实例是“对象"，但类不是“对象"的子类:这怎么可能? [英] Instance is an "object", but class is not a subclass of "object": how is this possible?

问题描述

如何有一个类为object的实例，而该类不是object的子类呢?这是一个例子:

How is it possible to have an instance of a class which is an object, without the class being a subclass of object? here is an example:

>>> class OldStyle(): pass
>>> issubclass(OldStyle, object)
False
>>> old_style = OldStyle()
>>> isinstance(old_style, object)
True

推荐答案

在Python 2中， type 和 class 不同，特别是对于旧样式类，type(obj) 与obj.__class__不同的对象 .因此这是可能的，因为老式类的实例实际上是与其类不同的类型(instance):

In Python 2, type and class are not the same thing, specifically, for old-style classes, type(obj) is not the same object as obj.__class__. So it is possible because instances of old-style classes are actually of a different type (instance) than their class:

>>> class A(): pass
>>> class B(A): pass
>>> b = B()

>>> assert b.__class__ is B
>>> issubclass(b.__class__, A) # same as issubclass(B, A)
True
>>> issubclass(type(b), A)
False

>>> type(b)
<type 'instance'>
>>> b.__class__
<class __main__.B at 0x10043aa10>

这在新式的类中得到解决:

This is resolved in new-style classes:

>>> class NA(object): pass
>>> class NB(NA): pass
>>> nb = NB()
>>> issubclass(type(nb), NA)
True
>>> type(nb)
<class '__main__.NB'>
>>> nb.__class__
<class '__main__.NB'>

旧类不是类型，新类是:

Old-style class is not a type, new-style class is:

>>> isinstance(A, type)
False
>>> isinstance(NA, type)
True

旧样式类被声明为已弃用.在Python 3中，只有新型类. class A()等同于class A(object)，并且您的代码在两次检查中都将产生True.

Old style classes are declared deprecated. In Python 3, there are only new-style classes; class A() is equivalent to class A(object) and your code will yield True in both checks.

请看以下问题进行更多讨论:什么是python中旧样式类与新样式类之间的区别?

Take a look at this question for some more discussion: What is the difference between old style and new style classes in Python?

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