Make abc.ABC a regular class by making __instancecheck__ and __subclasscheck__ class methods

[blhsing (Ben Hsing)]

@mikeshardmind

I generally think the ABC method is worse than using protocols (runtime checkable ones, even, if needed), but ABC came first, and comes with runtime enforcement that subclasses implemented the abstract methods.

ABC and Protocol serve entirely different use cases. ABC enforces structure of a subclass, while Protocol enforces structure of an instance. Moreover, ABC implements actual logical behaviors while Protocol does none.

I think my recent use case of ABC demonstrates its purpose well:

Context manager protocol extension (2nd attempt)

Note that we can make it a mixin class too for easier usage: from abc import ABC, abstractmethod from contextlib import contextmanager class ContextManagerMixin(ABC): def __init__(self): self._contexts = [] def __init_subclass__(cls): cls.__with__ = contextmanager(cls.__with__) @abstractmethod def __with__(self): ... def __enter__(self): context = self.__with__() self._contexts.append(context) return context.__enter__() def __…

But now imagine how someone who wishes to use this mixin class to easily gain this small capability of a context manager, only to end up with a metaclass conflict with a big metaclass-based base class that actually legitimately needs a metaclass, all because of __instancecheck__ and __subclasscheck__ that aren’t even used.