Ignore Pydantic classes when evaluating mutable-class-default (RUF012)

crates/ruff/resources/test/fixtures/ruff/RUF012.py

@@ -31,3 +31,15 @@ class C:
     correct_code: list[int] = KNOWINGLY_MUTABLE_DEFAULT
     perfectly_fine: list[int] = field(default_factory=list)
     class_variable: ClassVar[list[int]] = []
+
+
+from pydantic import BaseModel
+
+
+class D(BaseModel):
+    mutable_default: list[int] = []
+    immutable_annotation: Sequence[int] = []
+    without_annotation = []
+    correct_code: list[int] = KNOWINGLY_MUTABLE_DEFAULT
+    perfectly_fine: list[int] = field(default_factory=list)
+    class_variable: ClassVar[list[int]] = []

crates/ruff/src/rules/ruff/rules/helpers.rs

@@ -28,3 +28,12 @@ pub(super) fn is_dataclass(class_def: &ast::StmtClassDef, semantic: &SemanticMod
             })
     })
 }
+
+/// Returns `true` if the given class is a Pydantic `BaseModel`.
+pub(super) fn is_pydantic_model(class_def: &ast::StmtClassDef, semantic: &SemanticModel) -> bool {
+    class_def.bases.iter().any(|expr| {
+        semantic.resolve_call_path(expr).map_or(false, |call_path| {
+            matches!(call_path.as_slice(), ["pydantic", "BaseModel"])
+        })
+    })
+}

crates/ruff/src/rules/ruff/rules/mutable_class_default.rs

@@ -5,7 +5,9 @@ use ruff_macros::{derive_message_formats, violation};
 use ruff_python_semantic::analyze::typing::{is_immutable_annotation, is_mutable_expr};
 
 use crate::checkers::ast::Checker;
-use crate::rules::ruff::rules::helpers::{is_class_var_annotation, is_dataclass};
+use crate::rules::ruff::rules::helpers::{
+    is_class_var_annotation, is_dataclass, is_pydantic_model,
+};
 
 /// ## What it does
 /// Checks for mutable default values in class attributes.
@@ -57,13 +59,23 @@ pub(crate) fn mutable_class_default(checker: &mut Checker, class_def: &ast::Stmt
                     && !is_immutable_annotation(annotation, checker.semantic())
                     && !is_dataclass(class_def, checker.semantic())
                 {
+                    // Avoid Pydantic models, which end up copying defaults on instance creation.
+                    if is_pydantic_model(class_def, checker.semantic()) {
+                        return;
+                    }
+
                     checker
                         .diagnostics
                         .push(Diagnostic::new(MutableClassDefault, value.range()));
                 }
             }
             Stmt::Assign(ast::StmtAssign { value, .. }) => {
                 if is_mutable_expr(value, checker.semantic()) {
+                    // Avoid Pydantic models, which end up copying defaults on instance creation.
+                    if is_pydantic_model(class_def, checker.semantic()) {
+                        return;
+                    }
+
                     checker
                         .diagnostics
                         .push(Diagnostic::new(MutableClassDefault, value.range()));