difflib sequence matcher missing common substrings

Question

In an attempt to find common substrings between two strings, SequenceMatcher does not return all expected common substrings.

s1 = '++%2F%2F+Prints+%22Hello%2C+World%22+to+the+terminal+window.%0A++++++++System.out.pr%29%3B%0A++++%7D%0A%7D%0ASample+program%0Apublic+static+voclass+id+main%28String%5B%5D+args%29+'
s2 = 'gs%29+%7B%0A++++++++%2F'
# The common substring are '+%', '%0A++++++++', '%s' and 'gs%29+'
# but 'gs%29+' is not matched.

import difflib as d

seqmatch = d.SequenceMatcher(None,s1,s2)
matches = seqmatch.get_matching_blocks()

for match in matches:
    apos, bpos, matchlen = match
    print(s1[apos:apos+matchlen])

Output:

+%
%0A++++++++
%2

"gs%29+" is a common substring between s1 and s2, but it is not found by SequenceMatcher.

Am I missing something?

Thanks

Your question is not clear. Please revise your sentence. What is the expected output? — yoonghm
"gs%29+" is a common substring between s1 and s2, it is not in the list of matches that SequenceMatcher produces — rroutsong
I believe it is a bug for difflib.get_matching_blocks(): it does not re-look the passed s2characters once it has found a match. It found s2[5:7], s2[9:20], s2[20:22], so it will not go back to find s2[0:5] — yoonghm

yoonghm yoonghm · Accepted Answer · 2018-10-06T00:14:34

Perhaps the junk characters have confused the algorithm. I added a lambda function for isjunk within SequenceMatcher()

s1 = '++%2F%2F+Prints+%22Hello%2C+World%22+to+the+terminal+window.%0A++++++++System.out.pr%29%3B%0A++++%7D%0A%7D%0ASample+program%0Apublic+static+voclass+id+main%28String%5B%5D+args%29+'
s2 = 'gs%29+%7B%0A++++++++%2F'
# The expected substring is 'gs%29+'

import difflib as d

seqmatch = d.SequenceMatcher(lambda x: x in "+", s1, s2)
matches = seqmatch.get_matching_blocks()

for match in matches:
    apos, bpos, matchlen = match
    print(s1[apos:apos+matchlen])

The output is now

gs%29+

difflib sequence matcher missing common substrings

1 Answers