Bug report

Bug description:

#!/usr/bin/python3

import difflib

def get_lines(filename):
    with open(filename, 'r', encoding='utf8') as fd:
        return fd.readlines()

old_new = list(difflib.unified_diff(
    get_lines('external.old'),
    get_lines('external.new'),
))
new_old = list(difflib.unified_diff(
    get_lines('external.new'),
    get_lines('external.old'),
))
print('diff -u external.old external.new.json | wc -l')
print(len(old_new))
print('diff -u external.new external.old.json | wc -l')
print(len(new_old))

$ ./difftest.py
diff -u external.old.json external.new.json | wc -l
30854
diff -u external.new.json external.old.json | wc -l
26

$ diff -u external.old.json external.new.json | wc -l
26

$ diff -u external.new.json external.old.json | wc -l
26

Running on RHEL9 with Python 3.9.18.

I have a JSON file that I'm diffing and I tend to get very large diffs when lines are removed from the file. When lines are added the produced diffs are simple. The JSON file is a list of DNS records.

external.old.json
external.new.json

Here are smaller files that reproduce the effect. In trying to create the smaller files I noticed that if I go too small then the diffs work fine. If you remove the "abcluster.eas.gatech.edu" from both files the diffs work. I could not get the file smaller.

small.external.old.json
small.external.new.json

CPython versions tested on:

3.9

Operating systems tested on:

Linux

Linked PRs

• gh-118150: warn in doc about results asymmetry for difflib junk #153892
• gh-118150: expose autojunk SequenceMathcher flag to public methods/functions of difflib #153947
• gh-118150: difflib: expose autojunk flag from SequenceMatcher to public methods and functions #153959
• [3.15] gh-118150: warn in doc about results asymmetry for difflib junk (GH-153892) #154658
• [3.14] gh-118150: warn in doc about results asymmetry for difflib junk (GH-153892) #154659
• [3.13] gh-118150: warn in doc about results asymmetry for difflib junk (GH-153892) #154660