docs: Add note in docs

docs/usage/handling_custom_types.rst

@@ -22,3 +22,7 @@ for handling dataclasses:
 .. literalinclude:: /examples/handling_custom_types/test_example_2.py
     :caption: Creating a custom dataclass factory with extended provider map
     :language: python
+
+.. note::
+    If extra configs values are defined for custom base classes, then ``__config_keys__`` should be extended so
+    that these values are correctly passed onto to concrete factories.

polyfactory/factories/base.py

@@ -175,7 +175,6 @@ class BaseFactory(ABC, Generic[T]):
     """
     Flag indicating whether to use the default value on a specific field, if provided.
     """
-    __extra_providers__: dict[Any, Callable[[], Any]] | None = None
 
     __config_keys__: tuple[str, ...] = (
         "__check_model__",
@@ -196,6 +195,9 @@ class BaseFactory(ABC, Generic[T]):
     _factory_type_mapping: ClassVar[dict[Any, type[BaseFactory[Any]]]]
     _base_factories: ClassVar[list[type[BaseFactory[Any]]]]
 
+    # Non-public attributes
+    _extra_providers: dict[Any, Callable[[], Any]] | None = None
+
     def __init_subclass__(cls, *args: Any, **kwargs: Any) -> None:  # noqa: C901
         super().__init_subclass__(*args, **kwargs)
 
@@ -363,7 +365,7 @@ def _handle_factory_field_coverage(
     def _get_config(cls) -> dict[str, Any]:
         return {
             **{key: getattr(cls, key) for key in cls.__config_keys__},
-            "__extra_providers__": cls.get_provider_map(),
+            "_extra_providers": cls.get_provider_map(),
         }
 
     @classmethod
@@ -526,7 +528,7 @@ def _create_generic_fn() -> Callable:
             Callable: _create_generic_fn,
             abc.Callable: _create_generic_fn,
             Counter: lambda: Counter(cls.__faker__.pystr()),
-            **(cls.__extra_providers__ or {}),
+            **(cls._extra_providers or {}),
         }
 
     @classmethod