"RuntimeError: generator raised StopIteration" in difflib.mdiff

[JeffKaufman]

BPO	33224
Nosy	@rhettinger
PRs	bpo-33224: PEP 479 fix for difflib.mdiff() #6381 [3.7] bpo-33224: PEP 479 fix for difflib.mdiff() (GH-6381) #6390 [3.6] bpo-33224: PEP 479 fix for difflib.mdiff() (GH-6381) #6391

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = 'https://github.com/rhettinger'
closed_at = <Date 2018-04-05.19:23:50.574>
created_at = <Date 2018-04-04.15:18:24.095>
labels = ['3.7', '3.8', 'type-bug', 'library']
title = '"RuntimeError: generator raised StopIteration" in difflib.mdiff'
updated_at = <Date 2018-04-05.19:23:50.573>
user = 'https://bugs.python.org/JeffKaufman'

bugs.python.org fields:

activity = <Date 2018-04-05.19:23:50.573>
actor = 'rhettinger'
assignee = 'rhettinger'
closed = True
closed_date = <Date 2018-04-05.19:23:50.574>
closer = 'rhettinger'
components = ['Library (Lib)']
creation = <Date 2018-04-04.15:18:24.095>
creator = 'Jeff.Kaufman'
dependencies = []
files = []
hgrepos = []
issue_num = 33224
keywords = ['patch']
message_count = 5.0
messages = ['314936', '314999', '315001', '315003', '315004']
nosy_count = 2.0
nosy_names = ['rhettinger', 'Jeff.Kaufman']
pr_nums = ['6381', '6390', '6391']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue33224'
versions = ['Python 3.6', 'Python 3.7', 'Python 3.8']

[JeffKaufman]

With python built at HEAD (c51d8c9) and at 3.7b3 (fcd4e03) 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?

[rhettinger]

New changeset 01b731f by Raymond Hettinger in branch 'master':
bpo-33224: PEP-479 fix for difflib.mdiff() (GH-6381)
01b731f

[rhettinger]

New changeset 28c1790 by Raymond Hettinger (Miss Islington (bot)) in branch '3.7':
bpo-33224: PEP-479 fix for difflib.mdiff() (GH-6381) (GH-6390)
28c1790

[rhettinger]

New changeset 8da15f0 by Raymond Hettinger (Miss Islington (bot)) in branch '3.6':
bpo-33224: PEP-479 fix for difflib.mdiff() (GH-6381) (GH-6391)
8da15f0

[rhettinger]

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