Description

assignee = None
closed_at = None
created_at = <Date 2016-08-08.15:40:57.110>
labels = ['type-bug', 'library']
title = 'difflib.HtmlDiff produces different output from difflib.ndiff'
updated_at = <Date 2018-09-22.18:00:18.520>
user = 'https://bugs.python.org/SilentGhost'

activity = <Date 2018-09-22.18:00:18.520>
actor = 'xtreak'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Library (Lib)']
creation = <Date 2016-08-08.15:40:57.110>
creator = 'SilentGhost'
dependencies = []
files = []
hgrepos = []
issue_num = 27709
keywords = ['patch']
message_count = 2.0
messages = ['272173', '272196']
nosy_count = 6.0
nosy_names = ['loewis', 'terry.reedy', 'eli.bendersky', 'SilentGhost', 'jlwing', 'xtreak']
pr_nums = []
priority = 'normal'
resolution = None
stage = 'needs patch'
status = 'open'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue27709'
versions = ['Python 3.5', 'Python 3.6']

Activity

1. SilentGhost commented on Aug 8, 2016

   

   SilentGhostmannequin

   on Aug 8, 2016

   MannequinAuthor

   More actions

   msg264842 in bpo-26945 reports an odd results of HtmlDiff.make_file, digging into it I've noticed couple of things: different output to one generated directly from difflib.ndiff and underlying issue further up the stack that generates same faulty output with a slightly modified files.

   The patch I'm attaching fixes the issue with different outputs, from make_file and ndiff, by not removing EOL characters, I had to re-generate expected test output file.
2. SilentGhost

   added

   stdlibStandard Library Python modules in the Lib/ directoryStandard Library Python modules in the Lib/ directory

   type-bugAn unexpected behavior, bug, or errorAn unexpected behavior, bug, or error

   on Aug 8, 2016
3. SilentGhost commented on Aug 8, 2016

   

   SilentGhostmannequin

   on Aug 8, 2016

   MannequinAuthor

   More actions

   The degenerate behaviour appears ultimately due to the autojunk heuristic. Given that autojunk defaults to True and there isn't any way to change via most user-facing functions, I don't think there is an easy resolution that can be found here. I'm going to withdraw the earlier patch, since it feels like a half-measure which enable correct behaviour by accident.
4. ezio-melotti

   transferred this issue fromon Apr 10, 2022
5. Lenormju commented on Jul 18, 2026

   

   Lenormju

   on Jul 18, 2026

   Contributor

   More actions

   As described in the #118150 issue and handled there, yes the autojunk behavior is surprising at times.
   But there is also another issue here (reproduced using 3.13, 3.14 and 3.15):

   import difflib
   
   A = ["same\n", "diffme\n"]
   B = ["same", "diffme\n"]   # line 0 differs only by trailing newline
   
   print(difflib.SequenceMatcher(None, A, B).get_opcodes())  # [('replace', 0, 1, 0, 1), ('equal', 1, 2, 1, 2)]
   print(list(difflib.ndiff(A, B)))  # ['- same\n', '?     -\n', '+ same', '  diffme\n']
   print(list(difflib.unified_diff(A, B, lineterm='')))  # [..., '-same\n', '+same', ' diffme\n']
   print(list(difflib.context_diff(A, B, lineterm='')))  # [..., '! same\n', ..., '! same', '  diffme\n']
   print(difflib.HtmlDiff().make_file(A, B))  # no line is marked as changed/added/removed (classes diff_add/diff_chg/diff_sub)
   
   A_stripped = [elem.rstrip() for elem in A]
   B_stripped = [elem.rstrip() for elem in B]
   
   print(difflib.SequenceMatcher(None, A_stripped, B_stripped).get_opcodes())  # [('equal', 0, 2, 0, 2)]
   print(list(difflib.ndiff(A_stripped, B_stripped)))  # ['  same', '  diffme']
   print(list(difflib.unified_diff(A_stripped, B_stripped, lineterm='')))  # []
   print(list(difflib.context_diff(A_stripped, B_stripped, lineterm='')))  # []
   print(difflib.HtmlDiff().make_file(A_stripped, B_stripped))  # still no line is marked as changed/added/removed

   The HtmlDiff is the only one that does end-of-line stripping from the whole difflib. To me, this is a bug because it hides actual differences.
   It is caused by the _tab_newline_replace method being called before _mdiff.
6. Lenormju commented on Jul 18, 2026

   

   Lenormju

   on Jul 18, 2026

   Contributor

   More actions

   Petr Viktorin just told me we should NOT fix the issue (which dates back tot he original implementation in 2004, 22 years ago, cf e064b41). Instead, I will add a warning about it, and an automated test to prevent unexpected regression (if someone would fix the bug).
7. ZeroIntensity

   added a commit that references this issue on Jul 18, 2026

   gh-71896: Add a docs warning about trailing newlines ignored by `diff…

   755d971
8. ZeroIntensity

   added

   sprint

   on Jul 18, 2026
9. github-project-automation

   added this to Sprinton Jul 18, 2026
10. github-project-automation

    moved this to Todo in Sprinton Jul 18, 2026
11. ZeroIntensity

    added 3 commits that reference this issue on Jul 18, 2026

    [3.15] gh-71896: Add a docs warning about trailing newlines ignored b…

    b6a140b

    [3.13] gh-71896: Add a docs warning about trailing newlines ignored b…

    75b6789

    [3.14] gh-71896: Add a docs warning about trailing newlines ignored b…

    1f3814f
12. encukou

    closed this as completedon Aug 10, 2026
13. github-project-automation

    moved this from Todo to Done in Sprinton Aug 10, 2026

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