Fix inheritance of hash function for frozen models (#6789)

pydantic/_internal/_model_construction.py

@@ -116,12 +116,8 @@ def wrapped_model_post_init(self: BaseModel, __context: Any) -> None:
             namespace['__class_vars__'] = class_vars
             namespace['__private_attributes__'] = {**base_private_attributes, **private_attributes}
 
-            if '__hash__' not in namespace and config_wrapper.frozen:
-
-                def hash_func(self: Any) -> int:
-                    return hash(self.__class__) + hash(tuple(self.__dict__.values()))
-
-                namespace['__hash__'] = hash_func
+            if config_wrapper.frozen:
+                set_default_hash_func(namespace, bases)
 
             cls: type[BaseModel] = super().__new__(mcs, cls_name, bases, namespace, **kwargs)  # type: ignore
 
@@ -359,6 +355,26 @@ def inspect_namespace(  # noqa C901
     return private_attributes
 
 
+def set_default_hash_func(namespace: dict[str, Any], bases: tuple[type[Any], ...]) -> None:
+    if '__hash__' in namespace:
+        return
+
+    base_hash_func = None
+    for base in bases:
+        base_hash_func = getattr(base, '__hash__', PydanticUndefined)
+        if base_hash_func is not PydanticUndefined:
+            break
+
+    if base_hash_func is None:
+        # This will be the case for `BaseModel` since it defines `__eq__` but not `__hash__`.
+        # In this case, we generate a standard hash function, generally for use with frozen models.
+
+        def hash_func(self: Any) -> int:
+            return hash(self.__class__) + hash(tuple(self.__dict__.values()))
+
+        namespace['__hash__'] = hash_func
+
+
 def set_model_fields(
     cls: type[BaseModel], bases: tuple[type[Any], ...], config_wrapper: ConfigWrapper, types_namespace: dict[str, Any]
 ) -> None:

tests/test_main.py

@@ -574,6 +574,31 @@ class TestModel(BaseModel):
     assert hash(m) != hash(m4)
 
 
+def test_hash_method_is_inherited_for_frozen_models():
+    from functools import lru_cache
+
+    class MyBaseModel(BaseModel):
+        """A base model with sensible configurations."""
+
+        model_config = ConfigDict(frozen=True)
+
+        def __hash__(self):
+            return hash(id(self))
+
+    class MySubClass(MyBaseModel):
+        x: Dict[str, int]
+
+        @lru_cache(maxsize=None)
+        def cached_method(self):
+            return len(self.x)
+
+    my_instance = MySubClass(x={'a': 1, 'b': 2})
+    assert my_instance.cached_method() == 2
+
+    object.__setattr__(my_instance, 'x', {})  # can't change the "normal" way due to frozen
+    assert my_instance.cached_method() == 2
+
+
 @pytest.fixture(name='ValidateAssignmentModel', scope='session')
 def validate_assignment_fixture():
     class ValidateAssignmentModel(BaseModel):