[pylint] Implement singledispatch-method (E1519)

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

@@ -0,0 +1,39 @@
+from functools import singledispatch, singledispatchmethod
+
+
+@singledispatch
+def convert_position(position):
+    pass
+
+
+class Board:
+    @singledispatch  # [singledispatch-method]
+    @classmethod
+    def convert_position(cls, position):
+        pass
+
+    @singledispatch  # [singledispatch-method]
+    def move(self, position):
+        pass
+
+    @singledispatchmethod
+    def place(self, position):
+        pass
+
+    @singledispatch
+    @staticmethod
+    def do(position):
+        pass
+
+    # False negative (flagged by Pylint).
+    @convert_position.register
+    @classmethod
+    def _(cls, position: str) -> tuple:
+        position_a, position_b = position.split(",")
+        return (int(position_a), int(position_b))
+
+    # False negative (flagged by Pylint).
+    @convert_position.register
+    @classmethod
+    def _(cls, position: tuple) -> str:
+        return f"{position[0]},{position[1]}"

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

@@ -40,6 +40,7 @@ pub(crate) fn deferred_scopes(checker: &mut Checker) {
         Rule::UnusedPrivateTypedDict,
         Rule::UnusedStaticMethodArgument,
         Rule::UnusedVariable,
+        Rule::SingledispatchMethod,
     ]) {
         return;
     }
@@ -389,6 +390,10 @@ pub(crate) fn deferred_scopes(checker: &mut Checker) {
             if checker.enabled(Rule::TooManyLocals) {
                 pylint::rules::too_many_locals(checker, scope, &mut diagnostics);
             }
+
+            if checker.enabled(Rule::SingledispatchMethod) {
+                pylint::rules::singledispatch_method(checker, scope, &mut diagnostics);
+            }
         }
     }
     checker.diagnostics.extend(diagnostics);

crates/ruff_linter/src/codes.rs

@@ -252,6 +252,7 @@ pub fn code_to_rule(linter: Linter, code: &str) -> Option<(RuleGroup, Rule)> {
         (Pylint, "E1307") => (RuleGroup::Stable, rules::pylint::rules::BadStringFormatType),
         (Pylint, "E1310") => (RuleGroup::Stable, rules::pylint::rules::BadStrStripCall),
         (Pylint, "E1507") => (RuleGroup::Stable, rules::pylint::rules::InvalidEnvvarValue),
+        (Pylint, "E1519") => (RuleGroup::Preview, rules::pylint::rules::SingledispatchMethod),
         (Pylint, "E1700") => (RuleGroup::Stable, rules::pylint::rules::YieldFromInAsyncFunction),
         (Pylint, "E2502") => (RuleGroup::Stable, rules::pylint::rules::BidirectionalUnicode),
         (Pylint, "E2510") => (RuleGroup::Stable, rules::pylint::rules::InvalidCharacterBackspace),

crates/ruff_linter/src/rules/flake8_type_checking/helpers.rs

@@ -143,6 +143,7 @@ pub(crate) fn is_dataclass_meta_annotation(annotation: &Expr, semantic: &Semanti
 /// ```python
 /// from functools import singledispatch
 ///
+///
 /// @singledispatch
 /// def fun(arg, verbose=False):
 ///     ...
@@ -167,6 +168,7 @@ pub(crate) fn is_singledispatch_interface(
 /// ```python
 /// from functools import singledispatch
 ///
+///
 /// @singledispatch
 /// def fun(arg, verbose=False):
 ///     ...

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

@@ -20,6 +20,7 @@ mod tests {
     use crate::settings::LinterSettings;
     use crate::test::test_path;
 
+    #[test_case(Rule::SingledispatchMethod, Path::new("singledispatch_method.py"))]
     #[test_case(Rule::AssertOnStringLiteral, Path::new("assert_on_string_literal.py"))]
     #[test_case(Rule::AwaitOutsideAsync, Path::new("await_outside_async.py"))]
     #[test_case(Rule::BadOpenMode, Path::new("bad_open_mode.py"))]

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

@@ -54,6 +54,7 @@ pub(crate) use repeated_keyword_argument::*;
 pub(crate) use return_in_init::*;
 pub(crate) use self_assigning_variable::*;
 pub(crate) use single_string_slots::*;
+pub(crate) use singledispatch_method::*;
 pub(crate) use subprocess_popen_preexec_fn::*;
 pub(crate) use subprocess_run_without_check::*;
 pub(crate) use super_without_brackets::*;
@@ -140,6 +141,7 @@ mod repeated_keyword_argument;
 mod return_in_init;
 mod self_assigning_variable;
 mod single_string_slots;
+mod singledispatch_method;
 mod subprocess_popen_preexec_fn;
 mod subprocess_run_without_check;
 mod super_without_brackets;

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

@@ -0,0 +1,119 @@
+use ruff_diagnostics::{Diagnostic, Edit, Fix, FixAvailability, Violation};
+use ruff_macros::{derive_message_formats, violation};
+use ruff_python_ast as ast;
+use ruff_python_semantic::analyze::function_type;
+use ruff_python_semantic::Scope;
+use ruff_text_size::Ranged;
+
+use crate::checkers::ast::Checker;
+use crate::importer::ImportRequest;
+
+/// ## What it does
+/// Checks for `@singledispatch` decorators on class and instance methods.
+///
+/// ## Why is this bad?
+/// The `@singledispatch` decorator is intended for use with functions, not methods.
+///
+/// Instead, use the `@singledispatchmethod` decorator, or migrate the method to a
+/// standalone function or `@staticmethod`.
+///
+/// ## Example
+/// ```python
+/// from functools import singledispatch
+///
+///
+/// class Class:
+///     @singledispatch
+///     def method(self, arg):
+///         ...
+/// ```
+///
+/// Use instead:
+/// ```python
+/// from functools import singledispatchmethod
+///
+///
+/// class Class:
+///     @singledispatchmethod
+///     def method(self, arg):
+///         ...
+/// ```
+///
+/// ## Fix safety
+/// This rule's fix is marked as unsafe, as migrating from `@singledispatch` to
+/// `@singledispatchmethod` may change the behavior of the code.
+#[violation]
+pub struct SingledispatchMethod;
+
+impl Violation for SingledispatchMethod {
+    const FIX_AVAILABILITY: FixAvailability = FixAvailability::Sometimes;
+
+    #[derive_message_formats]
+    fn message(&self) -> String {
+        format!("`@singledispatch` decorator should not be used on methods")
+    }
+
+    fn fix_title(&self) -> Option<String> {
+        Some("Replace with `@singledispatchmethod`".to_string())
+    }
+}
+
+/// E1519
+pub(crate) fn singledispatch_method(
+    checker: &Checker,
+    scope: &Scope,
+    diagnostics: &mut Vec<Diagnostic>,
+) {
+    let Some(func) = scope.kind.as_function() else {
+        return;
+    };
+
+    let ast::StmtFunctionDef {
+        name,
+        decorator_list,
+        ..
+    } = func;
+
+    let Some(parent) = &checker.semantic().first_non_type_parent_scope(scope) else {
+        return;
+    };
+
+    let type_ = function_type::classify(
+        name,
+        decorator_list,
+        parent,
+        checker.semantic(),
+        &checker.settings.pep8_naming.classmethod_decorators,
+        &checker.settings.pep8_naming.staticmethod_decorators,
+    );
+    if !matches!(
+        type_,
+        function_type::FunctionType::Method | function_type::FunctionType::ClassMethod
+    ) {
+        return;
+    }
+
+    for decorator in decorator_list {
+        if checker
+            .semantic()
+            .resolve_call_path(&decorator.expression)
+            .is_some_and(|call_path| {
+                matches!(call_path.as_slice(), ["functools", "singledispatch"])
+            })
+        {
+            let mut diagnostic = Diagnostic::new(SingledispatchMethod, decorator.range());
+            diagnostic.try_set_fix(|| {
+                let (import_edit, binding) = checker.importer().get_or_import_symbol(
+                    &ImportRequest::import("functools", "singledispatchmethod"),
+                    decorator.start(),
+                    checker.semantic(),
+                )?;
+                Ok(Fix::unsafe_edits(
+                    Edit::range_replacement(binding, decorator.expression.range()),
+                    [import_edit],
+                ))
+            });
+            diagnostics.push(diagnostic);
+        }
+    }
+}

crates/ruff_linter/src/rules/pylint/snapshots/ruff_linter__rules__pylint__tests__PLE1519_singledispatch_method.py.snap

@@ -0,0 +1,43 @@
+---
+source: crates/ruff_linter/src/rules/pylint/mod.rs
+---
+singledispatch_method.py:10:5: PLE1519 [*] `@singledispatch` decorator should not be used on methods
+   |
+ 9 | class Board:
+10 |     @singledispatch  # [singledispatch-method]
+   |     ^^^^^^^^^^^^^^^ PLE1519
+11 |     @classmethod
+12 |     def convert_position(cls, position):
+   |
+   = help: Replace with `@singledispatchmethod`
+
+ℹ Unsafe fix
+7  7  | 
+8  8  | 
+9  9  | class Board:
+10    |-    @singledispatch  # [singledispatch-method]
+   10 |+    @singledispatchmethod  # [singledispatch-method]
+11 11 |     @classmethod
+12 12 |     def convert_position(cls, position):
+13 13 |         pass
+
+singledispatch_method.py:15:5: PLE1519 [*] `@singledispatch` decorator should not be used on methods
+   |
+13 |         pass
+14 | 
+15 |     @singledispatch  # [singledispatch-method]
+   |     ^^^^^^^^^^^^^^^ PLE1519
+16 |     def move(self, position):
+17 |         pass
+   |
+   = help: Replace with `@singledispatchmethod`
+
+ℹ Unsafe fix
+12 12 |     def convert_position(cls, position):
+13 13 |         pass
+14 14 | 
+15    |-    @singledispatch  # [singledispatch-method]
+   15 |+    @singledispatchmethod  # [singledispatch-method]
+16 16 |     def move(self, position):
+17 17 |         pass
+18 18 |