New submission from Marien Zwart <m_zw...@123mail.org>: 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()) ---------- components: Library (Lib) messages: 117612 nosy: marienz priority: normal severity: normal status: open title: difflib.SequenceMatcher has slightly buggy and undocumented caching behavior type: behavior versions: Python 2.6, Python 2.7, Python 3.1, Python 3.2 _______________________________________ Python tracker <rep...@bugs.python.org> <http://bugs.python.org/issue9985> _______________________________________ _______________________________________________ Python-bugs-list mailing list Unsubscribe: http://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com
