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Fix syntax error false positive on alternative match patterns #21362

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Merged
ntBre merged 1 commit into from
Nov 10, 2025
Merged

Fix syntax error false positive on alternative match patterns #21362

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2 changes: 2 additions & 0 deletions crates/ruff_python_parser/resources/inline/ok/nested_alternative_patterns.py
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Original file line number Diff line number Diff line change
Expand Up @@ -5,3 +5,5 @@
case [[x] | [x]] | x: ...
match 42:
case [[x | x] | [x]] | x: ...
match 42:
case ast.Subscript(n, ast.Constant() | ast.Slice()) | ast.Attribute(n): ...
4 changes: 3 additions & 1 deletion crates/ruff_python_parser/src/semantic_errors.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -1868,14 +1868,16 @@ impl<'a, Ctx: SemanticSyntaxContext> MatchPatternVisitor<'a, Ctx> {
// case [[x] | [x]] | x: ...
// match 42:
// case [[x | x] | [x]] | x: ...
// match 42:
// case ast.Subscript(n, ast.Constant() | ast.Slice()) | ast.Attribute(n): ...
SemanticSyntaxChecker::add_error(
self.ctx,
SemanticSyntaxErrorKind::DifferentMatchPatternBindings,
*range,
);
break;
}
self.names = visitor.names;
self.names.extend(visitor.names);
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@MichaReiser MichaReiser Nov 10, 2025

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Can you say a bit more about the fix. My understanding (and why I recommended it) was that using union or intersection would only matter for the error recovery case. But it seems that using union over intersection is important for valid cases too or are we only fixing a symptom here?

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@ntBre ntBre Nov 10, 2025

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I think the union is correct because there can be existing patterns in self.names before visiting nested alternative patterns. We need to compare all of the names we visit in that arm to all of the names we visit in the next arm, not just the set of names visited in the last, innermost alternative pattern.

That's what's happening in this case because we first visit the outer class pattern, which captures n, then we were overwriting that n with the empty set when visiting the Constant | Slice part of the pattern. Instead we need the union of n and the empty set to compare to the Attribute(n) branch.

match 42:
    case ast.Subscript(n, ast.Constant() | ast.Slice()) 
       | ast.Attribute(n): ...

}
}
}
Expand Down
212 changes: 211 additions & 1 deletion crates/ruff_python_parser/tests/snapshots/valid_syntax@nested_alternative_patterns.py.snap
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Original file line number Diff line number Diff line change
Expand Up @@ -8,7 +8,7 @@ input_file: crates/ruff_python_parser/resources/inline/ok/nested_alternative_pat
Module(
ModModule {
node_index: NodeIndex(None),
range: 0..181,
range: 0..271,
body: [
Match(
StmtMatch {
Expand Down Expand Up @@ -489,6 +489,216 @@ Module(
],
},
),
Match(
StmtMatch {
node_index: NodeIndex(None),
range: 181..270,
subject: NumberLiteral(
ExprNumberLiteral {
node_index: NodeIndex(None),
range: 187..189,
value: Int(
42,
),
},
),
cases: [
MatchCase {
range: 195..270,
node_index: NodeIndex(None),
pattern: MatchOr(
PatternMatchOr {
node_index: NodeIndex(None),
range: 200..265,
patterns: [
MatchClass(
PatternMatchClass {
node_index: NodeIndex(None),
range: 200..246,
cls: Attribute(
ExprAttribute {
node_index: NodeIndex(None),
range: 200..213,
value: Name(
ExprName {
node_index: NodeIndex(None),
range: 200..203,
id: Name("ast"),
ctx: Load,
},
),
attr: Identifier {
id: Name("Subscript"),
range: 204..213,
node_index: NodeIndex(None),
},
ctx: Load,
},
),
arguments: PatternArguments {
range: 213..246,
node_index: NodeIndex(None),
patterns: [
MatchAs(
PatternMatchAs {
node_index: NodeIndex(None),
range: 214..215,
pattern: None,
name: Some(
Identifier {
id: Name("n"),
range: 214..215,
node_index: NodeIndex(None),
},
),
},
),
MatchOr(
PatternMatchOr {
node_index: NodeIndex(None),
range: 217..245,
patterns: [
MatchClass(
PatternMatchClass {
node_index: NodeIndex(None),
range: 217..231,
cls: Attribute(
ExprAttribute {
node_index: NodeIndex(None),
range: 217..229,
value: Name(
ExprName {
node_index: NodeIndex(None),
range: 217..220,
id: Name("ast"),
ctx: Load,
},
),
attr: Identifier {
id: Name("Constant"),
range: 221..229,
node_index: NodeIndex(None),
},
ctx: Load,
},
),
arguments: PatternArguments {
range: 229..231,
node_index: NodeIndex(None),
patterns: [],
keywords: [],
},
},
),
MatchClass(
PatternMatchClass {
node_index: NodeIndex(None),
range: 234..245,
cls: Attribute(
ExprAttribute {
node_index: NodeIndex(None),
range: 234..243,
value: Name(
ExprName {
node_index: NodeIndex(None),
range: 234..237,
id: Name("ast"),
ctx: Load,
},
),
attr: Identifier {
id: Name("Slice"),
range: 238..243,
node_index: NodeIndex(None),
},
ctx: Load,
},
),
arguments: PatternArguments {
range: 243..245,
node_index: NodeIndex(None),
patterns: [],
keywords: [],
},
},
),
],
},
),
],
keywords: [],
},
},
),
MatchClass(
PatternMatchClass {
node_index: NodeIndex(None),
range: 249..265,
cls: Attribute(
ExprAttribute {
node_index: NodeIndex(None),
range: 249..262,
value: Name(
ExprName {
node_index: NodeIndex(None),
range: 249..252,
id: Name("ast"),
ctx: Load,
},
),
attr: Identifier {
id: Name("Attribute"),
range: 253..262,
node_index: NodeIndex(None),
},
ctx: Load,
},
),
arguments: PatternArguments {
range: 262..265,
node_index: NodeIndex(None),
patterns: [
MatchAs(
PatternMatchAs {
node_index: NodeIndex(None),
range: 263..264,
pattern: None,
name: Some(
Identifier {
id: Name("n"),
range: 263..264,
node_index: NodeIndex(None),
},
),
},
),
],
keywords: [],
},
},
),
],
},
),
guard: None,
body: [
Expr(
StmtExpr {
node_index: NodeIndex(None),
range: 267..270,
value: EllipsisLiteral(
ExprEllipsisLiteral {
node_index: NodeIndex(None),
range: 267..270,
},
),
},
),
],
},
],
},
),
],
},
)
Expand Down