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

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Merged
merged 8 commits into from
Oct 22, 2025
+671 −514
Merged

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

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0368491
Add and use `invalid_mapping_pattern`
bswck Oct 17, 2025
b203743
Add syntax error message tests
bswck Oct 17, 2025
a4a49d7
Add syntax error offset tests
bswck Oct 17, 2025
bc0eb7d
Add news entry
bswck Oct 17, 2025
49876df
Merge branch 'main' into invalid-double-star-mapping
bswck Oct 17, 2025
78ad464
Require a comma in the preceding items sub-pattern
bswck Oct 17, 2025
6f3559e
Regenerate the parser
bswck Oct 17, 2025
2a91f3f
Add nested mapping test
bswck Oct 18, 2025
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5 changes: 5 additions & 0 deletions Grammar/python.gram
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Original file line number Diff line number Diff line change
Expand Up @@ -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+
Expand Down Expand Up @@ -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:
Expand Down
11 changes: 10 additions & 1 deletion Lib/test/test_exceptions.py
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Expand Up @@ -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)
Comment on lines +255 to +264
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I made it multi-line with dedent so it's easier to read--these tests were missing a case where some items were placed before the double-star pattern.

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@bswck bswck Oct 17, 2025 •
edited
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I'm using rest1 and rest2 names here so that they differ from what is in the grammar file and perhaps in other places. Just my intuition.

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)
Expand Down
26 changes: 19 additions & 7 deletions Lib/test/test_syntax.py
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Expand Up @@ -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 {**_}:
... ...
Expand Down Expand Up @@ -2240,7 +2234,7 @@
Traceback (most recent call last):
SyntaxError: invalid character '£' (U+00A3)

Invalid pattern matching constructs:
Invalid pattern matching constructs:
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@bswck bswck Oct 17, 2025 •
edited
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This was likely an accidental indentation, fixed it by the way.


>>> match ...:
... case 42 as _:
Expand Down Expand Up @@ -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]
Expand Down
2 changes: 2 additions & 0 deletions Misc/NEWS.d/next/Core_and_Builtins/2025-10-17-14-38-10.gh-issue-140253.gCqFaL.rst
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@@ -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`.
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