我可以在 Python 3.6 中编写 abc.ABC 而不求助于元类吗？

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 不使用元类？

Answer 1

如果您只关心抽象方法的检查，那么是的。只需将 abstract method set computation 移动到 __init_subclass__ 方法：

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)

基础 object.__new__ implementation 然后使用非空 __abstractmethods__ 集来防止实例化。

但是ABCs也支持virtual subclass registration; the two hook methods this requires有在元类上实现

我可以在 Python 3.6 中编写 abc.ABC 而不求助于元类吗？

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

python

metaclass

abc