=True>:
Traceback (most recent call last):
  File "/srv/buildbot/buildarea/3.x.bolen-ubuntu/build/Lib/unittest/case.py", line 666, in run
    with outcome.testPartExecutor(self):
         ~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^
ResourceWarning: unclosed file <_io.TextIOWrapper name=10 encoding='utf-8'>
Warning -- Unraisable exception
Exception ignored while finalizing file <_io.FileIO name=8 mode='rb' closefd=True>:
Traceback (most recent call last):
  File "/srv/buildbot/buildarea/3.x.bolen-ubuntu/build/Lib/unittest/case.py", line 666, in run
    with outcome.testPartExecutor(self):
         ~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^
ResourceWarning: unclosed file <_io.TextIOWrapper name=8 encoding='utf-8'>


Traceback (most recent call last):
  File "/srv/buildbot/buildarea/3.x.bolen-ubuntu/build/Lib/subprocess.py", line 1191, in __del__
    _warn("subprocess %s is still running" % self.pid,
    ~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
          ResourceWarning, source=self)
          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
ResourceWarning: subprocess 1109411 is still running
Warning -- Unraisable exception
Exception ignored while finalizing file <_io.FileIO name=10 mode='rb' closefd=True>:
Traceback (most recent call last):
  File "/srv/buildbot/buildarea/3.x.bolen-ubuntu/build/Lib/unittest/case.py", line 666, in run
    with outcome.testPartExecutor(self):
         ~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^
ResourceWarning: unclosed file <_io.TextIOWrapper name=10 encoding='utf-8'>
Warning -- Unraisable exception
Exception ignored while finalizing file <_io.FileIO name=8 mode='rb' closefd=True>:
Traceback (most recent call last):
  File "/srv/buildbot/buildarea/3.x.bolen-ubuntu/build/Lib/unittest/case.py", line 666, in run
    with outcome.testPartExecutor(self):
         ~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^
ResourceWarning: unclosed file <_io.TextIOWrapper name=8 encoding='utf-8'>


Traceback (most recent call last):
  File "/srv/buildbot/buildarea/3.x.bolen-ubuntu/build/Lib/test/test_pyrepl/test_unix_console.py", line 390, in test_repl_eio
    _, err = proc.communicate(timeout=support.LONG_TIMEOUT)
             ~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/srv/buildbot/buildarea/3.x.bolen-ubuntu/build/Lib/subprocess.py", line 1273, in communicate
    stdout, stderr = self._communicate(input, endtime, timeout)
                     ~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/srv/buildbot/buildarea/3.x.bolen-ubuntu/build/Lib/subprocess.py", line 2293, in _communicate
    self._check_timeout(endtime, orig_timeout, stdout, stderr)
    ~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/srv/buildbot/buildarea/3.x.bolen-ubuntu/build/Lib/subprocess.py", line 1320, in _check_timeout
    raise TimeoutExpired(
    ...<2 lines>...
            stderr=b''.join(stderr_seq) if stderr_seq else None)
subprocess.TimeoutExpired: Command '['/srv/buildbot/buildarea/3.x.bolen-ubuntu/build/python', '-E', '-S', '/srv/buildbot/buildarea/3.x.bolen-ubuntu/build/Lib/test/test_pyrepl/eio_test_script.py']' timed out after 300.0 seconds

brijkapadia pushed a commit to brijkapadia/cpython that referenced this pull request Feb 28, 2026
@dg-pb @brijkapadia
pythongh-145131: difflib.Differ._fancy_replace performance (re-use ra…
60c0567 
…tio) (python#145133)

re-use calculated ratio
ljfp pushed a commit to ljfp/cpython that referenced this pull request Apr 25, 2026
@dg-pb @ljfp
pythongh-145131: difflib.Differ._fancy_replace performance (re-use ra…
b5ee497 
…tio) (python#145133)

re-use calculated ratio
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@dg-pb @bedevere-bot @tim-one