C implementation of parts of difflib

difflib, c, diff
pip install cdifflib==1.2.5



Python difflib sequence matcher reimplemented in C.

Actually only contains reimplemented parts. Creates a CSequenceMatcher type which inherets most functions from difflib.SequenceMatcher.

cdifflib is about 4x the speed of the pure python difflib when diffing large streams.


The C part of the code can only work on list rather than generic iterables, so anything that isn't a list will be converted to list in the CSequenceMatcher constructor. This may cause undesirable behavior if you're not expecting it.

Works with Python 2.7 and 3.6 (Should work on all 3.3+)


Can be used just like the difflib.SequenceMatcher as long as you pass lists. These examples are right out of the difflib docs:

>>> from cdifflib import CSequenceMatcher
>>> s = CSequenceMatcher(None, ' abcd', 'abcd abcd')
>>> s.find_longest_match(0, 5, 0, 9)
Match(a=1, b=0, size=4)
>>> s = CSequenceMatcher(lambda x: x == " ",
...                      "private Thread currentThread;",
...                      "private volatile Thread currentThread;")
>>> print round(s.ratio(), 3)

It's completely compatible, so you can replace the difflib version on startup and then other libraries will use CSequenceMatcher too, eg:

from cdifflib import CSequenceMatcher
import difflib
difflib.SequenceMatcher = CSequenceMatcher
import library_that_uses_difflib

# Now the library will transparantely be using the C SequenceMatcher - other
# things remain the same


To install:

python setup.py install

To test:

python setup.py test

License etc

This code lives at https://github.com/mduggan. See LICENSE for the license.


  • 1.2.6 - Clear state correctly when replacing seq1 (#10)
  • 1.2.5 - Fix some memory leaks (#7)
  • 1.2.4 - Repackage yet again using twine for pypi upload (no binary changes)
  • 1.2.3 - Repackage again with changelog update and corrected src package (no binary changes)
  • 1.2.2 - Repackage to add README.md in a way pypi supports (no binary changes)
  • 1.2.1 - Fix bug for longer sequences with "autojunk"
  • 1.2.0 - Python 3 support for other versions
  • 1.1.0 - Added Python 3.6 support (thanks Bclavie)
  • 1.0.4 - Changes to make it compile on MSVC++ compiler, no change for other platforms
  • 1.0.2 - Bugfix - also replace set_seq1 implementation so difflib.compare works with a CSequenceMatcher
  • 1.0.1 - Implement more bits in c to squeeze a bit more speed out
  • 1.0.0 - First release