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[syntax-errors] Tuple unpacking in for statement iterator clause before Python 3.9 #16558

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Mar 13, 2025
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[syntax-errors] Tuple unpacking in for statement iterator clause before Python 3.9 #16558

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9ed44c7
[syntax-errors] Tuple unpacking in `for` statement iterator clause
ntBre Mar 7, 2025
93379e4
fix doc link and rewrap line
ntBre Mar 7, 2025
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4 changes: 4 additions & 0 deletions crates/ruff_python_parser/resources/inline/err/for_iter_unpack_py38.py
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@@ -0,0 +1,4 @@
# parse_options: {"target-version": "3.8"}
for x in *a, b: ...
for x in a, *b: ...
for x in *a, *b: ...
4 changes: 4 additions & 0 deletions crates/ruff_python_parser/resources/inline/ok/for_iter_unpack_py38.py
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# parse_options: {"target-version": "3.8"}
for x in (*a, b): ...
for x in ( a, *b): ...
for x in (*a, *b): ...
4 changes: 4 additions & 0 deletions crates/ruff_python_parser/resources/inline/ok/for_iter_unpack_py39.py
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# parse_options: {"target-version": "3.9"}
for x in *a, b: ...
for x in a, *b: ...
for x in *a, *b: ...
33 changes: 33 additions & 0 deletions crates/ruff_python_parser/src/error.rs
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Expand Up @@ -616,6 +616,33 @@ pub enum UnsupportedSyntaxErrorKind {
TypeParameterList,
TypeAliasStatement,
TypeParamDefault,

/// Represents the use of tuple unpacking in a `for` statement iterator clause before Python
/// 3.9.
///
/// ## Examples
///
/// Like [`UnsupportedSyntaxErrorKind::StarTuple`] in `return` and `yield` statements, prior to
/// Python 3.9, tuple unpacking in the iterator clause of a `for` statement required
/// parentheses:
///
/// ```python
/// # valid on Python 3.8 and earlier
/// for i in (*a, *b): ...
/// ```
///
/// Omitting the parentheses was invalid:
///
/// ```python
/// for i in *a, *b: ... # SyntaxError
/// ```
///
/// This was changed as part of the [PEG parser rewrite] included in Python 3.9 but not
/// documented directly until the [Python 3.11 release].
///
/// [PEG parser rewrite]: https://peps.python.org/pep-0617/
/// [Python 3.11 release]: https://docs.python.org/3/whatsnew/3.11.html#other-language-changes
UnparenthesizedUnpackInFor,
}

impl Display for UnsupportedSyntaxError {
Expand All @@ -642,6 +669,9 @@ impl Display for UnsupportedSyntaxError {
UnsupportedSyntaxErrorKind::TypeParamDefault => {
"Cannot set default type for a type parameter"
}
UnsupportedSyntaxErrorKind::UnparenthesizedUnpackInFor => {
"Cannot use iterable unpacking in `for` statements"
}
};

write!(
Expand Down Expand Up @@ -687,6 +717,9 @@ impl UnsupportedSyntaxErrorKind {
UnsupportedSyntaxErrorKind::TypeParameterList => Change::Added(PythonVersion::PY312),
UnsupportedSyntaxErrorKind::TypeAliasStatement => Change::Added(PythonVersion::PY312),
UnsupportedSyntaxErrorKind::TypeParamDefault => Change::Added(PythonVersion::PY313),
UnsupportedSyntaxErrorKind::UnparenthesizedUnpackInFor => {
Change::Added(PythonVersion::PY39)
}
}
}

Expand Down
5 changes: 4 additions & 1 deletion crates/ruff_python_parser/src/parser/expression.rs
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Expand Up @@ -2130,7 +2130,10 @@ impl<'src> Parser<'src> {
// rest = (4, 5, 6)
// def g(): yield 1, 2, 3, *rest
// def h(): yield 1, (yield 2, *rest), 3
self.check_tuple_unpacking(&parsed_expr, StarTupleKind::Yield);
self.check_tuple_unpacking(
&parsed_expr,
UnsupportedSyntaxErrorKind::StarTuple(StarTupleKind::Yield),
);

Box::new(parsed_expr.expr)
});
Expand Down
37 changes: 32 additions & 5 deletions crates/ruff_python_parser/src/parser/statement.rs
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Expand Up @@ -406,7 +406,10 @@ impl<'src> Parser<'src> {
// # parse_options: {"target-version": "3.7"}
// rest = (4, 5, 6)
// def f(): return 1, 2, 3, *rest
self.check_tuple_unpacking(&parsed_expr, StarTupleKind::Return);
self.check_tuple_unpacking(
&parsed_expr,
UnsupportedSyntaxErrorKind::StarTuple(StarTupleKind::Return),
);

Box::new(parsed_expr.expr)
});
Expand All @@ -420,10 +423,12 @@ impl<'src> Parser<'src> {
/// Report [`UnsupportedSyntaxError`]s for each starred element in `expr` if it is an
/// unparenthesized tuple.
///
/// This method can be used to check for tuple unpacking in return and yield statements, which
/// are only allowed in Python 3.8 and later: <https://github.com/python/cpython/issues/76298>.
pub(crate) fn check_tuple_unpacking(&mut self, expr: &Expr, kind: StarTupleKind) {
let kind = UnsupportedSyntaxErrorKind::StarTuple(kind);
/// This method can be used to check for tuple unpacking in `return`, `yield`, and `for`
/// statements, which are only allowed after [Python 3.8] and [Python 3.9], respectively.
///
/// [Python 3.8]: https://github.com/python/cpython/issues/76298
/// [Python 3.9]: https://github.com/python/cpython/issues/90881
pub(super) fn check_tuple_unpacking(&mut self, expr: &Expr, kind: UnsupportedSyntaxErrorKind) {
if kind.is_supported(self.options.target_version) {
return;
}
Expand Down Expand Up @@ -1732,6 +1737,28 @@ impl<'src> Parser<'src> {
// for target in x := 1: ...
let iter = self.parse_expression_list(ExpressionContext::starred_bitwise_or());

// test_ok for_iter_unpack_py39
// # parse_options: {"target-version": "3.9"}
// for x in *a, b: ...
// for x in a, *b: ...
// for x in *a, *b: ...

// test_ok for_iter_unpack_py38
// # parse_options: {"target-version": "3.8"}
// for x in (*a, b): ...
// for x in ( a, *b): ...
// for x in (*a, *b): ...

// test_err for_iter_unpack_py38
// # parse_options: {"target-version": "3.8"}
// for x in *a, b: ...
// for x in a, *b: ...
// for x in *a, *b: ...
self.check_tuple_unpacking(
&iter,
UnsupportedSyntaxErrorKind::UnparenthesizedUnpackInFor,
);

self.expect(TokenKind::Colon);

let body = self.parse_body(Clause::For);
Expand Down
220 changes: 220 additions & 0 deletions crates/ruff_python_parser/tests/snapshots/invalid_syntax@for_iter_unpack_py38.py.snap
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---
source: crates/ruff_python_parser/tests/fixtures.rs
input_file: crates/ruff_python_parser/resources/inline/err/for_iter_unpack_py38.py
---
## AST

```
Module(
ModModule {
range: 0..106,
body: [
For(
StmtFor {
range: 43..63,
is_async: false,
target: Name(
ExprName {
range: 47..48,
id: Name("x"),
ctx: Store,
},
),
iter: Tuple(
ExprTuple {
range: 52..58,
elts: [
Starred(
ExprStarred {
range: 52..54,
value: Name(
ExprName {
range: 53..54,
id: Name("a"),
ctx: Load,
},
),
ctx: Load,
},
),
Name(
ExprName {
range: 57..58,
id: Name("b"),
ctx: Load,
},
),
],
ctx: Load,
parenthesized: false,
},
),
body: [
Expr(
StmtExpr {
range: 60..63,
value: EllipsisLiteral(
ExprEllipsisLiteral {
range: 60..63,
},
),
},
),
],
orelse: [],
},
),
For(
StmtFor {
range: 64..84,
is_async: false,
target: Name(
ExprName {
range: 68..69,
id: Name("x"),
ctx: Store,
},
),
iter: Tuple(
ExprTuple {
range: 74..79,
elts: [
Name(
ExprName {
range: 74..75,
id: Name("a"),
ctx: Load,
},
),
Starred(
ExprStarred {
range: 77..79,
value: Name(
ExprName {
range: 78..79,
id: Name("b"),
ctx: Load,
},
),
ctx: Load,
},
),
],
ctx: Load,
parenthesized: false,
},
),
body: [
Expr(
StmtExpr {
range: 81..84,
value: EllipsisLiteral(
ExprEllipsisLiteral {
range: 81..84,
},
),
},
),
],
orelse: [],
},
),
For(
StmtFor {
range: 85..105,
is_async: false,
target: Name(
ExprName {
range: 89..90,
id: Name("x"),
ctx: Store,
},
),
iter: Tuple(
ExprTuple {
range: 94..100,
elts: [
Starred(
ExprStarred {
range: 94..96,
value: Name(
ExprName {
range: 95..96,
id: Name("a"),
ctx: Load,
},
),
ctx: Load,
},
),
Starred(
ExprStarred {
range: 98..100,
value: Name(
ExprName {
range: 99..100,
id: Name("b"),
ctx: Load,
},
),
ctx: Load,
},
),
],
ctx: Load,
parenthesized: false,
},
),
body: [
Expr(
StmtExpr {
range: 102..105,
value: EllipsisLiteral(
ExprEllipsisLiteral {
range: 102..105,
},
),
},
),
],
orelse: [],
},
),
],
},
)
```
## Unsupported Syntax Errors

|
1 | # parse_options: {"target-version": "3.8"}
2 | for x in *a, b: ...
| ^^ Syntax Error: Cannot use iterable unpacking in `for` statements on Python 3.8 (syntax was added in Python 3.9)
3 | for x in a, *b: ...
4 | for x in *a, *b: ...
|


|
1 | # parse_options: {"target-version": "3.8"}
2 | for x in *a, b: ...
3 | for x in a, *b: ...
| ^^ Syntax Error: Cannot use iterable unpacking in `for` statements on Python 3.8 (syntax was added in Python 3.9)
4 | for x in *a, *b: ...
|


|
2 | for x in *a, b: ...
3 | for x in a, *b: ...
4 | for x in *a, *b: ...
| ^^ Syntax Error: Cannot use iterable unpacking in `for` statements on Python 3.8 (syntax was added in Python 3.9)
|


|
2 | for x in *a, b: ...
3 | for x in a, *b: ...
4 | for x in *a, *b: ...
| ^^ Syntax Error: Cannot use iterable unpacking in `for` statements on Python 3.8 (syntax was added in Python 3.9)
|
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