Python - extra keyword(?) and inheritance

Question

typing.py (from Python 3.6.6 as bundled with Anaconda) declares List class as follows:

class List(list, MutableSequence[T], extra=list):

As far as my understanding goes, it means that List class inherits from list and MutableSequence[T]). What is the meaning of extra assignment in the inheritance list?

Answer 1

In typing.py class GenericMeta takes the extra keyword argument. The extra argument is simply one of the arguments that GenericMeta takes for internal bookeeping. The updates happen here in the __new__ of GenericMeta:

namespace.update({'__origin__': origin, '__extra__': extra,
                      '_gorg': None if not origin else origin._gorg})

From this point on cls.__extra__ becomes part of Typing's internal API, much like __getattr__ or __len__. From the source code, it looks like __extra__ is used to help set attributes for the class that it's passed into:

def __init__(self, *args, **kwargs):
    super(GenericMeta, self).__init__(*args, **kwargs)
    if isinstance(self.__extra__, abc.ABCMeta):
        self._abc_registry = self.__extra__._abc_registry
        self._abc_cache = self.__extra__._abc_cache
    elif self.__origin__ is not None:
        self._abc_registry = self.__origin__._abc_registry
        self._abc_cache = self.__origin__._abc_cache

This code uses __extra__ to set _abc_registry and _abc_cache