Can I write abc.ABC without resorting to metaclasses in Python 3.6?

Question

Python 3.6 added PEP 487, which adds an __init_subclass__ method among other things. Is it possible to write a version of ABC that doesn't use a metaclass?

Answer 1

If all you care about is the check for abstract methods, then yes. Just move the abstract method set computation to the __init_subclass__ method:

def __init_subclass__(cls, **kwargs):
    super().__init_subclass__(**kwargs)
    # Compute set of abstract method names
    abstracts = {name
                 for name, value in vars(cls).items()
                 if getattr(value, "__isabstractmethod__", False)}
    for base in cls.__bases__:
        for name in getattr(base, "__abstractmethods__", set()):
            value = getattr(cls, name, None)
            if getattr(value, "__isabstractmethod__", False):
                abstracts.add(name)
    cls.__abstractmethods__ = frozenset(abstracts)

The base object.__new__ implementation then uses an non-empty __abstractmethods__ set to prevent instantiation.

But ABCs also support virtual subclass registration; the two hook methods this requires have to be implemented on the metaclass.