Abstract base class for generic types.
On Python 3.12 and newer, generic classes implicitly inherit from
Generic when they declare a parameter list after the class's name::
class Mapping[KT, VT]:
def __getitem__(self, key: KT) -> VT:
...
# Etc.
On older versions of Python, however, generic classes have to
explicitly inherit from Generic.
After a class has been declared to be generic, it can then be used as
follows::
def lookup_name[KT, VT](mapping: Mapping[KT, VT], key: KT, default: VT) -> VT:
try:
return mapping[key]
except KeyError:
return default