implement E307 for pylint invalid str return type

crates/ruff/resources/test/fixtures/pylint/invalid_return_type_str.py

@@ -0,0 +1,28 @@
+class Str:
+    def __str__(self):
+        return 1
+
+class Float:
+    def __str__(self):
+        return 3.05
+
+class Int:
+    def __str__(self):
+        return 0
+
+class Bool:
+    def __str__(self):
+        return False
+
+class Str2:
+    def __str__(self):
+        x = "ruff"
+        return x
+
+# TODO fixme once Ruff has better type checking
+def return_int():
+    return 3
+
+class ComplexReturn:
+    def __str__(self):
+        return return_int()

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

@@ -382,6 +382,10 @@ where
                     }
                 }
 
+                if self.enabled(Rule::InvalidStrReturnType) {
+                    pylint::rules::invalid_str_return(self, name, body);
+                }
+
                 if self.enabled(Rule::InvalidFunctionName) {
                     if let Some(diagnostic) = pep8_naming::rules::invalid_function_name(
                         stmt,

crates/ruff/src/codes.rs

@@ -167,6 +167,7 @@ pub fn code_to_rule(linter: Linter, code: &str) -> Option<(RuleGroup, Rule)> {
         (Pylint, "E0118") => (RuleGroup::Unspecified, rules::pylint::rules::LoadBeforeGlobalDeclaration),
         (Pylint, "E0241") => (RuleGroup::Unspecified, rules::pylint::rules::DuplicateBases),
         (Pylint, "E0302") => (RuleGroup::Unspecified, rules::pylint::rules::UnexpectedSpecialMethodSignature),
+        (Pylint, "E0307") => (RuleGroup::Unspecified, rules::pylint::rules::InvalidStrReturnType),
         (Pylint, "E0604") => (RuleGroup::Unspecified, rules::pylint::rules::InvalidAllObject),
         (Pylint, "E0605") => (RuleGroup::Unspecified, rules::pylint::rules::InvalidAllFormat),
         (Pylint, "E1142") => (RuleGroup::Unspecified, rules::pylint::rules::AwaitOutsideAsync),

crates/ruff/src/rules/flake8_annotations/rules/definition.rs

@@ -418,12 +418,14 @@ impl Violation for AnyType {
 fn is_none_returning(body: &[Stmt]) -> bool {
     let mut visitor = ReturnStatementVisitor::default();
     visitor.visit_body(body);
-    for expr in visitor.returns.into_iter().flatten() {
-        if !matches!(
-            expr,
-            Expr::Constant(ref constant) if constant.value.is_none()
-        ) {
-            return false;
+    for stmt in visitor.returns {
+        if let Some(value) = stmt.value.as_deref() {
+            if !matches!(
+                value,
+                Expr::Constant(constant) if constant.value.is_none()
+            ) {
+                return false;
+            }
         }
     }
     true

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

@@ -52,6 +52,7 @@ mod tests {
     #[test_case(Rule::ImportSelf, Path::new("import_self/module.py"))]
     #[test_case(Rule::InvalidAllFormat, Path::new("invalid_all_format.py"))]
     #[test_case(Rule::InvalidAllObject, Path::new("invalid_all_object.py"))]
+    #[test_case(Rule::InvalidStrReturnType, Path::new("invalid_return_type_str.py"))]
     #[test_case(Rule::DuplicateBases, Path::new("duplicate_bases.py"))]
     #[test_case(Rule::DuplicateValue, Path::new("duplicate_value.py"))]
     #[test_case(Rule::InvalidCharacterBackspace, Path::new("invalid_characters.py"))]

crates/ruff/src/rules/pylint/rules/invalid_str_return.rs

@@ -0,0 +1,75 @@
+use rustpython_parser::ast::{self, Constant, Expr, Ranged, Stmt};
+
+use ruff_diagnostics::{Diagnostic, Violation};
+use ruff_macros::{derive_message_formats, violation};
+use ruff_python_ast::{helpers::ReturnStatementVisitor, statement_visitor::StatementVisitor};
+
+use crate::checkers::ast::Checker;
+
+/// ## What it does
+/// Checks for `__str__` implementations that return a type other than `str`.
+///
+/// ## Why is this bad?
+/// The `__str__` method should return a `str` object. Returning a different
+/// type may cause unexpected behavior.
+#[violation]
+pub struct InvalidStrReturnType;
+
+impl Violation for InvalidStrReturnType {
+    #[derive_message_formats]
+    fn message(&self) -> String {
+        format!("`__str__` does not return `str`")
+    }
+}
+
+/// E0307
+pub(crate) fn invalid_str_return(checker: &mut Checker, name: &str, body: &[Stmt]) {
+    if name != "__str__" {
+        return;
+    }
+
+    if !checker.semantic_model().scope().kind.is_class() {
+        return;
+    }
+
+    let returns = {
+        let mut visitor = ReturnStatementVisitor::default();
+        visitor.visit_body(body);
+        visitor.returns
+    };
+
+    for stmt in returns {
+        // Disallow implicit `None`.
+        let Some(value) = stmt.value.as_deref() else {
+            checker.diagnostics.push(Diagnostic::new(InvalidStrReturnType, stmt.range()));
+            continue;
+        };
+
+        // Disallow other constants.
+        if matches!(
+            value,
+            Expr::List(_)
+                | Expr::Dict(_)
+                | Expr::Set(_)
+                | Expr::ListComp(_)
+                | Expr::DictComp(_)
+                | Expr::SetComp(_)
+                | Expr::GeneratorExp(_)
+                | Expr::Constant(ast::ExprConstant {
+                    value: Constant::None
+                        | Constant::Bool(_)
+                        | Constant::Bytes(_)
+                        | Constant::Int(_)
+                        | Constant::Tuple(_)
+                        | Constant::Float(_)
+                        | Constant::Complex { .. }
+                        | Constant::Ellipsis,
+                    ..
+                })
+        ) {
+            checker
+                .diagnostics
+                .push(Diagnostic::new(InvalidStrReturnType, value.range()));
+        }
+    }
+}

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

@@ -17,6 +17,7 @@ pub(crate) use invalid_all_format::{invalid_all_format, InvalidAllFormat};
 pub(crate) use invalid_all_object::{invalid_all_object, InvalidAllObject};
 pub(crate) use invalid_envvar_default::{invalid_envvar_default, InvalidEnvvarDefault};
 pub(crate) use invalid_envvar_value::{invalid_envvar_value, InvalidEnvvarValue};
+pub(crate) use invalid_str_return::{invalid_str_return, InvalidStrReturnType};
 pub(crate) use invalid_string_characters::{
     invalid_string_characters, InvalidCharacterBackspace, InvalidCharacterEsc, InvalidCharacterNul,
     InvalidCharacterSub, InvalidCharacterZeroWidthSpace,
@@ -73,6 +74,7 @@ mod invalid_all_format;
 mod invalid_all_object;
 mod invalid_envvar_default;
 mod invalid_envvar_value;
+mod invalid_str_return;
 mod invalid_string_characters;
 mod iteration_over_set;
 mod load_before_global_declaration;

crates/ruff/src/rules/pylint/snapshots/ruff__rules__pylint__tests__PLE0307_invalid_return_type_str.py.snap

@@ -0,0 +1,44 @@
+---
+source: crates/ruff/src/rules/pylint/mod.rs
+---
+invalid_return_type_str.py:3:16: PLE0307 `__str__` does not return `str`
+  |
+3 | class Str:
+4 |     def __str__(self):
+5 |         return 1
+  |                ^ PLE0307
+6 | 
+7 | class Float:
+  |
+
+invalid_return_type_str.py:7:16: PLE0307 `__str__` does not return `str`
+   |
+ 7 | class Float:
+ 8 |     def __str__(self):
+ 9 |         return 3.05
+   |                ^^^^ PLE0307
+10 |     
+11 | class Int:
+   |
+
+invalid_return_type_str.py:11:16: PLE0307 `__str__` does not return `str`
+   |
+11 | class Int:
+12 |     def __str__(self):
+13 |         return 0
+   |                ^ PLE0307
+14 |     
+15 | class Bool:
+   |
+
+invalid_return_type_str.py:15:16: PLE0307 `__str__` does not return `str`
+   |
+15 | class Bool:
+16 |     def __str__(self):
+17 |         return False
+   |                ^^^^^ PLE0307
+18 |     
+19 | class Str2:
+   |
+
+

crates/ruff_python_ast/src/helpers.rs

@@ -907,7 +907,7 @@ pub fn resolve_imported_module_path<'a>(
 /// A [`StatementVisitor`] that collects all `return` statements in a function or method.
 #[derive(Default)]
 pub struct ReturnStatementVisitor<'a> {
-    pub returns: Vec<Option<&'a Expr>>,
+    pub returns: Vec<&'a ast::StmtReturn>,
 }
 
 impl<'a, 'b> StatementVisitor<'b> for ReturnStatementVisitor<'a>
@@ -919,10 +919,7 @@ where
             Stmt::FunctionDef(_) | Stmt::AsyncFunctionDef(_) => {
                 // Don't recurse.
             }
-            Stmt::Return(ast::StmtReturn {
-                value,
-                range: _range,
-            }) => self.returns.push(value.as_deref()),
+            Stmt::Return(stmt) => self.returns.push(stmt),
             _ => walk_stmt(self, stmt),
         }
     }