[pylint] Implement self-cls-assignment (W0642)

crates/ruff_linter/resources/test/fixtures/pylint/self_or_cls_assignment.py

@@ -0,0 +1,43 @@
+class Fruit:
+    @classmethod
+    def list_fruits(cls) -> None:
+        cls = "apple"  # PLW0642
+        cls: Fruit = "apple"  # PLW0642
+        cls += "orange"  # PLW0642
+        *cls = "banana"  # PLW0642
+        cls, blah = "apple", "orange"  # PLW0642
+        blah, (cls, blah2) = "apple", ("orange", "banana")  # PLW0642
+        blah, [cls, blah2] = "apple", ("orange", "banana")  # PLW0642
+
+    @classmethod
+    def add_fruits(cls, fruits, /) -> None:
+        cls = fruits  # PLW0642
+
+    def print_color(self) -> None:
+        self = "red"  # PLW0642
+        self: Self = "red"  # PLW0642
+        self += "blue"  # PLW0642
+        *self = "blue"  # PLW0642
+        self, blah = "red", "blue"  # PLW0642
+        blah, (self, blah2) = "apple", ("orange", "banana")  # PLW0642
+        blah, [self, blah2] = "apple", ("orange", "banana")  # PLW0642
+
+    def print_color(self, color, /) -> None:
+        self = color
+
+    def ok(self) -> None:
+        cls = None  # OK because the rule looks for the name in the signature
+
+    @classmethod
+    def ok(cls) -> None:
+        self = None
+
+    @staticmethod
+    def list_fruits_static(self, cls) -> None:
+        self = "apple"  # Ok
+        cls = "banana"  # Ok
+
+
+def list_fruits(self, cls) -> None:
+    self = "apple"  # Ok
+    cls = "banana"  # Ok

crates/ruff_linter/src/checkers/ast/analyze/statement.rs

@@ -1055,6 +1055,9 @@ pub(crate) fn statement(stmt: &Stmt, checker: &mut Checker) {
             }
         }
         Stmt::AugAssign(aug_assign @ ast::StmtAugAssign { target, .. }) => {
+            if checker.enabled(Rule::SelfOrClsAssignment) {
+                pylint::rules::self_or_cls_assignment(checker, target);
+            }
             if checker.enabled(Rule::GlobalStatement) {
                 if let Expr::Name(ast::ExprName { id, .. }) = target.as_ref() {
                     pylint::rules::global_statement(checker, id);
@@ -1410,6 +1413,11 @@ pub(crate) fn statement(stmt: &Stmt, checker: &mut Checker) {
             }
         }
         Stmt::Assign(assign @ ast::StmtAssign { targets, value, .. }) => {
+            if checker.enabled(Rule::SelfOrClsAssignment) {
+                for target in targets {
+                    pylint::rules::self_or_cls_assignment(checker, target);
+                }
+            }
             if checker.enabled(Rule::RedeclaredAssignedName) {
                 pylint::rules::redeclared_assigned_name(checker, targets);
             }
@@ -1547,6 +1555,9 @@ pub(crate) fn statement(stmt: &Stmt, checker: &mut Checker) {
                     );
                 }
             }
+            if checker.enabled(Rule::SelfOrClsAssignment) {
+                pylint::rules::self_or_cls_assignment(checker, target);
+            }
             if checker.enabled(Rule::SelfAssigningVariable) {
                 pylint::rules::self_annotated_assignment(checker, assign_stmt);
             }

crates/ruff_linter/src/codes.rs

@@ -311,6 +311,7 @@ pub fn code_to_rule(linter: Linter, code: &str) -> Option<(RuleGroup, Rule)> {
         (Pylint, "W0602") => (RuleGroup::Stable, rules::pylint::rules::GlobalVariableNotAssigned),
         (Pylint, "W0603") => (RuleGroup::Stable, rules::pylint::rules::GlobalStatement),
         (Pylint, "W0604") => (RuleGroup::Preview, rules::pylint::rules::GlobalAtModuleLevel),
+        (Pylint, "W0642") => (RuleGroup::Preview, rules::pylint::rules::SelfOrClsAssignment),
         (Pylint, "W0711") => (RuleGroup::Stable, rules::pylint::rules::BinaryOpException),
         (Pylint, "W1501") => (RuleGroup::Preview, rules::pylint::rules::BadOpenMode),
         (Pylint, "W1508") => (RuleGroup::Stable, rules::pylint::rules::InvalidEnvvarDefault),

crates/ruff_linter/src/rules/pylint/mod.rs

@@ -180,6 +180,7 @@ mod tests {
     #[test_case(Rule::NoStaticmethodDecorator, Path::new("no_method_decorator.py"))]
     #[test_case(Rule::PotentialIndexError, Path::new("potential_index_error.py"))]
     #[test_case(Rule::SuperWithoutBrackets, Path::new("super_without_brackets.py"))]
+    #[test_case(Rule::SelfOrClsAssignment, Path::new("self_or_cls_assignment.py"))]
     #[test_case(Rule::TooManyNestedBlocks, Path::new("too_many_nested_blocks.py"))]
     #[test_case(Rule::DictIterMissingItems, Path::new("dict_iter_missing_items.py"))]
     #[test_case(

crates/ruff_linter/src/rules/pylint/rules/mod.rs

@@ -61,6 +61,7 @@ pub(crate) use repeated_isinstance_calls::*;
 pub(crate) use repeated_keyword_argument::*;
 pub(crate) use return_in_init::*;
 pub(crate) use self_assigning_variable::*;
+pub(crate) use self_or_cls_assignment::*;
 pub(crate) use single_string_slots::*;
 pub(crate) use singledispatch_method::*;
 pub(crate) use singledispatchmethod_function::*;
@@ -158,6 +159,7 @@ mod repeated_isinstance_calls;
 mod repeated_keyword_argument;
 mod return_in_init;
 mod self_assigning_variable;
+mod self_or_cls_assignment;
 mod single_string_slots;
 mod singledispatch_method;
 mod singledispatchmethod_function;

crates/ruff_linter/src/rules/pylint/rules/self_or_cls_assignment.rs

@@ -0,0 +1,148 @@
+use ruff_diagnostics::{Diagnostic, Violation};
+use ruff_macros::{derive_message_formats, violation};
+use ruff_python_ast::{self as ast, Expr, ParameterWithDefault};
+use ruff_python_semantic::analyze::function_type::{self as function_type, FunctionType};
+use ruff_python_semantic::ScopeKind;
+use ruff_text_size::Ranged;
+
+use crate::checkers::ast::Checker;
+
+/// ## What it does
+/// Checks for assignment of `self` and `cls` in instance and class methods respectively.
+///
+/// ## Why is this bad?
+/// The identifiers `self` and `cls` are conventional in Python for the first argument of instance
+/// methods and class methods, respectively.
+///
+/// ## Example
+///
+/// ```python
+/// class Versions:
+///     def add(self, version):
+///         self = version
+///
+///     @classmethod
+///     def from_list(cls, versions):
+///         cls = versions
+/// ```
+///
+/// Use instead:
+/// ```python
+/// class Versions:
+///     def add(self, version):
+///         self.versions.append(version)
+///
+///     @classmethod
+///     def from_list(cls, versions):
+///         return cls(versions)
+/// ```
+#[violation]
+pub struct SelfOrClsAssignment {
+    method_type: MethodType,
+}
+
+impl Violation for SelfOrClsAssignment {
+    #[derive_message_formats]
+    fn message(&self) -> String {
+        let SelfOrClsAssignment { method_type } = self;
+
+        format!(
+            "Invalid assignment to `{}` argument in {method_type} method",
+            method_type.arg_name(),
+        )
+    }
+}
+
+/// PLW0127
+pub(crate) fn self_or_cls_assignment(checker: &mut Checker, target: &Expr) {
+    let ScopeKind::Function(ast::StmtFunctionDef {
+        name,
+        decorator_list,
+        parameters,
+        ..
+    }) = checker.semantic().current_scope().kind
+    else {
+        return;
+    };
+
+    let Some(parent) = &checker
+        .semantic()
+        .first_non_type_parent_scope(checker.semantic().current_scope())
+    else {
+        return;
+    };
+
+    let Some(ParameterWithDefault {
+        parameter: self_or_cls,
+        ..
+    }) = parameters
+        .posonlyargs
+        .first()
+        .or_else(|| parameters.args.first())
+    else {
+        return;
+    };
+
+    let function_type = function_type::classify(
+        name,
+        decorator_list,
+        parent,
+        checker.semantic(),
+        &checker.settings.pep8_naming.classmethod_decorators,
+        &checker.settings.pep8_naming.staticmethod_decorators,
+    );
+
+    let method_type = match (function_type, self_or_cls.name.as_str()) {
+        (FunctionType::Method { .. }, "self") => MethodType::Instance,
+        (FunctionType::ClassMethod { .. }, "cls") => MethodType::Class,
+        _ => return,
+    };
+
+    check_expr(checker, target, method_type);
+}
+
+fn check_expr(checker: &mut Checker, target: &Expr, method_type: MethodType) {
+    match target {
+        Expr::Name(_) => {
+            if let Expr::Name(ast::ExprName { id, .. }) = target {
+                if id.as_str() == method_type.arg_name() {
+                    checker.diagnostics.push(Diagnostic::new(
+                        SelfOrClsAssignment { method_type },
+                        target.range(),
+                    ));
+                }
+            }
+        }
+        Expr::Tuple(ast::ExprTuple { elts, .. }) | Expr::List(ast::ExprList { elts, .. }) => {
+            for element in elts {
+                check_expr(checker, element, method_type);
+            }
+        }
+        Expr::Starred(ast::ExprStarred { value, .. }) => check_expr(checker, value, method_type),
+        _ => {}
+    }
+}
+
+#[derive(Debug, Copy, Clone, PartialEq, Eq)]
+enum MethodType {
+    Instance,
+    Class,
+}
+
+impl MethodType {
+    fn arg_name(self) -> &'static str {
+        match self {
+            MethodType::Instance => "self",
+            MethodType::Class => "cls",
+        }
+    }
+}
+
+impl std::fmt::Display for MethodType {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        match self {
+            MethodType::Instance => f.write_str("instance"),
+            MethodType::Class => f.write_str("class"),
+        }
+    }
+}