fix: DataclassInstance and AttrInstance protocols (#108)

* refactor: rename DataclassesProtocol -> DataclassInstance

* fix: instance protocol classvars

* fix: 3.8 hint

* chore: black

Author: msto

fgpyo/util/inspect.py

@@ -2,6 +2,7 @@
 import types as python_types
 import typing
 from typing import Any
+from typing import ClassVar
 from typing import Dict
 from typing import FrozenSet
 from typing import Iterable
@@ -68,19 +69,20 @@ def get_attr_fields_dict(cls: type) -> Dict[str, dataclasses.Field]:  # type: ig
     MISSING = frozenset({DATACLASSES_MISSING})
 
 if TYPE_CHECKING:  # pragma: no cover
-    from _typeshed import DataclassInstance as DataclassesProtocol
+    from _typeshed import DataclassInstance
 else:
-
-    class DataclassesProtocol(Protocol):
-        __dataclasses_fields__: Dict[str, dataclasses.Field]
+    # https://github.com/python/typeshed/blob/727f3c4320d2af3af2f16695e24dd78e79b7c070/stdlib/_typeshed/__init__.pyi#L348
+    # TODO: update the hint to `Field[Any]` when we drop support for 3.8
+    class DataclassInstance(Protocol):
+        __dataclasses_fields__: ClassVar[Dict[str, dataclasses.Field]]
 
 
 if TYPE_CHECKING and _use_attr:  # pragma: no cover
     from attr import AttrsInstance
 else:
-
+    # https://github.com/python-attrs/attrs/blob/f7f317ae4c3790f23ae027db626593d50e8a4e88/src/attr/_typing_compat.pyi#L9
     class AttrsInstance(Protocol):  # type: ignore[no-redef]
-        __attrs_attrs__: Dict[str, Any]
+        __attrs_attrs__: ClassVar[Any]
 
 
 def is_attr_class(cls: type) -> bool:  # type: ignore[arg-type]
@@ -90,7 +92,7 @@ def is_attr_class(cls: type) -> bool:  # type: ignore[arg-type]
 
 _MISSING_OR_NONE: FrozenSet[Any] = frozenset({*MISSING, None})
 """Set of values that are considered missing or None for dataclasses or attr classes"""
-_DataclassesOrAttrClass: TypeAlias = Union[DataclassesProtocol, AttrsInstance]
+_DataclassesOrAttrClass: TypeAlias = Union[DataclassInstance, AttrsInstance]
 """
 TypeAlias for dataclasses or attr classes. Mostly nonsense because they are not true types, they
 are traits, but there is no python trait-tester.
@@ -103,7 +105,7 @@ def is_attr_class(cls: type) -> bool:  # type: ignore[arg-type]
 
 
 def _get_dataclasses_fields_dict(
-    class_or_instance: Union[DataclassesProtocol, Type[DataclassesProtocol]],
+    class_or_instance: Union[DataclassInstance, Type[DataclassInstance]],
 ) -> Dict[str, dataclasses.Field]:
     """Get a dict from field name to Field for a dataclass class or instance."""
     return {field.name: field for field in get_dataclasses_fields(class_or_instance)}