Have BaseModel inherit from Representation (#1319)

* Have BaseModel inherit from Representation

This changes the inheritance so that `BaseModel` *actually* inherits from `Representation`, rather than just having the same methods as it. While this doesn't actually affect the runtime, it does make mypy happier.

Fixes https://github.com/samuelcolvin/pydantic/issues/1310

* Add '__dict__' back into slots

changes/1310-FuegoFro.md

@@ -0,0 +1 @@
+Have `BaseModel` inherit from `Representation` to make mypy happy when overriding `__str__`.

pydantic/main.py

@@ -184,6 +184,12 @@ def validate_custom_root_type(fields: Dict[str, ModelField]) -> None:
 
 UNTOUCHED_TYPES = FunctionType, property, type, classmethod, staticmethod
 
+# Note `ModelMetaclass` refers to `BaseModel`, but is also used to *create* `BaseModel`, so we need to add this extra
+# (somewhat hacky) boolean to keep track of whether we've created the `BaseModel` class yet, and therefore whether it's
+# safe to refer to it. If it *hasn't* been created, we assume that the `__new__` call we're in the middle of is for
+# the `BaseModel` class, since that's defined immediately after the metaclass.
+_is_base_model_class_defined = False
+
 
 class ModelMetaclass(ABCMeta):
     @no_type_check  # noqa C901
@@ -195,7 +201,7 @@ class ModelMetaclass(ABCMeta):
 
         pre_root_validators, post_root_validators = [], []
         for base in reversed(bases):
-            if issubclass(base, BaseModel) and base != BaseModel:
+            if _is_base_model_class_defined and issubclass(base, BaseModel) and base != BaseModel:
                 fields.update(deepcopy(base.__fields__))
                 config = inherit_config(base.__config__, config)
                 validators = inherit_validators(base.__validators__, validators)
@@ -297,7 +303,7 @@ class ModelMetaclass(ABCMeta):
         return cls
 
 
-class BaseModel(metaclass=ModelMetaclass):
+class BaseModel(Representation, metaclass=ModelMetaclass):
     if TYPE_CHECKING:
         # populated by the metaclass, defined here to help IDEs only
         __fields__: Dict[str, ModelField] = {}
@@ -314,12 +320,7 @@ class BaseModel(metaclass=ModelMetaclass):
 
     Config = BaseConfig
     __slots__ = ('__dict__', '__fields_set__')
-    # equivalent of inheriting from Representation
-    __repr_name__ = Representation.__repr_name__
-    __repr_str__ = Representation.__repr_str__
-    __pretty__ = Representation.__pretty__
-    __str__ = Representation.__str__
-    __repr__ = Representation.__repr__
+    __doc__ = ''  # Null out the Representation docstring
 
     def __init__(__pydantic_self__, **data: Any) -> None:
         """
@@ -768,6 +769,9 @@ class BaseModel(metaclass=ModelMetaclass):
         return self.__dict__
 
 
+_is_base_model_class_defined = True
+
+
 def create_model(
     model_name: str,
     *,

pydantic/utils.py

@@ -220,6 +220,8 @@ class Representation:
     of objects.
     """
 
+    __slots__: Tuple[str, ...] = tuple()
+
     def __repr_args__(self) -> 'ReprArgs':
         """
         Returns the attributes to show in __str__, __repr__, and __pretty__ this is generally overridden.

tests/mypy/modules/success.py

@@ -16,6 +16,9 @@ from pydantic.generics import GenericModel
 class Flags(BaseModel):
     strict_bool: StrictBool = False
 
+    def __str__(self) -> str:
+        return f'flag={self.strict_bool}'
+
 
 class Model(BaseModel):
     age: int