difflib empty list IndexError fix (Bug #980117)

[roccomoretti]

BPO	980120
Nosy	@brettcannon
Files	difflib patch.txt: Patch for Bug 980117

^{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 = None
closed_at = <Date 2004-07-10.23:59:27.000>
created_at = <Date 2004-06-26.02:52:16.000>
labels = ['library']
title = 'difflib empty list IndexError fix (Bug python/issues-test-cpython#980117)'
updated_at = <Date 2004-07-10.23:59:27.000>
user = 'https://bugs.python.org/roccomoretti'

bugs.python.org fields:

activity = <Date 2004-07-10.23:59:27.000>
actor = 'brett.cannon'
assignee = 'none'
closed = True
closed_date = None
closer = None
components = ['Library (Lib)']
creation = <Date 2004-06-26.02:52:16.000>
creator = 'roccomoretti'
dependencies = []
files = ['6053']
hgrepos = []
issue_num = 980120
keywords = ['patch']
message_count = 2.0
messages = ['46241', '46242']
nosy_count = 2.0
nosy_names = ['brett.cannon', 'roccomoretti']
pr_nums = []
priority = 'normal'
resolution = 'fixed'
stage = None
status = 'closed'
superseder = None
type = None
url = 'https://bugs.python.org/issue980120'
versions = []

[roccomoretti]

This is a potential fix for the Bug bpo-980117.

The functions unified_diff() and context_diff() give an
IndexError when called with two empty lists due to an
unmasked index of the opcode lists in
SequenceMatcher.get_grouped_opcodes(). In the case of
two empty lists passed to the functions,
SequenceMatcher.codes is an empty list.

[brettcannon]

Logged In: YES
user_id=357491

Fixed in rev. 1.21 of Lib/difflib.py .