gh-140253: Improve the syntax error from an ill-positioned double-star subpattern (#140254)

Author: bswck

Grammar/python.gram

@@ -626,6 +626,7 @@ mapping_pattern[pattern_ty]:
             CHECK(asdl_pattern_seq*, _PyPegen_get_patterns(p, items)),
             NULL,
             EXTRA) }
+    | invalid_mapping_pattern
 
 items_pattern[asdl_seq*]:
     | ','.key_value_pattern+
@@ -1490,6 +1491,10 @@ invalid_class_pattern:
         PyPegen_first_item(a, pattern_ty),
         PyPegen_last_item(a, pattern_ty),
         "positional patterns follow keyword patterns") }
+invalid_mapping_pattern:
+    | '{' (items_pattern ',')? rest=double_star_pattern ',' items_pattern ','? '}' { RAISE_SYNTAX_ERROR_KNOWN_LOCATION(
+        rest,
+        "double star pattern must be the last (right-most) subpattern in the mapping pattern") }
 invalid_class_argument_pattern[asdl_pattern_seq*]:
     | [positional_patterns ','] keyword_patterns ',' a=positional_patterns { a }
 invalid_if_stmt:

Lib/test/test_exceptions.py

@@ -252,7 +252,16 @@ def testSyntaxErrorOffset(self):
         check('[\nfile\nfor str(file)\nin\n[]\n]', 3, 5)
         check('[file for\n str(file) in []]', 2, 2)
         check("ages = {'Alice'=22, 'Bob'=23}", 1, 9)
-        check('match ...:\n    case {**rest, "key": value}:\n        ...', 2, 19)
+        check(dedent("""\
+          match ...:
+            case {**rest1, "after": after}:
+              ...
+        """), 2, 11)
+        check(dedent("""\
+          match ...:
+            case {"before": before, **rest2, "after": after}:
+              ...
+        """), 2, 29)
         check("[a b c d e f]", 1, 2)
         check("for x yfff:", 1, 7)
         check("f(a for a in b, c)", 1, 3, 1, 15)

Lib/test/test_syntax.py

@@ -370,12 +370,6 @@
 Traceback (most recent call last):
 SyntaxError: invalid syntax
 
->>> match ...:
-...     case {**rest, "key": value}:
-...        ...
-Traceback (most recent call last):
-SyntaxError: invalid syntax
-
 >>> match ...:
 ...     case {**_}:
 ...        ...
@@ -2240,7 +2234,7 @@
     Traceback (most recent call last):
     SyntaxError: invalid character '£' (U+00A3)
 
-  Invalid pattern matching constructs:
+Invalid pattern matching constructs:
 
     >>> match ...:
     ...   case 42 as _:
@@ -2302,6 +2296,24 @@
     Traceback (most recent call last):
     SyntaxError: positional patterns follow keyword patterns
 
+    >>> match ...:
+    ...   case {**double_star, "spam": "eggs"}:
+    ...     ...
+    Traceback (most recent call last):
+    SyntaxError: double star pattern must be the last (right-most) subpattern in the mapping pattern
+
+    >>> match ...:
+    ...   case {"foo": 1, **double_star, "spam": "eggs"}:
+    ...     ...
+    Traceback (most recent call last):
+    SyntaxError: double star pattern must be the last (right-most) subpattern in the mapping pattern
+
+    >>> match ...:
+    ...   case {"spam": "eggs", "b": {**d, "ham": "bacon"}}:
+    ...     ...
+    Traceback (most recent call last):
+    SyntaxError: double star pattern must be the last (right-most) subpattern in the mapping pattern
+
 Uses of the star operator which should fail:
 
 A[:*b]

Misc/NEWS.d/next/Core_and_Builtins/2025-10-17-14-38-10.gh-issue-140253.gCqFaL.rst

@@ -0,0 +1,2 @@
+Wrong placement of a double-star pattern inside a mapping pattern now throws a specialized syntax error.
+Contributed by Bartosz Sławecki in :gh:`140253`.