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gh-146582: Improve syntax error for non-case statement inside match statement #146583

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gh-146582: Improve syntax error for non-case statement inside match statement #146583

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7873c8c
Improve syntax error for non-`case` statement inside `match` statement
brianschubert Mar 28, 2026
b640ab6
Address review comments
brianschubert Mar 28, 2026
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1 change: 1 addition & 0 deletions Grammar/python.gram
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Original file line number Diff line number Diff line change
Expand Up @@ -1503,6 +1503,7 @@ invalid_case_block:
| "case" patterns guard? NEWLINE { RAISE_SYNTAX_ERROR("expected ':'") }
| a="case" patterns guard? ':' NEWLINE !INDENT {
RAISE_INDENTATION_ERROR("expected an indented block after 'case' statement on line %d", a->lineno) }
| !"case" { RAISE_SYNTAX_ERROR_ON_NEXT_TOKEN("expected 'case' statement inside 'match' statement") }
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@pablogsal pablogsal Mar 28, 2026 •
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Unless I'm missing something, is this still incomplete for invalid statements that start with soft-keyword case?

For example with

compile("match 1:\n    case = 1\n", "<repro>", "exec")
compile("match 1:\n    case += 1\n", "<repro>", "exec")
compile("match 1:\n    case, x = 1, 2\n", "<repro>", "exec")

you get:

SyntaxError: invalid syntax @ line 2, col 10
SyntaxError: invalid syntax @ line 2, col 10
SyntaxError: invalid syntax @ line 2, col 9

So the new diagnostic still doesn't seem to catch these case-starting non-case statements inside a match body. Should those be covered too?

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@brianschubert brianschubert Mar 28, 2026

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Should those be covered too?

Hmm, possibly? It occurred that me that this wouldn't catch cases like this, but I figured that this would be tricky to do right and only represents a vanishingly small set of statements in practice, so I didn't give it much thought.

Do you think it's worth trying to catch these too? I'd worry that a rule that matches these cases would also run the risk of matching an actual malformed case statement, which could make for a rather confusing error for the user

invalid_as_pattern:
| or_pattern 'as' a="_" { RAISE_SYNTAX_ERROR_KNOWN_LOCATION(a, "cannot use '_' as a target") }
| or_pattern 'as' a=expression {
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34 changes: 33 additions & 1 deletion Lib/test/test_syntax.py
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Original file line number Diff line number Diff line change
Expand Up @@ -356,7 +356,7 @@
>>> match x:
... y = 3
Traceback (most recent call last):
SyntaxError: invalid syntax
SyntaxError: expected 'case' statement inside 'match' statement

>>> match x:
... case y:
Expand Down Expand Up @@ -3466,6 +3466,38 @@ def test_match_stmt_invalid_as_expr(self):
end_offset=15 + len("obj.attr"),
)

def test_match_stmt_contains_invalid_stmt(self):
for stmt in [
"a",
"a = 1",
"if a:",
"else:",
"match 1:"
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"pass",
"return",
"return 2",
"raise Exception('a')",
"del a",
"yield 2",
"assert False",
"break",
"continue",
"import",
"import ast",
"from",
"from ast import *"
]:
self._check_error(
textwrap.dedent(
f"""
match 1:
{stmt}
"""
),
errtext="expected 'case' statement inside 'match' statement",
lineno=3,
)

def test_ifexp_else_stmt(self):
msg = "expected expression after 'else', but statement is given"

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3 changes: 3 additions & 0 deletions Misc/NEWS.d/next/Core_and_Builtins/2026-03-28-14-09-16.gh-issue-146582.wlpY3D.rst
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Original file line number Diff line number Diff line change
@@ -0,0 +1,3 @@
Improve :exc:`SyntaxError` message when a statement other than a
:keyword:`case` statement appears inside a :keyword:`match` statement.
Patch by Brian Schubert.
24 changes: 24 additions & 0 deletions Parser/parser.c
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