Do not autofix E731 in class bodies (#3050)

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

@@ -9,6 +9,9 @@
 f = lambda: (yield 1)
 #: E731
 f = lambda: (yield from g())
+#: E731
+class F:
+    f = lambda x: 2 * x
 
 f = object()
 f.method = lambda: "Method"

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

@@ -2,64 +2,84 @@ use ruff_macros::{define_violation, derive_message_formats};
 use rustpython_parser::ast::{Arguments, Expr, ExprKind, Location, Stmt, StmtKind};
 
 use crate::ast::helpers::{match_leading_content, match_trailing_content, unparse_stmt};
-use crate::ast::types::Range;
+use crate::ast::types::{Range, ScopeKind};
 use crate::ast::whitespace::leading_space;
 use crate::checkers::ast::Checker;
 use crate::fix::Fix;
 use crate::registry::Diagnostic;
 use crate::source_code::Stylist;
-use crate::violation::AlwaysAutofixableViolation;
+use crate::violation::{AutofixKind, Availability, Violation};
 
 define_violation!(
-    pub struct LambdaAssignment(pub String);
+    pub struct LambdaAssignment {
+        pub name: String,
+        pub fixable: bool,
+    }
 );
-impl AlwaysAutofixableViolation for LambdaAssignment {
+impl Violation for LambdaAssignment {
+    const AUTOFIX: Option<AutofixKind> = Some(AutofixKind::new(Availability::Sometimes));
+
     #[derive_message_formats]
     fn message(&self) -> String {
         format!("Do not assign a `lambda` expression, use a `def`")
     }
 
-    fn autofix_title(&self) -> String {
-        let LambdaAssignment(name) = self;
-        format!("Rewrite `{name}` as a `def`")
+    fn autofix_title_formatter(&self) -> Option<fn(&Self) -> String> {
+        self.fixable
+            .then_some(|v| format!("Rewrite `{}` as a `def`", v.name))
     }
 }
 
 /// E731
 pub fn lambda_assignment(checker: &mut Checker, target: &Expr, value: &Expr, stmt: &Stmt) {
     if let ExprKind::Name { id, .. } = &target.node {
         if let ExprKind::Lambda { args, body } = &value.node {
-            let mut diagnostic =
-                Diagnostic::new(LambdaAssignment(id.to_string()), Range::from_located(stmt));
-            if checker.patch(diagnostic.kind.rule()) {
-                if !match_leading_content(stmt, checker.locator)
-                    && !match_trailing_content(stmt, checker.locator)
+            // 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.
+            // See https://github.com/charliermarsh/ruff/issues/3046
+            let fixable = !matches!(checker.current_scope().kind, ScopeKind::Class(_));
+
+            let mut diagnostic = Diagnostic::new(
+                LambdaAssignment {
+                    name: id.to_string(),
+                    fixable,
+                },
+                Range::from_located(stmt),
+            );
+
+            if checker.patch(diagnostic.kind.rule())
+                && fixable
+                && !match_leading_content(stmt, checker.locator)
+                && !match_trailing_content(stmt, checker.locator)
+            {
+                let first_line = checker.locator.slice(&Range::new(
+                    Location::new(stmt.location.row(), 0),
+                    Location::new(stmt.location.row() + 1, 0),
+                ));
+                let indentation = &leading_space(first_line);
+                let mut indented = String::new();
+                for (idx, line) in function(id, args, body, checker.stylist)
+                    .lines()
+                    .enumerate()
                 {
-                    let first_line = checker.locator.slice(&Range::new(
-                        Location::new(stmt.location.row(), 0),
-                        Location::new(stmt.location.row() + 1, 0),
-                    ));
-                    let indentation = &leading_space(first_line);
-                    let mut indented = String::new();
-                    for (idx, line) in function(id, args, body, checker.stylist)
-                        .lines()
-                        .enumerate()
-                    {
-                        if idx == 0 {
-                            indented.push_str(line);
-                        } else {
-                            indented.push_str(checker.stylist.line_ending().as_str());
-                            indented.push_str(indentation);
-                            indented.push_str(line);
-                        }
+                    if idx == 0 {
+                        indented.push_str(line);
+                    } else {
+                        indented.push_str(checker.stylist.line_ending().as_str());
+                        indented.push_str(indentation);
+                        indented.push_str(line);
                     }
-                    diagnostic.amend(Fix::replacement(
-                        indented,
-                        stmt.location,
-                        stmt.end_location.unwrap(),
-                    ));
                 }
+                diagnostic.amend(Fix::replacement(
+                    indented,
+                    stmt.location,
+                    stmt.end_location.unwrap(),
+                ));
             }
+
             checker.diagnostics.push(diagnostic);
         }
     }

crates/ruff/src/rules/pycodestyle/snapshots/ruff__rules__pycodestyle__tests__E731_E731.py.snap

@@ -3,7 +3,9 @@ source: crates/ruff/src/rules/pycodestyle/mod.rs
 expression: diagnostics
 ---
 - kind:
-    LambdaAssignment: f
+    LambdaAssignment:
+      name: f
+      fixable: true
   location:
     row: 2
     column: 0
@@ -22,7 +24,9 @@ expression: diagnostics
       column: 19
   parent: ~
 - kind:
-    LambdaAssignment: f
+    LambdaAssignment:
+      name: f
+      fixable: true
   location:
     row: 4
     column: 0
@@ -41,7 +45,9 @@ expression: diagnostics
       column: 19
   parent: ~
 - kind:
-    LambdaAssignment: this
+    LambdaAssignment:
+      name: this
+      fixable: true
   location:
     row: 7
     column: 4
@@ -60,7 +66,9 @@ expression: diagnostics
       column: 29
   parent: ~
 - kind:
-    LambdaAssignment: f
+    LambdaAssignment:
+      name: f
+      fixable: true
   location:
     row: 9
     column: 0
@@ -79,7 +87,9 @@ expression: diagnostics
       column: 21
   parent: ~
 - kind:
-    LambdaAssignment: f
+    LambdaAssignment:
+      name: f
+      fixable: true
   location:
     row: 11
     column: 0
@@ -97,4 +107,16 @@ expression: diagnostics
       row: 11
       column: 28
   parent: ~
+- kind:
+    LambdaAssignment:
+      name: f
+      fixable: false
+  location:
+    row: 14
+    column: 4
+  end_location:
+    row: 14
+    column: 23
+  fix: ~
+  parent: ~