Warn if a class inherits from Generic before BaseModel (#7891)

pydantic/_internal/_model_construction.py

@@ -14,7 +14,7 @@
 
 from ..errors import PydanticUndefinedAnnotation, PydanticUserError
 from ..plugin._schema_validator import create_schema_validator
-from ..warnings import PydanticDeprecatedSince20
+from ..warnings import GenericBeforeBaseModelWarning, PydanticDeprecatedSince20
 from ._config import ConfigWrapper
 from ._decorators import DecoratorInfos, PydanticDescriptorProxy, get_attribute_from_bases
 from ._fields import collect_model_fields, is_valid_field_name, is_valid_privateattr_name
@@ -115,6 +115,16 @@ def wrapped_model_post_init(self: BaseModel, __context: Any) -> None:
 
             from ..main import BaseModel
 
+            mro = cls.__mro__
+            if Generic in mro and mro.index(Generic) < mro.index(BaseModel):
+                warnings.warn(
+                    GenericBeforeBaseModelWarning(
+                        'Classes should inherit from `BaseModel` before generic classes (e.g. `typing.Generic[T]`) '
+                        'for pydantic generics to work properly.'
+                    ),
+                    stacklevel=2,
+                )
+
             cls.__pydantic_custom_init__ = not getattr(cls.__init__, '__pydantic_base_init__', False)
             cls.__pydantic_post_init__ = None if cls.model_post_init is BaseModel.model_post_init else 'model_post_init'
 

pydantic/warnings.py

@@ -45,3 +45,7 @@ class PydanticDeprecatedSince20(PydanticDeprecationWarning):
 
     def __init__(self, message: str, *args: object) -> None:
         super().__init__(message, *args, since=(2, 0), expected_removal=(3, 0))
+
+
+class GenericBeforeBaseModelWarning(Warning):
+    pass

tests/test_annotated.py

@@ -247,7 +247,7 @@ class _(BaseModel):
 
     T = TypeVar('T')
 
-    class GenericModel(Generic[T], BaseModel):
+    class GenericModel(BaseModel, Generic[T]):
         y: T
 
     class _(BaseModel):

tests/test_generics.py

@@ -72,6 +72,7 @@
     recursively_defined_type_refs,
     replace_types,
 )
+from pydantic.warnings import GenericBeforeBaseModelWarning
 
 
 @pytest.fixture()
@@ -2053,9 +2054,14 @@ def test_generic_subclass_with_extra_type_with_hint_message():
     E = TypeVar('E', bound=BaseModel)
     D = TypeVar('D')
 
-    class BaseGenericClass(Generic[E, D], BaseModel):
-        uid: str
-        name: str
+    with pytest.warns(
+        GenericBeforeBaseModelWarning,
+        match='Classes should inherit from `BaseModel` before generic classes',
+    ):
+
+        class BaseGenericClass(Generic[E, D], BaseModel):
+            uid: str
+            name: str
 
     with pytest.raises(
         TypeError,
@@ -2065,9 +2071,13 @@ class BaseGenericClass(Generic[E, D], BaseModel):
             ' `class ChildGenericClass(BaseGenericClass, typing.Generic[~E, ~D]): ...`'
         ),
     ):
+        with pytest.warns(
+            GenericBeforeBaseModelWarning,
+            match='Classes should inherit from `BaseModel` before generic classes',
+        ):
 
-        class ChildGenericClass(BaseGenericClass[E, Dict[str, Any]]):
-            ...
+            class ChildGenericClass(BaseGenericClass[E, Dict[str, Any]]):
+                ...
 
 
 def test_multi_inheritance_generic_binding():
@@ -2645,9 +2655,14 @@ class C(B[int]):
 def test_reverse_order_generic_hashability():
     T = TypeVar('T')
 
-    class Model(Generic[T], BaseModel):
-        x: T
-        model_config = dict(frozen=True)
+    with pytest.warns(
+        GenericBeforeBaseModelWarning,
+        match='Classes should inherit from `BaseModel` before generic classes',
+    ):
+
+        class Model(Generic[T], BaseModel):
+            x: T
+            model_config = dict(frozen=True)
 
     m1 = Model[int](x=1)
     m2 = Model[int](x=1)