Allow __slots__ assignments in mutable-class-default (#5314)

Closes #5309.

crates/ruff/resources/test/fixtures/ruff/RUF012.py

@@ -1,23 +1,14 @@
-import typing
 from typing import ClassVar, Sequence, Final
 
-KNOWINGLY_MUTABLE_DEFAULT = []
-
 
 class A:
-    mutable_default: list[int] = []
-    immutable_annotation: typing.Sequence[int] = []
-    without_annotation = []
-    correct_code: list[int] = KNOWINGLY_MUTABLE_DEFAULT
-    class_variable: typing.ClassVar[list[int]] = []
-    final_variable: typing.Final[list[int]] = []
+    __slots__ = {
+        "mutable_default": "A mutable default value",
+    }
 
-
-class B:
     mutable_default: list[int] = []
     immutable_annotation: Sequence[int] = []
     without_annotation = []
-    correct_code: list[int] = KNOWINGLY_MUTABLE_DEFAULT
     class_variable: ClassVar[list[int]] = []
     final_variable: Final[list[int]] = []
 
@@ -30,7 +21,6 @@ class C:
     mutable_default: list[int] = []
     immutable_annotation: Sequence[int] = []
     without_annotation = []
-    correct_code: list[int] = KNOWINGLY_MUTABLE_DEFAULT
     perfectly_fine: list[int] = field(default_factory=list)
     class_variable: ClassVar[list[int]] = []
     final_variable: Final[list[int]] = []
@@ -43,7 +33,5 @@ class D(BaseModel):
     mutable_default: list[int] = []
     immutable_annotation: Sequence[int] = []
     without_annotation = []
-    correct_code: list[int] = KNOWINGLY_MUTABLE_DEFAULT
-    perfectly_fine: list[int] = field(default_factory=list)
     class_variable: ClassVar[list[int]] = []
     final_variable: Final[list[int]] = []

crates/ruff/src/rules/ruff/rules/helpers.rs

@@ -3,6 +3,22 @@ use rustpython_parser::ast::{self, Expr};
 use ruff_python_ast::helpers::map_callable;
 use ruff_python_semantic::SemanticModel;
 
+/// Return `true` if the given [`Expr`] is a special class attribute, like `__slots__`.
+///
+/// While `__slots__` is typically defined via a tuple, Python accepts any iterable and, in
+/// particular, allows the use of a dictionary to define the attribute names (as keys) and
+/// docstrings (as values).
+pub(super) fn is_special_attribute(value: &Expr) -> bool {
+    if let Expr::Name(ast::ExprName { id, .. }) = value {
+        matches!(
+            id.as_str(),
+            "__slots__" | "__dict__" | "__weakref__" | "__annotations__"
+        )
+    } else {
+        false
+    }
+}
+
 /// Returns `true` if the given [`Expr`] is a `dataclasses.field` call.
 pub(super) fn is_dataclass_field(func: &Expr, semantic: &SemanticModel) -> bool {
     semantic.resolve_call_path(func).map_or(false, |call_path| {

crates/ruff/src/rules/ruff/rules/mutable_class_default.rs

@@ -7,6 +7,7 @@ use ruff_python_semantic::analyze::typing::{is_immutable_annotation, is_mutable_
 use crate::checkers::ast::Checker;
 use crate::rules::ruff::rules::helpers::{
     is_class_var_annotation, is_dataclass, is_final_annotation, is_pydantic_model,
+    is_special_attribute,
 };
 
 /// ## What it does
@@ -51,10 +52,12 @@ pub(crate) fn mutable_class_default(checker: &mut Checker, class_def: &ast::Stmt
         match statement {
             Stmt::AnnAssign(ast::StmtAnnAssign {
                 annotation,
+                target,
                 value: Some(value),
                 ..
             }) => {
-                if is_mutable_expr(value, checker.semantic())
+                if !is_special_attribute(target)
+                    && is_mutable_expr(value, checker.semantic())
                     && !is_class_var_annotation(annotation, checker.semantic())
                     && !is_final_annotation(annotation, checker.semantic())
                     && !is_immutable_annotation(annotation, checker.semantic())
@@ -70,8 +73,10 @@ pub(crate) fn mutable_class_default(checker: &mut Checker, class_def: &ast::Stmt
                         .push(Diagnostic::new(MutableClassDefault, value.range()));
                 }
             }
-            Stmt::Assign(ast::StmtAssign { value, .. }) => {
-                if is_mutable_expr(value, checker.semantic()) {
+            Stmt::Assign(ast::StmtAssign { value, targets, .. }) => {
+                if !targets.iter().all(is_special_attribute)
+                    && is_mutable_expr(value, checker.semantic())
+                {
                     // Avoid Pydantic models, which end up copying defaults on instance creation.
                     if is_pydantic_model(class_def, checker.semantic()) {
                         return;

crates/ruff/src/rules/ruff/snapshots/ruff__rules__ruff__tests__RUF012_RUF012.py.snap

@@ -1,52 +1,34 @@
 ---
 source: crates/ruff/src/rules/ruff/mod.rs
 ---
-RUF012.py:8:34: RUF012 Mutable class attributes should be annotated with `typing.ClassVar`
+RUF012.py:9:34: RUF012 Mutable class attributes should be annotated with `typing.ClassVar`
    |
- 7 | class A:
- 8 |     mutable_default: list[int] = []
+ 7 |     }
+ 8 | 
+ 9 |     mutable_default: list[int] = []
    |                                  ^^ RUF012
- 9 |     immutable_annotation: typing.Sequence[int] = []
-10 |     without_annotation = []
+10 |     immutable_annotation: Sequence[int] = []
+11 |     without_annotation = []
    |
 
-RUF012.py:10:26: RUF012 Mutable class attributes should be annotated with `typing.ClassVar`
+RUF012.py:11:26: RUF012 Mutable class attributes should be annotated with `typing.ClassVar`
    |
- 8 |     mutable_default: list[int] = []
- 9 |     immutable_annotation: typing.Sequence[int] = []
-10 |     without_annotation = []
+ 9 |     mutable_default: list[int] = []
+10 |     immutable_annotation: Sequence[int] = []
+11 |     without_annotation = []
    |                          ^^ RUF012
-11 |     correct_code: list[int] = KNOWINGLY_MUTABLE_DEFAULT
-12 |     class_variable: typing.ClassVar[list[int]] = []
+12 |     class_variable: ClassVar[list[int]] = []
+13 |     final_variable: Final[list[int]] = []
    |
 
-RUF012.py:17:34: RUF012 Mutable class attributes should be annotated with `typing.ClassVar`
+RUF012.py:23:26: RUF012 Mutable class attributes should be annotated with `typing.ClassVar`
    |
-16 | class B:
-17 |     mutable_default: list[int] = []
-   |                                  ^^ RUF012
-18 |     immutable_annotation: Sequence[int] = []
-19 |     without_annotation = []
-   |
-
-RUF012.py:19:26: RUF012 Mutable class attributes should be annotated with `typing.ClassVar`
-   |
-17 |     mutable_default: list[int] = []
-18 |     immutable_annotation: Sequence[int] = []
-19 |     without_annotation = []
-   |                          ^^ RUF012
-20 |     correct_code: list[int] = KNOWINGLY_MUTABLE_DEFAULT
-21 |     class_variable: ClassVar[list[int]] = []
-   |
-
-RUF012.py:32:26: RUF012 Mutable class attributes should be annotated with `typing.ClassVar`
-   |
-30 |     mutable_default: list[int] = []
-31 |     immutable_annotation: Sequence[int] = []
-32 |     without_annotation = []
+21 |     mutable_default: list[int] = []
+22 |     immutable_annotation: Sequence[int] = []
+23 |     without_annotation = []
    |                          ^^ RUF012
-33 |     correct_code: list[int] = KNOWINGLY_MUTABLE_DEFAULT
-34 |     perfectly_fine: list[int] = field(default_factory=list)
+24 |     perfectly_fine: list[int] = field(default_factory=list)
+25 |     class_variable: ClassVar[list[int]] = []
    |