Python使用abstractmethod的不同行为 [英] Python different behaviour with abstractmethod

问题描述

我有两个继承自同一父 P 的类：

I have two classes inheriting from the same parent P:

from abc import ABCMeta, abstractmethod

class P(object):

    __metaclass__ = ABCMeta

    @abstractmethod  
    def foo(self):
        pass

class C(P):
    pass

class D(tuple, P):
    pass

唯一的区别是 D 继承自元组和 P ，而 C 则继承自 P

The only difference is that D inherited from tuple and P while C inherits from P only.

现在这是行为： c = C（）出现错误，按预期：

Now this is the behavior: c = C() got error, as expected:

TypeError: Can't instantiate abstract class C with abstract methods foo

但 d = D（）可以正常工作！

我什至可以呼叫 d.foo（）。

推荐答案

在 object .__ new __ 中测试了抽象方法。 code>方法；当您从具有自己的 __ new __ 方法的 tuple 继承时， object .__ new __ ，也不会对抽象方法进行测试。

Abstract methods are tested for in the object.__new__ method; when you inherit from tuple, which has its own __new__ method, object.__new__ is not called and the test for abstract methods is not made.

换句话说，将抽象方法与 any 混合内置的不可变类型将导致此问题。

In other words, mixing abstract methods with any of the built-in immutable types will cause this problem.

唯一有效的解决方案是在 __ new __ 中进行 own 测试仅当在子类的两个基中混合时将抽象类放在之前 元组。

The only solution that works is to do your own test in __new__ and then only if you put your abstract class before tuple when mixing in the two bases in a subclass.

class P(object):
    __metaclass__ = ABCMeta

    def __new__(cls, *args, **kwargs):
        super_new = super(P, cls).__new__
        if super_new.__self__ is not object:
            # immutable mix-in used, test for abstract methods
            if getattr(cls, '__abstractmethods__'):
                raise TypeError(
                    "Can't instantiate abstract class %s "
                    "with abstract methods %s" % (
                        cls.__name__,
                        ', '.join(sorted(cls.__abstractmethods__))))

        return super_new(cls, *args, **kwargs)

    @abstractmethod  
    def foo(self):
        pass

class D(P, tuple):
    pass

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