为什么从对象继承会在Python中产生影响？

Why does inheriting from object make a difference in Python?

本问题已经有最佳答案，请猛点这里访问。

当类从任何东西继承时，我有一个实例类型的对象。

1
2
3
4
5
6
7
8

>>> class A(): pass;
>>> a = A()
>>> type(a)
<type 'instance'>
>>> type(a) is A
False
>>> type(A)
<type 'classobj'>

但是，当我从一个对象继承同一个类时，创建的对象是A类型。

1
2
3
4
5
6
7
8

>>> class A(object): pass;
>>> a = A()
>>> type(a)
<class '__main__.A'>
>>> type(a) is A
True
>>> type(A)
<type 'type'>

这背后的逻辑是什么？这是否意味着每个类都应该从对象继承？

相关讨论

在Python3中，这两者是相同的。但是，在Python2中：

1
2
3

class A: pass # old-style class

class B(object): pass # new-style class

从文档中的新样式和经典类：

Up to Python 2.1, old-style classes were the only flavour available to the user. The concept of (old-style) class is unrelated to the concept of type: if x is an instance of an old-style class, then x.__class__ designates the class of x, but type(x) is always . This reflects the fact that all old-style instances, independently of their class, are implemented with a single built-in type, called instance.

New-style classes were introduced in Python 2.2 to unify classes and types. A new-style class is neither more nor less than a user-defined type. If x is an instance of a new-style class, then type(x) is the same as x.__class__.

The major motivation for introducing new-style classes is to provide a unified object model with a full meta-model. It also has a number of immediate benefits, like the ability to subclass most built-in types, or the introduction of"descriptors", which enable computed properties.

出于这些原因，随时使用新样式的类是个好主意。在python2.2+中存在旧样式类的唯一原因是为了向后兼容；在python3中，旧样式类被删除。