[flake8-pyi] Implement PYI035 (#4820)

crates/ruff/resources/test/fixtures/flake8_pyi/PYI035.py

@@ -0,0 +1,15 @@
+__all__: list[str]
+
+__all__: list[str] = ["foo"]
+
+
+class Foo:
+    __all__: list[str]
+    __match_args__: tuple[str, ...]
+    __slots__: tuple[str, ...]
+
+
+class Bar:
+    __all__: list[str] = ["foo"]
+    __match_args__: tuple[str, ...] = (1,)
+    __slots__: tuple[str, ...] = "foo"

crates/ruff/resources/test/fixtures/flake8_pyi/PYI035.pyi

@@ -0,0 +1,13 @@
+__all__: list[str]  # Error: PYI035
+
+__all__: list[str] = ["foo"]
+
+class Foo:
+    __all__: list[str]
+    __match_args__: tuple[str, ...]  # Error: PYI035
+    __slots__: tuple[str, ...]  # Error: PYI035
+
+class Bar:
+    __all__: list[str] = ["foo"]
+    __match_args__: tuple[str, ...] = (1,)
+    __slots__: tuple[str, ...] = "foo"

crates/ruff/src/checkers/ast/mod.rs

@@ -1785,6 +1785,12 @@ where
                                 );
                             }
                         }
+                    } else {
+                        if self.enabled(Rule::UnassignedSpecialVariableInStub) {
+                            flake8_pyi::rules::unassigned_special_variable_in_stub(
+                                self, target, stmt,
+                            );
+                        }
                     }
                     if self
                         .semantic_model

crates/ruff/src/codes.rs

@@ -607,6 +607,7 @@ pub fn code_to_rule(linter: Linter, code: &str) -> Option<(RuleGroup, Rule)> {
         (Flake8Pyi, "032") => (RuleGroup::Unspecified, rules::flake8_pyi::rules::AnyEqNeAnnotation),
         (Flake8Pyi, "033") => (RuleGroup::Unspecified, rules::flake8_pyi::rules::TypeCommentInStub),
         (Flake8Pyi, "034") => (RuleGroup::Unspecified, rules::flake8_pyi::rules::NonSelfReturnType),
+        (Flake8Pyi, "035") => (RuleGroup::Unspecified, rules::flake8_pyi::rules::UnassignedSpecialVariableInStub),
         (Flake8Pyi, "042") => (RuleGroup::Unspecified, rules::flake8_pyi::rules::SnakeCaseTypeAlias),
         (Flake8Pyi, "043") => (RuleGroup::Unspecified, rules::flake8_pyi::rules::TSuffixedTypeAlias),
         (Flake8Pyi, "045") => (RuleGroup::Unspecified, rules::flake8_pyi::rules::IterMethodReturnIterable),

crates/ruff/src/rules/flake8_pyi/mod.rs

@@ -44,6 +44,8 @@ mod tests {
     #[test_case(Rule::QuotedAnnotationInStub, Path::new("PYI020.pyi"))]
     #[test_case(Rule::SnakeCaseTypeAlias, Path::new("PYI042.py"))]
     #[test_case(Rule::SnakeCaseTypeAlias, Path::new("PYI042.pyi"))]
+    #[test_case(Rule::UnassignedSpecialVariableInStub, Path::new("PYI035.py"))]
+    #[test_case(Rule::UnassignedSpecialVariableInStub, Path::new("PYI035.pyi"))]
     #[test_case(Rule::StubBodyMultipleStatements, Path::new("PYI048.py"))]
     #[test_case(Rule::StubBodyMultipleStatements, Path::new("PYI048.pyi"))]
     #[test_case(Rule::TSuffixedTypeAlias, Path::new("PYI043.py"))]

crates/ruff/src/rules/flake8_pyi/rules/mod.rs

@@ -20,8 +20,9 @@ pub(crate) use prefix_type_params::{prefix_type_params, UnprefixedTypeParam};
 pub(crate) use quoted_annotation_in_stub::{quoted_annotation_in_stub, QuotedAnnotationInStub};
 pub(crate) use simple_defaults::{
     annotated_assignment_default_in_stub, argument_simple_defaults, assignment_default_in_stub,
-    typed_argument_simple_defaults, unannotated_assignment_in_stub, ArgumentDefaultInStub,
-    AssignmentDefaultInStub, TypedArgumentDefaultInStub, UnannotatedAssignmentInStub,
+    typed_argument_simple_defaults, unannotated_assignment_in_stub,
+    unassigned_special_variable_in_stub, ArgumentDefaultInStub, AssignmentDefaultInStub,
+    TypedArgumentDefaultInStub, UnannotatedAssignmentInStub, UnassignedSpecialVariableInStub,
 };
 pub(crate) use string_or_bytes_too_long::{string_or_bytes_too_long, StringOrBytesTooLong};
 pub(crate) use stub_body_multiple_statements::{

crates/ruff/src/rules/flake8_pyi/rules/simple_defaults.rs

@@ -1,4 +1,4 @@
-use rustpython_parser::ast::{self, Arguments, Constant, Expr, Operator, Ranged, Unaryop};
+use rustpython_parser::ast::{self, Arguments, Constant, Expr, Operator, Ranged, Stmt, Unaryop};
 
 use ruff_diagnostics::{AlwaysAutofixableViolation, Diagnostic, Edit, Fix, Violation};
 use ruff_macros::{derive_message_formats, violation};
@@ -64,6 +64,37 @@ impl Violation for UnannotatedAssignmentInStub {
     }
 }
 
+#[violation]
+pub struct UnassignedSpecialVariableInStub {
+    name: String,
+}
+
+/// ## What it does
+/// Checks that `__all__`, `__match_args__`, and `__slots__` variables are
+/// assigned to values when defined in stub files.
+///
+/// ## Why is this bad?
+/// Special variables like `__all__` have the same semantics in stub files
+/// as they do in Python modules, and so should be consistent with their
+/// runtime counterparts.
+///
+/// ## Example
+/// ```python
+/// __all__: list[str]
+/// ```
+///
+/// Use instead:
+/// ```python
+/// __all__: list[str] = ["foo", "bar"]
+/// ```
+impl Violation for UnassignedSpecialVariableInStub {
+    #[derive_message_formats]
+    fn message(&self) -> String {
+        let UnassignedSpecialVariableInStub { name } = self;
+        format!("`{name}` in a stub file must have a value, as it has the same semantics as `{name}` at runtime")
+    }
+}
+
 const ALLOWED_MATH_ATTRIBUTES_IN_DEFAULTS: &[&[&str]] = &[
     &["math", "inf"],
     &["math", "nan"],
@@ -528,3 +559,25 @@ pub(crate) fn unannotated_assignment_in_stub(
         value.range(),
     ));
 }
+
+/// PYI035
+pub(crate) fn unassigned_special_variable_in_stub(
+    checker: &mut Checker,
+    target: &Expr,
+    stmt: &Stmt,
+) {
+    let Expr::Name(ast::ExprName { id, .. }) = target else {
+        return;
+    };
+
+    if !is_special_assignment(checker.semantic_model(), target) {
+        return;
+    }
+
+    checker.diagnostics.push(Diagnostic::new(
+        UnassignedSpecialVariableInStub {
+            name: id.to_string(),
+        },
+        stmt.range(),
+    ));
+}

crates/ruff/src/rules/flake8_pyi/snapshots/ruff__rules__flake8_pyi__tests__PYI035_PYI035.py.snap

@@ -0,0 +1,4 @@
+---
+source: crates/ruff/src/rules/flake8_pyi/mod.rs
+---
+

crates/ruff/src/rules/flake8_pyi/snapshots/ruff__rules__flake8_pyi__tests__PYI035_PYI035.pyi.snap

@@ -0,0 +1,31 @@
+---
+source: crates/ruff/src/rules/flake8_pyi/mod.rs
+---
+PYI035.pyi:1:1: PYI035 `__all__` in a stub file must have a value, as it has the same semantics as `__all__` at runtime
+  |
+1 | __all__: list[str]  # Error: PYI035
+  | ^^^^^^^^^^^^^^^^^^ PYI035
+2 | 
+3 | __all__: list[str] = ["foo"]
+  |
+
+PYI035.pyi:7:5: PYI035 `__match_args__` in a stub file must have a value, as it has the same semantics as `__match_args__` at runtime
+   |
+ 7 | class Foo:
+ 8 |     __all__: list[str]
+ 9 |     __match_args__: tuple[str, ...]  # Error: PYI035
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ PYI035
+10 |     __slots__: tuple[str, ...]  # Error: PYI035
+   |
+
+PYI035.pyi:8:5: PYI035 `__slots__` in a stub file must have a value, as it has the same semantics as `__slots__` at runtime
+   |
+ 8 |     __all__: list[str]
+ 9 |     __match_args__: tuple[str, ...]  # Error: PYI035
+10 |     __slots__: tuple[str, ...]  # Error: PYI035
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^ PYI035
+11 | 
+12 | class Bar:
+   |
+
+