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[syntax-errors] Tuple unpacking in for statement iterator clause before Python 3.9 (#16558)
Summary -- This PR reuses a slightly modified version of the `check_tuple_unpacking` method added for detecting unpacking in `return` and `yield` statements to detect the same issue in the iterator clause of `for` loops. I ran into the same issue with a bare `for x in *rest: ...` example (invalid even on Python 3.13) and added it as a comment on #16520. I considered just making this an additional `StarTupleKind` variant as well, but this change was in a different version of Python, so I kept it separate. Test Plan -- New inline tests.
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‎crates/ruff_python_parser/resources/inline/err/for_iter_unpack_py38.py

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# parse_options: {"target-version": "3.8"}
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for x in *a, b: ...
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for x in a, *b: ...
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for x in *a, *b: ...

‎crates/ruff_python_parser/resources/inline/ok/for_iter_unpack_py38.py

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

‎crates/ruff_python_parser/resources/inline/ok/for_iter_unpack_py39.py

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

‎crates/ruff_python_parser/src/error.rs

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@@ -616,6 +616,33 @@ pub enum UnsupportedSyntaxErrorKind {
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TypeParameterList,
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TypeAliasStatement,
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TypeParamDefault,
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/// Represents the use of tuple unpacking in a `for` statement iterator clause before Python
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/// 3.9.
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///
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/// ## Examples
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///
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/// Like [`UnsupportedSyntaxErrorKind::StarTuple`] in `return` and `yield` statements, prior to
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/// Python 3.9, tuple unpacking in the iterator clause of a `for` statement required
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/// parentheses:
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///
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/// ```python
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/// # valid on Python 3.8 and earlier
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/// for i in (*a, *b): ...
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/// ```
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///
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/// Omitting the parentheses was invalid:
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///
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/// ```python
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/// for i in *a, *b: ... # SyntaxError
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/// ```
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///
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/// This was changed as part of the [PEG parser rewrite] included in Python 3.9 but not
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/// documented directly until the [Python 3.11 release].
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///
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/// [PEG parser rewrite]: https://peps.python.org/pep-0617/
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/// [Python 3.11 release]: https://docs.python.org/3/whatsnew/3.11.html#other-language-changes
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UnparenthesizedUnpackInFor,
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}
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impl Display for UnsupportedSyntaxError {
@@ -642,6 +669,9 @@ impl Display for UnsupportedSyntaxError {
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UnsupportedSyntaxErrorKind::TypeParamDefault => {
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"Cannot set default type for a type parameter"
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}
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UnsupportedSyntaxErrorKind::UnparenthesizedUnpackInFor => {
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"Cannot use iterable unpacking in `for` statements"
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}
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};
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write!(
@@ -687,6 +717,9 @@ impl UnsupportedSyntaxErrorKind {
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UnsupportedSyntaxErrorKind::TypeParameterList => Change::Added(PythonVersion::PY312),
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UnsupportedSyntaxErrorKind::TypeAliasStatement => Change::Added(PythonVersion::PY312),
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UnsupportedSyntaxErrorKind::TypeParamDefault => Change::Added(PythonVersion::PY313),
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UnsupportedSyntaxErrorKind::UnparenthesizedUnpackInFor => {
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Change::Added(PythonVersion::PY39)
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}
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}
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}
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‎crates/ruff_python_parser/src/parser/expression.rs

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@@ -2130,7 +2130,10 @@ impl<'src> Parser<'src> {
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// rest = (4, 5, 6)
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// def g(): yield 1, 2, 3, *rest
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// def h(): yield 1, (yield 2, *rest), 3
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self.check_tuple_unpacking(&parsed_expr, StarTupleKind::Yield);
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self.check_tuple_unpacking(
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&parsed_expr,
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UnsupportedSyntaxErrorKind::StarTuple(StarTupleKind::Yield),
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);
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Box::new(parsed_expr.expr)
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});

‎crates/ruff_python_parser/src/parser/statement.rs

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@@ -406,7 +406,10 @@ impl<'src> Parser<'src> {
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// # parse_options: {"target-version": "3.7"}
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// rest = (4, 5, 6)
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// def f(): return 1, 2, 3, *rest
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self.check_tuple_unpacking(&parsed_expr, StarTupleKind::Return);
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self.check_tuple_unpacking(
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&parsed_expr,
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UnsupportedSyntaxErrorKind::StarTuple(StarTupleKind::Return),
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);
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Box::new(parsed_expr.expr)
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});
@@ -420,10 +423,12 @@ impl<'src> Parser<'src> {
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/// Report [`UnsupportedSyntaxError`]s for each starred element in `expr` if it is an
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/// unparenthesized tuple.
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///
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/// This method can be used to check for tuple unpacking in return and yield statements, which
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/// are only allowed in Python 3.8 and later: <https://github.com/python/cpython/issues/76298>.
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pub(crate) fn check_tuple_unpacking(&mut self, expr: &Expr, kind: StarTupleKind) {
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let kind = UnsupportedSyntaxErrorKind::StarTuple(kind);
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/// This method can be used to check for tuple unpacking in `return`, `yield`, and `for`
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/// statements, which are only allowed after [Python 3.8] and [Python 3.9], respectively.
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///
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/// [Python 3.8]: https://github.com/python/cpython/issues/76298
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/// [Python 3.9]: https://github.com/python/cpython/issues/90881
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pub(super) fn check_tuple_unpacking(&mut self, expr: &Expr, kind: UnsupportedSyntaxErrorKind) {
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if kind.is_supported(self.options.target_version) {
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return;
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}
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// for target in x := 1: ...
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let iter = self.parse_expression_list(ExpressionContext::starred_bitwise_or());
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// test_ok for_iter_unpack_py39
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// # parse_options: {"target-version": "3.9"}
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// for x in *a, b: ...
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// for x in a, *b: ...
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// for x in *a, *b: ...
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// test_ok for_iter_unpack_py38
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// # parse_options: {"target-version": "3.8"}
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// for x in (*a, b): ...
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// for x in ( a, *b): ...
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// for x in (*a, *b): ...
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// test_err for_iter_unpack_py38
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// # parse_options: {"target-version": "3.8"}
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// for x in *a, b: ...
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// for x in a, *b: ...
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// for x in *a, *b: ...
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self.check_tuple_unpacking(
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&iter,
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UnsupportedSyntaxErrorKind::UnparenthesizedUnpackInFor,
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);
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self.expect(TokenKind::Colon);
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let body = self.parse_body(Clause::For);

‎crates/ruff_python_parser/tests/snapshots/invalid_syntax@for_iter_unpack_py38.py.snap

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---
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source: crates/ruff_python_parser/tests/fixtures.rs
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input_file: crates/ruff_python_parser/resources/inline/err/for_iter_unpack_py38.py
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---
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## AST
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```
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Module(
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ModModule {
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range: 0..106,
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body: [
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For(
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StmtFor {
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range: 43..63,
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is_async: false,
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target: Name(
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ExprName {
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range: 47..48,
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id: Name("x"),
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ctx: Store,
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},
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),
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iter: Tuple(
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ExprTuple {
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range: 52..58,
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elts: [
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Starred(
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ExprStarred {
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range: 52..54,
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value: Name(
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ExprName {
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range: 53..54,
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id: Name("a"),
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ctx: Load,
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},
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),
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ctx: Load,
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},
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),
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Name(
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ExprName {
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range: 57..58,
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id: Name("b"),
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ctx: Load,
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},
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),
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],
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ctx: Load,
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parenthesized: false,
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},
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),
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body: [
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Expr(
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StmtExpr {
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range: 60..63,
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value: EllipsisLiteral(
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ExprEllipsisLiteral {
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range: 60..63,
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},
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),
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},
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),
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],
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orelse: [],
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},
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),
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For(
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StmtFor {
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range: 64..84,
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is_async: false,
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target: Name(
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ExprName {
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range: 68..69,
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id: Name("x"),
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ctx: Store,
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},
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),
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iter: Tuple(
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ExprTuple {
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range: 74..79,
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elts: [
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Name(
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ExprName {
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range: 74..75,
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id: Name("a"),
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ctx: Load,
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},
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),
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Starred(
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ExprStarred {
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range: 77..79,
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value: Name(
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ExprName {
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range: 78..79,
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id: Name("b"),
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ctx: Load,
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},
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),
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ctx: Load,
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},
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),
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],
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ctx: Load,
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parenthesized: false,
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},
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),
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body: [
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Expr(
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StmtExpr {
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range: 81..84,
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value: EllipsisLiteral(
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ExprEllipsisLiteral {
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range: 81..84,
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},
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),
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},
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),
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],
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orelse: [],
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},
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),
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For(
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StmtFor {
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range: 85..105,
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is_async: false,
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target: Name(
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ExprName {
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range: 89..90,
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id: Name("x"),
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ctx: Store,
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},
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),
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iter: Tuple(
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ExprTuple {
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range: 94..100,
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elts: [
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Starred(
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ExprStarred {
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range: 94..96,
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value: Name(
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ExprName {
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range: 95..96,
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id: Name("a"),
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ctx: Load,
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},
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),
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ctx: Load,
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},
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),
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Starred(
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ExprStarred {
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range: 98..100,
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value: Name(
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ExprName {
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range: 99..100,
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id: Name("b"),
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ctx: Load,
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},
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),
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ctx: Load,
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},
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),
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],
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ctx: Load,
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parenthesized: false,
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},
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),
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body: [
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Expr(
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StmtExpr {
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range: 102..105,
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value: EllipsisLiteral(
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ExprEllipsisLiteral {
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range: 102..105,
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},
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),
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},
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),
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],
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orelse: [],
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},
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),
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],
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},
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)
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```
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## Unsupported Syntax Errors
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|
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1 | # parse_options: {"target-version": "3.8"}
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2 | for x in *a, b: ...
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| ^^ Syntax Error: Cannot use iterable unpacking in `for` statements on Python 3.8 (syntax was added in Python 3.9)
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3 | for x in a, *b: ...
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4 | for x in *a, *b: ...
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|
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|
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1 | # parse_options: {"target-version": "3.8"}
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2 | for x in *a, b: ...
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3 | for x in a, *b: ...
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| ^^ Syntax Error: Cannot use iterable unpacking in `for` statements on Python 3.8 (syntax was added in Python 3.9)
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4 | for x in *a, *b: ...
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|
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|
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2 | for x in *a, b: ...
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3 | for x in a, *b: ...
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4 | for x in *a, *b: ...
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| ^^ Syntax Error: Cannot use iterable unpacking in `for` statements on Python 3.8 (syntax was added in Python 3.9)
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|
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|
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2 | for x in *a, b: ...
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3 | for x in a, *b: ...
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4 | for x in *a, *b: ...
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| ^^ Syntax Error: Cannot use iterable unpacking in `for` statements on Python 3.8 (syntax was added in Python 3.9)
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|

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