Ignore annotated lambdas in class scopes

crates/ruff_linter/resources/test/fixtures/pycodestyle/E731.py

@@ -60,7 +60,7 @@ class Scope:
 class Scope:
     from typing import Callable
 
-    # E731
+    # OK
     f: Callable[[int], int] = lambda x: 2 * x
 
 
@@ -147,3 +147,12 @@ def scope():
     f = lambda: (
         i := 1,
     )
+
+
+from dataclasses import dataclass
+from typing import Callable
+
+@dataclass
+class FilterDataclass:
+    # OK
+    filter: Callable[[str], bool] = lambda _: True

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

@@ -1,3 +1,4 @@
+/// Returns `true` if the name should be considered "ambiguous".
 pub(super) fn is_ambiguous_name(name: &str) -> bool {
     name == "l" || name == "I" || name == "O"
 }

crates/ruff_linter/src/rules/pycodestyle/rules/lambda_assignment.rs

@@ -1,14 +1,13 @@
+use ruff_diagnostics::{Diagnostic, Edit, Fix, FixAvailability, Violation};
+use ruff_macros::{derive_message_formats, violation};
 use ruff_python_ast::{
     self as ast, Expr, Identifier, Parameter, ParameterWithDefault, Parameters, Stmt,
 };
-use ruff_text_size::{Ranged, TextRange};
-
-use ruff_diagnostics::{Diagnostic, Edit, Fix, FixAvailability, Violation};
-use ruff_macros::{derive_message_formats, violation};
 use ruff_python_codegen::Generator;
 use ruff_python_semantic::SemanticModel;
 use ruff_python_trivia::{has_leading_content, has_trailing_content, leading_indentation};
 use ruff_source_file::UniversalNewlines;
+use ruff_text_size::{Ranged, TextRange};
 
 use crate::checkers::ast::Checker;
 
@@ -106,12 +105,17 @@ pub(crate) fn lambda_assignment(
             }
         }
 
-        // If the assignment is in a class body, it might not be safe to replace it because the
-        // assignment might be carrying a type annotation that will be used by some package like
-        // dataclasses, which wouldn't consider the rewritten function definition to be
-        // equivalent. Even if it _doesn't_ have an annotation, rewriting safely would require
-        // making this a static method.
-        // See: https://github.com/astral-sh/ruff/issues/3046
+        // If the assignment is a class attribute (with an annotation), ignore it.
+        //
+        // This is most common for, e.g., dataclasses and Pydantic models. Those libraries will
+        // treat the lambda as an assignable field, and the use of a lambda is almost certainly
+        // intentional.
+        if annotation.is_some() && checker.semantic().current_scope().kind.is_class() {
+            return;
+        }
+
+        // Otherwise, if the assignment is in a class body, flag it, but use a display-only fix.
+        // Rewriting safely would require making this a static method.
         //
         // Similarly, if the lambda is shadowing a variable in the current scope,
         // rewriting it as a function declaration may break type-checking.
@@ -179,6 +183,7 @@ fn extract_types(annotation: &Expr, semantic: &SemanticModel) -> Option<(Vec<Exp
     Some((params, return_type))
 }
 
+/// Generate a function definition from a `lambda` expression.
 fn function(
     name: &str,
     parameters: Option<&Parameters>,

crates/ruff_linter/src/rules/pycodestyle/snapshots/ruff_linter__rules__pycodestyle__tests__E731_E731.py.snap

@@ -120,25 +120,6 @@ E731.py:57:5: E731 Do not assign a `lambda` expression, use a `def`
 59 60 | 
 60 61 | class Scope:
 
-E731.py:64:5: E731 Do not assign a `lambda` expression, use a `def`
-   |
-63 |     # E731
-64 |     f: Callable[[int], int] = lambda x: 2 * x
-   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ E731
-   |
-   = help: Rewrite `f` as a `def`
-
-ℹ Display-only fix
-61 61 |     from typing import Callable
-62 62 | 
-63 63 |     # E731
-64    |-    f: Callable[[int], int] = lambda x: 2 * x
-   64 |+    def f(x: int) -> int:
-   65 |+        return 2 * x
-65 66 | 
-66 67 | 
-67 68 | def scope():
-
 E731.py:73:9: E731 Do not assign a `lambda` expression, use a `def`
    |
 71 |     x: Callable[[int], int]
@@ -383,5 +364,6 @@ E731.py:147:5: E731 [*] Do not assign a `lambda` expression, use a `def`
 149     |-    )
     147 |+    def f():
     148 |+        return (i := 1),
-
-
+150 149 | 
+151 150 | 
+152 151 | from dataclasses import dataclass