Add covariant mapping type

Author: harahu

src/typing_extensions.py

@@ -42,6 +42,7 @@
     'TypedDict',
 
     # Structural checks, a.k.a. protocols.
+    'MappingAuxiliary',
     'SupportsIndex',
 
     # One-off things.
@@ -145,7 +146,9 @@ def _collect_type_vars(types, typevar_types=None):
 # (These are not for export.)
 T = typing.TypeVar('T')  # Any type.
 KT = typing.TypeVar('KT')  # Key type.
+KT_co = typing.TypeVar('KT', covariant=True)  # Covariant key type.
 VT = typing.TypeVar('VT')  # Value type.
+VT_co = typing.TypeVar('VT', covariant=True)  # Value type.
 T_co = typing.TypeVar('T_co', covariant=True)  # Any type covariant containers.
 T_contra = typing.TypeVar('T_contra', contravariant=True)  # Ditto contravariant.
 
@@ -603,6 +606,19 @@ def runtime_checkable(cls):
 runtime = runtime_checkable
 
 
+# Not yet ported to typing, but who knows
+if hasattr(typing, 'MappingAuxiliary'):
+    MappingAuxiliary = typing.MappingAuxiliary
+else:
+    class MappingAuxiliary(typing.Collection[KT_co], Protocol[KT_co, VT_co]):
+        """Represents the covariant sub-protocol of the Mapping type.
+        """
+        def items(self) -> ItemsView[KT_co, VT_co]: ...
+        def keys(self) -> KeysView[KT_co]: ...
+        def values(self) -> ValuesView[VT_co]: ...
+        def __contains__(self, __o: object) -> bool: ...
+
+
 # 3.8+
 if hasattr(typing, 'SupportsIndex'):
     SupportsIndex = typing.SupportsIndex