Find common substring between two strings

For completeness, difflib in the standard-library provides loads of sequence-comparison utilities. For instance find_longest_match which finds the longest common substring when used on strings. Example use:

from difflib import SequenceMatcher

string1 = "apple pie available"
string2 = "come have some apple pies"

match = SequenceMatcher(None, string1, string2).find_longest_match()

print(match)  # -> Match(a=0, b=15, size=9)
print(string1[match.a:match.a + match.size])  # -> apple pie
print(string2[match.b:match.b + match.size])  # -> apple pie

If you're using a version older than 3.9, you'need to call find_longest_match() with the following arguments:

SequenceMatcher(None, string1, string2).find_longest_match(0, len(string1), 0, len(string2))

One might also consider os.path.commonprefix that works on characters and thus can be used for any strings.

import os
common = os.path.commonprefix(['apple pie available', 'apple pies'])
assert common == 'apple pie'

As the function name indicates, this only considers the common prefix of two strings.