Description

assignee = None
closed_at = None
created_at = <Date 2020-05-19.20:57:46.596>
labels = ['3.7', 'library']
title = 'misleading output from difflib unified_diff'
updated_at = <Date 2020-05-21.08:34:07.226>
user = 'https://github.com/cdanis'

activity = <Date 2020-05-21.08:34:07.226>
actor = 'DahlitzFlorian'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Library (Lib)']
creation = <Date 2020-05-19.20:57:46.596>
creator = 'cdanis'
dependencies = []
files = ['49171', '49172']
hgrepos = []
issue_num = 40691
keywords = []
message_count = 1.0
messages = ['369390']
nosy_count = 2.0
nosy_names = ['DahlitzFlorian', 'cdanis']
pr_nums = []
priority = 'normal'
resolution = None
stage = None
status = 'open'
superseder = None
type = None
url = 'https://bugs.python.org/issue40691'
versions = ['Python 3.7']

Activity

1. cdanis commented on May 19, 2020

   

   cdanismannequin

   on May 19, 2020

   MannequinAuthor

   More actions

   When difflib.unified_diff is asked to produce a diff of inputs a.txt and b.txt (attached), with 10 context lines, it produces the following:

   --- codfw/groupLoadsBySection live
   +++ codfw/groupLoadsBySection generated
   @@ -90,37 +90,31 @@
        },
        "s4": {
            "api": {
                "db2106": 300,
                "db2110": 100
            },
            "contributions": {
                "db2084:3314": 100,
                "db2091:3314": 100
            },
   -        "dump": {
   -            "db2073": 100
   -        },
   -        "logpager": {
   -            "db2084:3314": 100,
   -            "db2091:3314": 100
   -        },
   -        "recentchanges": {
   -            "db2084:3314": 100,
   -            "db2091:3314": 100
   -        },
   -        "recentchangeslinked": {
   -            "db2084:3314": 100,
   -            "db2091:3314": 100
   -        },
   -        "vslow": {
   -            "db2073": 100
   +        "logpager": {
   +            "db2084:3314": 100,
   +            "db2091:3314": 100
   +        },
   +        "recentchanges": {
   +            "db2084:3314": 100,
   +            "db2091:3314": 100
   +        },
   +        "recentchangeslinked": {
   +            "db2084:3314": 100,
   +            "db2091:3314": 100
            },
            "watchlist": {
                "db2084:3314": 100,
                "db2091:3314": 100
            }
        },
        "s5": {
            "api": {
                "db2075": 300,
                "db2128": 100

   However, this could simply and much more readably be rendered as:

   @@ -90,38 +90,32 @@
      },
      "s4": {
        "api": {
          "db2106": 3,
          "db2110": 1
        },
        "contributions": {
          "db2084:3314": 1,
          "db2091:3314": 1
        },
   -    "dump": {
   -      "db2073": 1
   -    },
        "logpager": {
          "db2084:3314": 1,
          "db2091:3314": 1
        },
        "recentchanges": {
          "db2084:3314": 1,
          "db2091:3314": 1
        },
        "recentchangeslinked": {
          "db2084:3314": 1,
          "db2091:3314": 1
        },
   -    "vslow": {
   -      "db2073": 1
   -    },
        "watchlist": {
          "db2084:3314": 1,
          "db2091:3314": 1
        }
      },
      "s5": {
        "api": {
          "db2059": 1,
          "db2066": 1
        },

   (This is what diff -u10 produces.)
2. cdanis

   added

   3.7 (EOL)end of lifeend of life

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

   on May 19, 2020
3. ezio-melotti

   transferred this issue fromon Apr 10, 2022
4. WeiHao97 commented on Jun 2, 2022

   

   WeiHao97

   on Jun 2, 2022

   More actions

   Hi @ezio-melotti @DahlitzFlorian, @cdanis, I have encountered the same problem when using the function. Is this solved ? Thank you
5. ezio-melotti

   added

   3.11only security fixesonly security fixes

   3.10 (EOL)end of lifeend of life

   3.12only security fixesonly security fixes

   and removed

   3.7 (EOL)end of lifeend of life

   on Jun 2, 2022
6. noscript commented on Nov 16, 2022

   

   noscript

   on Nov 16, 2022

   More actions

   The fix is simple, disable autojunk:

   cpython/Lib/difflib.py

   Line 1138 in 0a208d5

   for group in SequenceMatcher(None,a,b).get_grouped_opcodes(n):

   -    for group in SequenceMatcher(None,a,b).get_grouped_opcodes(n):
   +    for group in SequenceMatcher(None,a,b,False).get_grouped_opcodes(n):
7. encukou commented on Sep 7, 2026

   

   encukou

   on Sep 7, 2026

   Member

   More actions

   In #118150, the docs were rewritten to mention autojunk and its trade-offs more prominently.
   I don't think there's more to do for this issue, short of adding a better algorithm and/or changing defaults (which would bring backwards compatibility issues).
8. encukou

   closed this as completedon Sep 7, 2026
9. alkorsan

   added a commit that references this issue on Sep 21, 2026

   force unified_diff to accept autojunk=False

   b49b33a

Issue actions