[issue35079] difflib.SequenceMatcher.get_matching_blocks omits common strings

[Springem Springsbee]

New submission from Springem Springsbee <mrals89 at gmail.com>:

Hello, I'm using difflib's SequenceMatcher to locate common substrings. It seems like the matcher is missing a common substrings. I'm guessing this is a rather low-level issue in difflib. The autojunk parameter has no effect for obvious reasons. Alternate pairwise comparisons between the following 3 strings omit the 2-character match 'AC'

GATTACA
TAGACCA
ATACA

The following Github gist captures the issue, which I'll repeat here for redundancy https://gist.github.com/MatthewRalston/b0ab6ac1dbe322cb12063310ccdbb786

>import difflib
>string1 = "TAGACCA"
>string2 = "ATACA"
>s = difflib.SequenceMatcher(None, string1, string2)
>blox = s.get_matching_blocks()
>print(blox)
[Match(a=0, b=1, size=2), Match(a=5, b=3, size=2), Match(a=7, b=5, size=0)] # Missing Match(a=3, b=2, size=2)
>print([string1[x.a:x.a+x.size] for x in blox if x.size > 1])
['TA', 'CA'] # Missing the substring 'CA'

----------
components: Library (Lib)
messages: 328593
nosy: Springem Springsbee, terry.reedy
priority: normal
severity: normal
status: open
title: difflib.SequenceMatcher.get_matching_blocks omits common strings
type: behavior
versions: Python 2.7, Python 3.4, Python 3.5, Python 3.6, Python 3.7

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue35079>
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