[pylint] Implement invalid-index-returned (PLE0305)

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

@@ -0,0 +1,73 @@
+# These testcases should raise errors
+
+
+class Bool:
+    """pylint would not raise, but ruff does - see explanation in the docs"""
+
+    def __index__(self):
+        return True  # [invalid-index-return]
+
+
+class Float:
+    def __index__(self):
+        return 3.05  # [invalid-index-return]
+
+
+class Dict:
+    def __index__(self):
+        return {"1": "1"}  # [invalid-index-return]
+
+
+class Str:
+    def __index__(self):
+        return "ruff"  # [invalid-index-return]
+
+
+class IndexNoReturn:
+    def __index__(self):
+        print("ruff")  # [invalid-index-return]
+
+
+# TODO: Once Ruff has better type checking
+def return_index():
+    return "3"
+
+
+class ComplexReturn:
+    def __index__(self):
+        return return_index()  # [invalid-index-return]
+
+
+# These testcases should NOT raise errors
+
+
+class Index:
+    def __index__(self):
+        return 0
+
+
+class Index2:
+    def __index__(self):
+        x = 1
+        return x
+
+
+class Index3:
+    def __index__(self):
+        ...
+
+
+class Index4:
+    def __index__(self):
+        pass
+
+
+class Index5:
+    def __index__(self):
+        raise NotImplementedError
+
+
+class Index6:
+    def __index__(self):
+        print("raise some error")
+        raise NotImplementedError

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

@@ -103,6 +103,9 @@ pub(crate) fn statement(stmt: &Stmt, checker: &mut Checker) {
             if checker.enabled(Rule::InvalidBytesReturnType) {
                 pylint::rules::invalid_bytes_return(checker, function_def);
             }
+            if checker.enabled(Rule::InvalidIndexReturnType) {
+                pylint::rules::invalid_index_return(checker, function_def);
+            }
             if checker.enabled(Rule::InvalidHashReturnType) {
                 pylint::rules::invalid_hash_return(checker, function_def);
             }

crates/ruff_linter/src/codes.rs

@@ -243,6 +243,7 @@ pub fn code_to_rule(linter: Linter, code: &str) -> Option<(RuleGroup, Rule)> {
         (Pylint, "E0302") => (RuleGroup::Stable, rules::pylint::rules::UnexpectedSpecialMethodSignature),
         (Pylint, "E0303") => (RuleGroup::Preview, rules::pylint::rules::InvalidLengthReturnType),
         (Pylint, "E0304") => (RuleGroup::Preview, rules::pylint::rules::InvalidBoolReturnType),
+        (Pylint, "E0305") => (RuleGroup::Preview, rules::pylint::rules::InvalidIndexReturnType),
         (Pylint, "E0307") => (RuleGroup::Stable, rules::pylint::rules::InvalidStrReturnType),
         (Pylint, "E0308") => (RuleGroup::Preview, rules::pylint::rules::InvalidBytesReturnType),
         (Pylint, "E0309") => (RuleGroup::Preview, rules::pylint::rules::InvalidHashReturnType),

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

@@ -81,6 +81,10 @@ mod tests {
         Rule::InvalidBytesReturnType,
         Path::new("invalid_return_type_bytes.py")
     )]
+    #[test_case(
+        Rule::InvalidIndexReturnType,
+        Path::new("invalid_return_type_index.py")
+    )]
     #[test_case(Rule::InvalidHashReturnType, Path::new("invalid_return_type_hash.py"))]
     #[test_case(
         Rule::InvalidLengthReturnType,

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

@@ -0,0 +1,109 @@
+use ruff_diagnostics::{Diagnostic, Violation};
+use ruff_macros::{derive_message_formats, violation};
+use ruff_python_ast::helpers::ReturnStatementVisitor;
+use ruff_python_ast::identifier::Identifier;
+use ruff_python_ast::visitor::Visitor;
+use ruff_python_ast::{self as ast};
+use ruff_python_semantic::analyze::function_type::is_stub;
+use ruff_python_semantic::analyze::terminal::Terminal;
+use ruff_python_semantic::analyze::type_inference::{NumberLike, PythonType, ResolvedPythonType};
+use ruff_text_size::Ranged;
+
+use crate::checkers::ast::Checker;
+
+/// ## What it does
+/// Checks for `__index__` implementations that return a type other than `integer`.
+///
+/// ## Why is this bad?
+/// The `__index__` method should return an `integer`. Returning a different
+/// type may cause unexpected behavior.
+///
+/// Note: `bool` is a subclass of `int`, so it's technically valid for `__index__` to
+/// return `True` or `False`. However, a DeprecationWarning (`DeprecationWarning:
+/// __index__ returned non-int (type bool)`) for such cases was already introduced,
+/// thus this is a conscious difference between the original pylint rule and the
+/// current ruff implementation.
+///
+/// ## Example
+/// ```python
+/// class Foo:
+///     def __index__(self):
+///         return "2"
+/// ```
+///
+/// Use instead:
+/// ```python
+/// class Foo:
+///     def __index__(self):
+///         return 2
+/// ```
+///
+///
+/// ## References
+/// - [Python documentation: The `__index__` method](https://docs.python.org/3/reference/datamodel.html#object.__index__)
+#[violation]
+pub struct InvalidIndexReturnType;
+
+impl Violation for InvalidIndexReturnType {
+    #[derive_message_formats]
+    fn message(&self) -> String {
+        format!("`__index__` does not return an integer")
+    }
+}
+
+/// E0305
+pub(crate) fn invalid_index_return(checker: &mut Checker, function_def: &ast::StmtFunctionDef) {
+    if function_def.name.as_str() != "__index__" {
+        return;
+    }
+
+    if !checker.semantic().current_scope().kind.is_class() {
+        return;
+    }
+
+    if is_stub(function_def, checker.semantic()) {
+        return;
+    }
+
+    // Determine the terminal behavior (i.e., implicit return, no return, etc.).
+    let terminal = Terminal::from_function(function_def);
+
+    // If every control flow path raises an exception, ignore the function.
+    if terminal == Terminal::Raise {
+        return;
+    }
+
+    // If there are no return statements, add a diagnostic.
+    if terminal == Terminal::Implicit {
+        checker.diagnostics.push(Diagnostic::new(
+            InvalidIndexReturnType,
+            function_def.identifier(),
+        ));
+        return;
+    }
+
+    let returns = {
+        let mut visitor = ReturnStatementVisitor::default();
+        visitor.visit_body(&function_def.body);
+        visitor.returns
+    };
+
+    for stmt in returns {
+        if let Some(value) = stmt.value.as_deref() {
+            if !matches!(
+                ResolvedPythonType::from(value),
+                ResolvedPythonType::Unknown
+                    | ResolvedPythonType::Atom(PythonType::Number(NumberLike::Integer))
+            ) {
+                checker
+                    .diagnostics
+                    .push(Diagnostic::new(InvalidIndexReturnType, value.range()));
+            }
+        } else {
+            // Disallow implicit `None`.
+            checker
+                .diagnostics
+                .push(Diagnostic::new(InvalidIndexReturnType, stmt.range()));
+        }
+    }
+}

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

@@ -32,6 +32,7 @@ pub(crate) use invalid_bytes_return::*;
 pub(crate) use invalid_envvar_default::*;
 pub(crate) use invalid_envvar_value::*;
 pub(crate) use invalid_hash_return::*;
+pub(crate) use invalid_index_return::*;
 pub(crate) use invalid_length_return::*;
 pub(crate) use invalid_str_return::*;
 pub(crate) use invalid_string_characters::*;
@@ -133,6 +134,7 @@ mod invalid_bytes_return;
 mod invalid_envvar_default;
 mod invalid_envvar_value;
 mod invalid_hash_return;
+mod invalid_index_return;
 mod invalid_length_return;
 mod invalid_str_return;
 mod invalid_string_characters;

crates/ruff_linter/src/rules/pylint/snapshots/ruff_linter__rules__pylint__tests__PLE0305_invalid_return_type_index.py.snap

@@ -0,0 +1,41 @@
+---
+source: crates/ruff_linter/src/rules/pylint/mod.rs
+---
+invalid_return_type_index.py:8:16: PLE0305 `__index__` does not return an integer
+  |
+7 |     def __index__(self):
+8 |         return True  # [invalid-index-return]
+  |                ^^^^ PLE0305
+  |
+
+invalid_return_type_index.py:13:16: PLE0305 `__index__` does not return an integer
+   |
+11 | class Float:
+12 |     def __index__(self):
+13 |         return 3.05  # [invalid-index-return]
+   |                ^^^^ PLE0305
+   |
+
+invalid_return_type_index.py:18:16: PLE0305 `__index__` does not return an integer
+   |
+16 | class Dict:
+17 |     def __index__(self):
+18 |         return {"1": "1"}  # [invalid-index-return]
+   |                ^^^^^^^^^^ PLE0305
+   |
+
+invalid_return_type_index.py:23:16: PLE0305 `__index__` does not return an integer
+   |
+21 | class Str:
+22 |     def __index__(self):
+23 |         return "ruff"  # [invalid-index-return]
+   |                ^^^^^^ PLE0305
+   |
+
+invalid_return_type_index.py:27:9: PLE0305 `__index__` does not return an integer
+   |
+26 | class IndexNoReturn:
+27 |     def __index__(self):
+   |         ^^^^^^^^^ PLE0305
+28 |         print("ruff")  # [invalid-index-return]
+   |