Acts like a class method when called from a class, like an
instance method when called by an instance. The method should
take two arguments, 'self' and 'cls'; one of these will be None
depending on how the method was called.
The classinstancemethod class is accessible via the
__sizeof__() -> int
size of object in memory, in bytes
Abstract classes can override this to customize issubclass().
This is invoked early on by abc.ABCMeta.__subclasscheck__().
It should return True, False or NotImplemented. If it returns
NotImplemented, the normal algorithm is used. Otherwise, it
overrides the normal algorithm (and the outcome is cached).
for more information.