With python built at HEAD (c51d8c9b) and at 3.7b3 (fcd4e03e08) the code:

    import difflib
    for fromdata, todata, flag in difflib._mdiff(
        ["2"], ["3"], 1):
      pass

produces:

    Traceback (most recent call last):
      File "/home/jefftk/cpython/Lib/difflib.py", line 1638, in _mdiff
        from_line, to_line, found_diff = next(line_pair_iterator)
    StopIteration

    The above exception was the direct cause of the following exception:

    Traceback (most recent call last):
      File "/home/jefftk/icdiff/repro.py", line 3, in <module>
        ["2"], ["3"], 1):
    RuntimeError: generator raised StopIteration

In python 3.5 and 3.6 I don't get an error.

This is probably due to https://bugs.python.org/issue32670 which implements PEP 479, but I this this isn't supposed to happen in library code?

New changeset 01b731fc2b04744a11e32f93aba8bfb9ddb3dd29 by Raymond Hettinger in branch 'master':
bpo-33224: PEP 479 fix for difflib.mdiff() (GH-6381)
https://github.com/python/cpython/commit/01b731fc2b04744a11e32f93aba8bfb9ddb3dd29

New changeset 28c179094bcb7829d184fb3cfb3ef626505b9f77 by Raymond Hettinger (Miss Islington (bot)) in branch '3.7':
bpo-33224: PEP 479 fix for difflib.mdiff() (GH-6381) (GH-6390)
https://github.com/python/cpython/commit/28c179094bcb7829d184fb3cfb3ef626505b9f77

New changeset 8da15f09458fd4f4fe341861e41723892b25a11b by Raymond Hettinger (Miss Islington (bot)) in branch '3.6':
bpo-33224: PEP 479 fix for difflib.mdiff() (GH-6381) (GH-6391)
https://github.com/python/cpython/commit/8da15f09458fd4f4fe341861e41723892b25a11b

Thanks for timely report.  I appreciate you exercising the beta release to shake out bugs.