fix: do not overwrite declared __hash__ in subclasses of a model

changes/2422-PrettyWood.md

@@ -0,0 +1 @@
+do not overwrite declared `__hash__` in subclasses of a model

pydantic/main.py

@@ -231,6 +231,7 @@ def __new__(mcs, name, bases, namespace, **kwargs):  # noqa C901
         slots: SetStr = namespace.get('__slots__', ())
         slots = {slots} if isinstance(slots, str) else set(slots)
         class_vars: SetStr = set()
+        hash_func: Optional[Callable[[Any], int]] = None
 
         for base in reversed(bases):
             if _is_base_model_class_defined and issubclass(base, BaseModel) and base != BaseModel:
@@ -241,6 +242,7 @@ def __new__(mcs, name, bases, namespace, **kwargs):  # noqa C901
                 post_root_validators += base.__post_root_validators__
                 private_attributes.update(base.__private_attributes__)
                 class_vars.update(base.__class_vars__)
+                hash_func = base.__hash__
 
         config_kwargs = {key: kwargs.pop(key) for key in kwargs.keys() & BaseConfig.__dict__.keys()}
         config_from_namespace = namespace.get('Config')
@@ -332,6 +334,9 @@ def is_untouched(v: Any) -> bool:
             json_encoder = pydantic_encoder
         pre_rv_new, post_rv_new = extract_root_validators(namespace)
 
+        if hash_func is None:
+            hash_func = generate_hash_function(config.frozen)
+
         exclude_from_namespace = fields | private_attributes.keys() | {'__slots__'}
         new_namespace = {
             '__config__': config,
@@ -344,7 +349,7 @@ def is_untouched(v: Any) -> bool:
             '__custom_root_type__': _custom_root_type,
             '__private_attributes__': private_attributes,
             '__slots__': slots | private_attributes.keys(),
-            '__hash__': generate_hash_function(config.frozen),
+            '__hash__': hash_func,
             '__class_vars__': class_vars,
             **{n: v for n, v in namespace.items() if n not in exclude_from_namespace},
         }

tests/test_main.py

@@ -395,6 +395,27 @@ class TestModel(BaseModel):
     assert "unhashable type: 'TestModel'" in exc_info.value.args[0]
 
 
+def test_with_declared_hash():
+    class Foo(BaseModel):
+        x: int
+
+        def __hash__(self):
+            return self.x ** 2
+
+    class Bar(Foo):
+        y: int
+
+        def __hash__(self):
+            return self.y ** 3
+
+    class Buz(Bar):
+        z: int
+
+    assert hash(Foo(x=2)) == 4
+    assert hash(Bar(x=2, y=3)) == 27
+    assert hash(Buz(x=2, y=3, z=4)) == 27
+
+
 def test_frozen_with_hashable_fields_are_hashable():
     class TestModel(BaseModel):
         a: int = 10