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[WIP] gh-132166: Add check in find_longest_match for checking if sequences are identical #132167

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[WIP] gh-132166: Add check in find_longest_match for checking if sequences are identical #132167

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fc53f9e
initial commit
jurgenwigg Apr 6, 2025
6fce1a0
📜🤖 Added by blurb_it.
blurb-it[bot] Apr 6, 2025
132e938
fixed news lint fail
jurgenwigg Apr 6, 2025
7a2f27b
Update difflib.py
jurgenwigg Apr 6, 2025
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6 changes: 6 additions & 0 deletions Lib/difflib.py
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Original file line number Diff line number Diff line change
Expand Up @@ -365,6 +365,12 @@ def find_longest_match(self, alo=0, ahi=None, blo=0, bhi=None):
ahi = len(a)
if bhi is None:
bhi = len(b)

# Check if both sequences are the same before executing rest of the
# method.
if a[alo:ahi] == b[blo:bhi]:
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Is this covered in tests? if not, please add a test, otherwise no need to add a large one.

return Match(alo, blo, len(a[alo:ahi]))

besti, bestj, bestsize = alo, blo, 0
# find longest junk-free match
# during an iteration of the loop, j2len[j] = length of longest
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5 changes: 5 additions & 0 deletions Misc/NEWS.d/next/Core_and_Builtins/2025-04-06-17-31-44.gh-issue-132166.tbo9yR.rst
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@@ -0,0 +1,5 @@
Add checking if sequences ``a[alo:ahi]`` and ``b[blo:bhi]`` are the same on the
beginning of the method find_longest_match in SequenceMatcher. For identical
sequences there is no reason to run whole logic when simple check can be done.
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This NEWS should be summarized. I suggest something like (assumiong that the method is documented, if not use a plain

Add a fast path in :meth:`SequenceMatcher.find_longest_match
<difflib.SequenceMatcher.find_longest_match>` to handle the
case when ``a[alo:ahi] == b[blo:bhi]``.

It appears to fix issue when comparing two slightly different strings ends up
with waiting forever for the result.
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Suggested change
It appears to fix issue when comparing two slightly different strings ends up
with waiting forever for the result.
It appears to fix issue when comparing two slightly different strings ends up
with waiting forever for the result.

We should be more confident here. Does it fix the issue or not? if so, let's say it. Otherwise, let's not give tentative conclusions.

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