[New-bugs-announce] [issue40691] misleading output from difflib unified_diff

[Chris Danis]

New submission from Chris Danis <cdanis at wikimedia.org>:

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.)

----------
components: Library (Lib)
files: a.txt
messages: 369390
nosy: cdanis
priority: normal
severity: normal
status: open
title: misleading output from difflib unified_diff
versions: Python 3.7
Added file: https://bugs.python.org/file49171/a.txt

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue40691>
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