SequenceMatcher caches the result of get_matching_blocks and get_opcodes. There are some problems with this:

What get_matching_blocks caches is a list of tuples. The first call does not return that list: it returns map(Match._make, self.matching_blocks) (converting the tuples to namedtuples). Subsequent calls just return self.matching_blocks directly. Especially in python 3 and up this is weird, since the first call returns a map object while later calls return a list.

This caching behavior is not documented, so calling code may mutate the returned list. One example of calling code is difflib itself: get_grouped_opcodes mutates the result of get_opcodes (a cached list). I am not sure if the right fix is to have get_grouped_opcodes copy before it mutates or to have get_opcodes return a copy.

Snippet demonstrating both bugs:

matcher = difflib.SequenceMatcher(a='aaaaaaaabc', b='aaaaaaaadc')
print(list(matcher.get_matching_blocks()))
# This should print the same thing, but it does not:
print(list(matcher.get_matching_blocks()))

print(matcher.get_opcodes())
print(list(matcher.get_grouped_opcodes()))
# This should print the same thing as the previous get_opcodes()
# list, but it does not:
print(matcher.get_opcodes())

Here's a test case and a fix for get_matching_blocks() to return the same content on subsequent calls.

BTW, here's the commit that broke the behavior in the first place: http://svn.python.org/view/python/trunk/Lib/difflib.py?r1=54230&r2=59907

That fixes the first problem in python 2. It should do:

self.matching_blocks = [Match._make(t) for t in non_adjacent]

in python 3 though, or it will return an already-exhausted map object if it is called again.

This leaves the problem of other code mutating the cached list (not realizing it is cached). I think it would make sense for these functions to just return a shallow copy of their cached list, but I have not thought that through much.

(+nosy Raymond as committer of r59907 and removing python2.6 since this is not a security issue)

I believe this has been fixed by changesets f02a563ad1bf and ed73c127421c in #21635.

This was fixed for Python 2.7 and Python 3.4 and later.