gh-113149: Improve error message when JSON has trailing comma (GH-113227)

1 files changed, 7 insertions, 0 deletions

diff --git a/Lib/json/decoder.py b/Lib/json/decoder.py
index c5d9ae2..d69a45d 100644
--- a/Lib/json/decoder.py
+++ b/Lib/json/decoder.py
@@ -200,10 +200,13 @@ def JSONObject(s_and_end, strict, scan_once, object_hook, object_pairs_hook,
             break
         elif nextchar != ',':
             raise JSONDecodeError("Expecting ',' delimiter", s, end - 1)
+        comma_idx = end - 1
         end = _w(s, end).end()
         nextchar = s[end:end + 1]
         end += 1
         if nextchar != '"':
+            if nextchar == '}':
+                raise JSONDecodeError("Illegal trailing comma before end of object", s, comma_idx)
             raise JSONDecodeError(
                 "Expecting property name enclosed in double quotes", s, end - 1)
     if object_pairs_hook is not None:
@@ -240,13 +243,17 @@ def JSONArray(s_and_end, scan_once, _w=WHITESPACE.match, _ws=WHITESPACE_STR):
             break
         elif nextchar != ',':
             raise JSONDecodeError("Expecting ',' delimiter", s, end - 1)
+        comma_idx = end - 1
         try:
             if s[end] in _ws:
                 end += 1
                 if s[end] in _ws:
                     end = _w(s, end + 1).end()
+            nextchar = s[end:end + 1]
         except IndexError:
             pass
+        if nextchar == ']':
+            raise JSONDecodeError("Illegal trailing comma before end of array", s, comma_idx)
 
     return values, end