Improve error message for improper RootModel subclasses (#8857)

pydantic/_internal/_model_construction.py

@@ -137,22 +137,40 @@ def wrapped_model_post_init(self: BaseModel, __context: Any) -> None:
                 parent_parameters = getattr(cls, '__pydantic_generic_metadata__', {}).get('parameters', ())
                 parameters = getattr(cls, '__parameters__', None) or parent_parameters
                 if parameters and parent_parameters and not all(x in parameters for x in parent_parameters):
-                    combined_parameters = parent_parameters + tuple(x for x in parameters if x not in parent_parameters)
-                    parameters_str = ', '.join([str(x) for x in combined_parameters])
-                    generic_type_label = f'typing.Generic[{parameters_str}]'
-                    error_message = (
-                        f'All parameters must be present on typing.Generic;'
-                        f' you should inherit from {generic_type_label}.'
-                    )
-                    if Generic not in bases:  # pragma: no cover
-                        # We raise an error here not because it is desirable, but because some cases are mishandled.
-                        # It would be nice to remove this error and still have things behave as expected, it's just
-                        # challenging because we are using a custom `__class_getitem__` to parametrize generic models,
-                        # and not returning a typing._GenericAlias from it.
-                        bases_str = ', '.join([x.__name__ for x in bases] + [generic_type_label])
-                        error_message += (
-                            f' Note: `typing.Generic` must go last: `class {cls.__name__}({bases_str}): ...`)'
+                    from ..root_model import RootModelRootType
+
+                    missing_parameters = tuple(x for x in parameters if x not in parent_parameters)
+                    if RootModelRootType in parent_parameters and RootModelRootType not in parameters:
+                        # This is a special case where the user has subclassed `RootModel`, but has not parametrized
+                        # RootModel with the generic type identifiers being used. Ex:
+                        # class MyModel(RootModel, Generic[T]):
+                        #    root: T
+                        # Should instead just be:
+                        # class MyModel(RootModel[T]):
+                        #   root: T
+                        parameters_str = ', '.join([x.__name__ for x in missing_parameters])
+                        error_message = (
+                            f'{cls.__name__} is a subclass of `RootModel`, but does not include the generic type identifier(s) '
+                            f'{parameters_str} in its parameters. '
+                            f'You should parametrize RootModel directly, e.g., `class {cls.__name__}(RootModel[{parameters_str}]): ...`.'
                         )
+                    else:
+                        combined_parameters = parent_parameters + missing_parameters
+                        parameters_str = ', '.join([str(x) for x in combined_parameters])
+                        generic_type_label = f'typing.Generic[{parameters_str}]'
+                        error_message = (
+                            f'All parameters must be present on typing.Generic;'
+                            f' you should inherit from {generic_type_label}.'
+                        )
+                        if Generic not in bases:  # pragma: no cover
+                            # We raise an error here not because it is desirable, but because some cases are mishandled.
+                            # It would be nice to remove this error and still have things behave as expected, it's just
+                            # challenging because we are using a custom `__class_getitem__` to parametrize generic models,
+                            # and not returning a typing._GenericAlias from it.
+                            bases_str = ', '.join([x.__name__ for x in bases] + [generic_type_label])
+                            error_message += (
+                                f' Note: `typing.Generic` must go last: `class {cls.__name__}({bases_str}): ...`)'
+                            )
                     raise TypeError(error_message)
 
                 cls.__pydantic_generic_metadata__ = {

tests/test_root_model.py

@@ -1,11 +1,11 @@
 import pickle
 from datetime import date, datetime
-from typing import Any, Dict, List, Optional, Union
+from typing import Any, Dict, Generic, List, Optional, Union
 
 import pytest
 from pydantic_core import CoreSchema
 from pydantic_core.core_schema import SerializerFunctionWrapHandler
-from typing_extensions import Annotated, Literal
+from typing_extensions import Annotated, Literal, TypeVar
 
 from pydantic import (
     Base64Str,
@@ -648,3 +648,12 @@ def test_model_validate_strings(root_type, input_value, expected, raises_match,
             Model.model_validate_strings(input_value, strict=strict)
     else:
         assert Model.model_validate_strings(input_value, strict=strict).root == expected
+
+
+def test_model_construction_with_invalid_generic_specification() -> None:
+    T_ = TypeVar('T_', bound=BaseModel)
+
+    with pytest.raises(TypeError, match='You should parametrize RootModel directly'):
+
+        class GenericRootModel(RootModel, Generic[T_]):
+            root: Union[T_, int]