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gh-145239: Specialize match syntax error for unary addition in pattern #145658

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gh-145239: Specialize match syntax error for unary addition in pattern #145658

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3b7072e
gh-145239: Specialize match syntax error for unary addition in pattern
mvanhorn Mar 9, 2026
b07a507
Remove test_patma changes per review feedback
mvanhorn Mar 9, 2026
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3 changes: 3 additions & 0 deletions Grammar/python.gram
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Original file line number Diff line number Diff line change
Expand Up @@ -536,6 +536,7 @@ literal_pattern[pattern_ty]:
| 'None' { _PyAST_MatchSingleton(Py_None, EXTRA) }
| 'True' { _PyAST_MatchSingleton(Py_True, EXTRA) }
| 'False' { _PyAST_MatchSingleton(Py_False, EXTRA) }
| invalid_literal_pattern

# Literal expressions are used to restrict permitted mapping pattern keys
literal_expr[expr_ty]:
Expand Down Expand Up @@ -1519,6 +1520,8 @@ invalid_mapping_pattern:
"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_literal_pattern:
| a='+' NUMBER { RAISE_SYNTAX_ERROR_KNOWN_LOCATION(a, "cannot use unary '+' in a literal pattern") }
invalid_if_stmt:
| 'if' named_expression NEWLINE { RAISE_SYNTAX_ERROR("expected ':'") }
| a='if' a=named_expression ':' NEWLINE !INDENT {
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32 changes: 32 additions & 0 deletions Lib/test/test_syntax.py
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Expand Up @@ -2399,6 +2399,38 @@
Traceback (most recent call last):
SyntaxError: double star pattern must be the last (right-most) subpattern in the mapping pattern

Unary '+' is not allowed in match patterns:

>>> match ...:
... case +1:
... ...
Traceback (most recent call last):
SyntaxError: cannot use unary '+' in a literal pattern

>>> match ...:
... case 1 | +2 | -3:
... ...
Traceback (most recent call last):
SyntaxError: cannot use unary '+' in a literal pattern

>>> match ...:
... case [1, +2, -3]:
... ...
Traceback (most recent call last):
SyntaxError: cannot use unary '+' in a literal pattern

>>> match ...:
... case Foo(x=+1, y=-2):
... ...
Traceback (most recent call last):
SyntaxError: cannot use unary '+' in a literal pattern

>>> match ...:
... case {True: +1, False: -2}:
... ...
Traceback (most recent call last):
SyntaxError: cannot use unary '+' in a literal pattern

Uses of the star operator which should fail:

A[:*b]
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3 changes: 3 additions & 0 deletions .../NEWS.d/next/Core_and_Builtins/2026-03-08-00-00-00.gh-issue-145239.UnaryAdd.rst
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Original file line number Diff line number Diff line change
@@ -0,0 +1,3 @@
Improved the error message when using unary ``+`` in a :keyword:`match`
pattern. Instead of a generic "invalid syntax", Python now reports "cannot
use unary '+' in a literal pattern".
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