python - 没有抽象方法的抽象类在python中创建对象

Question

基本上，我知道抽象基类就像常规类一样被用作骨架类，但是强制抽象方法应该被子类/继承类覆盖，如果它有一个如下所示

Class AbstractClass(object):
    __metaclass__ = abc.ABCMeta

    @abstractmethod
    def absmethod(self):
        pass

class ChildClass(AbstractClass):
    def absmethod(self):
        print "Yeah!"

obj = ChildClass()

所以我们可以像上面那样创建一个ChildClass的对象

我们知道我们不能实例化一个抽象类，因为它意味着只是骨架，如果我们尝试如下实例化它，我们会得到一个错误

obj = AbstractClass()

*** TypeError: Can't instantiate abstract class AbstractClass with abstract methods absmethod

但我对发布此 StackOverflow 的实际查询是，如果我们使用 abc.ABCMeta 创建一个抽象类，没有抽象方法，我可以创建一个实例抽象类不应该是这样的(如果我错了请纠正我)

Class AbstractClass(object):
    __metaclass__ = abc.ABCMeta

obj = AbstractClass()

OOOPPPSS 它起作用了，我们真的可以创建一个没有抽象方法的抽象类对象吗？所以请让我知道这背后的关键点

Answer 1

来自 the docs :

A class that has a metaclass derived from ABCMeta cannot be instantiated unless all of its abstract methods and properties are overridden.

相反，这意味着任何没有抽象方法或属性的类(如您的 AbstractClass)都可以被实例化。

---

如果您想禁止最顶层父类的实例化，您可以编写一个自定义类，在其 __new__ 中执行类型检查。方法:

class SubclassOnlyABC(object):
    __metaclass__ = abc.ABCMeta

    def __new__(cls, *args, **kwargs):
        if cls.__bases__ == (SubclassOnlyABC,):
            msg = 'Abstract class {} cannot be instantiated'.format(cls.__name__)
            raise TypeError(msg)

        return super(SubclassOnlyABC, cls).__new__(cls, *args, **kwargs)

class AbstractClass(SubclassOnlyABC):
    pass

class ChildClass(AbstractClass):
    pass

ChildClass()  # works because it's a child class of an abstract class
AbstractClass()  # throws TypeError because its parent class is "object"

---

您还可以编写一个 __new__ 方法来防止实例化没有抽象方法的类:

class NonEmptyABC(object):
    __metaclass__ = abc.ABCMeta

    def __new__(cls, *args, **kwargs):
        # check if ANY abstractmethod exists
        for parentcls in cls.__mro__:
            if any(getattr(attr, '__isabstractmethod__', False)
                               for attr in vars(parentcls).values()):
                break
        else:
            msg = 'Abstract class {} cannot be instantiated'.format(cls.__name__)
            raise TypeError(msg)

        return super(NonEmptyABC, cls).__new__(cls, *args, **kwargs)

class EmptyAbstractClass(NonEmptyABC):
    pass

class NonemptyAbstractClass(NonEmptyABC):
    @abc.abstractmethod
    def foo(self):
        pass

class NonemptyChild(NonemptyAbstractClass):
    def foo(self):
        pass

NonemptyChild()  # works because "foo" is an abstractmethod
EmptyAbstractClass()  # throws TypeError because there are no abstractmethods