众所周知，Python 3.x中的所有类都继承自object Base Class.因此，当我创建这样的空类时:
class A:
pass
我可以拨打callable(A)或hasattr(A, '__call__')，他们会显示True.
>>> callable(A)
True
>>> hasattr(A, '__call__')
True
__call__方法显然应该在对象类中，但令人惊讶的是，它不是:
>>> object.__dict__
mappingproxy({'__new__': <built-in method __new__ of type object at 0x10a155940>, '__repr__': <slot wrapper '__repr__' of 'object' objects>, '__hash__': <slot wrapper '__hash__' of 'object' objects>, '__str__': <slot wrapper '__str__' of 'object' objects>, '__getattribute__': <slot wrapper '__getattribute__' of 'object' objects>, '__setattr__': <slot wrapper '__setattr__' of 'object' objects>, '__delattr__': <slot wrapper '__delattr__' of 'object' objects>, '__lt__': <slot wrapper '__lt__' of 'object' objects>, '__le__': <slot wrapper '__le__' of 'object' objects>, '__eq__': <slot wrapper '__eq__' of 'object' objects>, '__ne__': <slot wrapper '__ne__' of 'object' objects>, '__gt__': <slot wrapper '__gt__' of 'object' objects>, '__ge__': <slot wrapper '__ge__' of 'object' objects>, '__init__': <slot wrapper '__init__' of 'object' objects>, '__reduce_ex__': <method '__reduce_ex__' of 'object' objects>, '__reduce__': <method '__reduce__' of 'object' objects>, '__getstate__': <method '__getstate__' of 'object' objects>, '__subclasshook__': <method '__subclasshook__' of 'object' objects>, '__init_subclass__': <method '__init_subclass__' of 'object' objects>, '__format__': <method '__format__' of 'object' objects>, '__sizeof__': <method '__sizeof__' of 'object' objects>, '__dir__': <method '__dir__' of 'object' objects>, '__class__': <attribute '__class__' of 'object' objects>, '__doc__': 'The base class of the class hierarchy.\n\nWhen called, it accepts no arguments and returns a new featureless\ninstance that has no instance attributes and cannot be given any.\n'})
>>> dir(object)
['__class__', '__delattr__', '__dir__', '__doc__', '__eq__', '__format__', '__ge__', '__getattribute__', '__getstate__', '__gt__', '__hash__', '__init__', '__init_subclass__', '__le__', '__lt__', '__ne__', '__new__', '__reduce__', '__reduce_ex__', '__repr__', '__setattr__', '__sizeof__', '__str__', '__subclasshook__']
那么问题是:如果A类不实现__call__，为什么它可以调用？嗯，type班怎么样？
>>> type(A)
<class 'type'>
>>> type.__dict__
mappingproxy({'__new__': <built-in method __new__ of type object at 0x10a155110>, '__repr__': <slot wrapper '__repr__' of 'type' objects>, '__call__': <slot wrapper '__call__' of 'type' objects>, '__getattribute__': <slot wrapper '__getattribute__' of 'type' objects>, '__setattr__': <slot wrapper '__setattr__' of 'type' objects>, '__delattr__': <slot wrapper '__delattr__' of 'type' objects>, '__init__': <slot wrapper '__init__' of 'type' objects>, '__or__': <slot wrapper '__or__' of 'type' objects>, '__ror__': <slot wrapper '__ror__' of 'type' objects>, 'mro': <method 'mro' of 'type' objects>, '__subclasses__': <method '__subclasses__' of 'type' objects>, '__prepare__': <method '__prepare__' of 'type' objects>, '__instancecheck__': <method '__instancecheck__' of 'type' objects>, '__subclasscheck__': <method '__subclasscheck__' of 'type' objects>, '__dir__': <method '__dir__' of 'type' objects>, '__sizeof__': <method '__sizeof__' of 'type' objects>, '__basicsize__': <member '__basicsize__' of 'type' objects>, '__itemsize__': <member '__itemsize__' of 'type' objects>, '__flags__': <member '__flags__' of 'type' objects>, '__weakrefoffset__': <member '__weakrefoffset__' of 'type' objects>, '__base__': <member '__base__' of 'type' objects>, '__dictoffset__': <member '__dictoffset__' of 'type' objects>, '__name__': <attribute '__name__' of 'type' objects>, '__qualname__': <attribute '__qualname__' of 'type' objects>, '__bases__': <attribute '__bases__' of 'type' objects>, '__mro__': <attribute '__mro__' of 'type' objects>, '__module__': <attribute '__module__' of 'type' objects>, '__abstractmethods__': <attribute '__abstractmethods__' of 'type' objects>, '__dict__': <attribute '__dict__' of 'type' objects>, '__doc__': <attribute '__doc__' of 'type' objects>, '__text_signature__': <attribute '__text_signature__' of 'type' objects>, '__annotations__': <attribute '__annotations__' of 'type' objects>, '__type_params__': <attribute '__type_params__' of 'type' objects>})
>>> dir(type)
['__abstractmethods__', '__annotations__', '__base__', '__bases__', '__basicsize__', '__call__', '__class__', '__delattr__', '__dict__', '__dictoffset__', '__dir__', '__doc__', '__eq__', '__flags__', '__format__', '__ge__', '__getattribute__', '__getstate__', '__gt__', '__hash__', '__init__', '__init_subclass__', '__instancecheck__', '__itemsize__', '__le__', '__lt__', '__module__', '__mro__', '__name__', '__ne__', '__new__', '__or__', '__prepare__', '__qualname__', '__reduce__', '__reduce_ex__', '__repr__', '__ror__', '__setattr__', '__sizeof__', '__str__', '__subclasscheck__', '__subclasses__', '__subclasshook__', '__text_signature__', '__type_params__', '__weakrefoffset__', 'mro']
正是在这里，给出了__call__方法，是否意味着A类实际上继承自类型类？
>>> isinstance(A, type)
True
>>> issubclass(A, type)
False
搞什么？好吧，A可能是类型类型的类对象本身(对于解释器)，但不是类型的子集，好吧，让我们判断object、type和A的MRO:
>>> A.mro()
[<class '__main__.A'>, <class 'object'>]
>>> object.mro()
[<class 'object'>]
>>> type.mro(object)
[<class 'object'>]
>>> type.mro(A)
[<class '__main__.A'>, <class 'object'>]
>>> type.mro(type)
[<class 'type'>, <class 'object'>]
我在这里:既然类型类是对象的子集，而A类是类型类型，为什么MRO不在列表中显示类型类？如果object base class没有__call__方法并且type类既不是A的超类也不是对象，为什么A仍然是callable？