Python 中的类型和类
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Types and classes in Python
提问by sasuke
I'm a bit confused about types and classes in Python. For e.g. the following REPL conversation confuses me:
我对 Python 中的类型和类有点困惑。例如，以下 REPL 对话让我感到困惑：
>>> class A: pass
...
>>> a = A()
>>> type(a)
<type 'instance'>
>>> a.__class__
<class __main__.A at 0xb770756c>
>>> type([])
<type 'list'>
>>> [].__class__
<type 'list'>
>>> type(list)
<type 'type'>
>>> list.__class__
<type 'type'>
>>> type(A)
<type 'classobj'>
>>> A.__class__
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
AttributeError: class A has no attribute '__class__'
- Why is the type and class for inbuilt things (e.g. list here) the same but different for user classes/types?
- Isn't every class an instance of some other class (like Class in Java)? Why no
__class__for user defined classes?
- 为什么内置事物的类型和类（例如此处列出）与用户类/类型相同但不同？
- 不是每个类都是其他类的实例（如 Java 中的类）吗？为什么不是
__class__用户定义的类？
Any explanation/further reading which can clarify this behaviour would be much appreciated. TIA.
任何可以澄清这种行为的解释/进一步阅读将不胜感激。TIA。
采纳答案by wheaties
You're encountering the different behavior for new style classes versus classic classes. For further reading read this: Python Data Model. Specifically read the section on classes and the difference between new style and classic classes.
您会遇到新样式类与经典类的不同行为。如需进一步阅读，请阅读：Python 数据模型。具体阅读类和新式与经典类的区别部分。
Try typing the following into your REPL:
尝试在您的 REPL 中输入以下内容：
class A: pass
class B(object): pass
and you'll see that you get different results. Here you're dealing with the difference between new style and old style classes. Using Python 2.6.1 here's what I get:
你会看到你得到不同的结果。在这里，您正在处理新样式类和旧样式类之间的区别。使用 Python 2.6.1 这是我得到的：
> type(A)
<type "classobj">
> type(B)
<type "type">
which tells you that lists are new style classes and not old style classes. We can further play around with things using listas well:
它告诉您列表是新样式类而不是旧样式类。我们还可以进一步使用以下内容list：
> type(list)
<type "type">
same as our class B(object): passresult. And also
和我们的class B(object): pass结果一样。并且
> c = []
> type(c)
<type "list">
which is telling you about the instance of the object and not it's definition.
这是告诉你对象的实例而不是它的定义。
回答by Lennart Regebro
It's "Hystorical reasons". Or possible "Histerical". It's all fixed in Python 3:
这是“历史原因”。或者可能的“历史”。这一切都在 Python 3 中修复了：
>>> class A: pass
...
>>> a = A()
>>> type(a)
<class '__main__.A'>
>>> a.__class__
<class '__main__.A'>
>>> type([])
<class 'list'>
>>> [].__class__
<class 'list'>
>>> type(list)
<class 'type'>
>>> list.__class__
<class 'type'>
>>> type(A)
<class 'type'>
>>> A.__class__
<class 'type'>
>>> class B(object): pass
...
>>> type(B)
<class 'type'>
>>> b = B()
>>> type(b)
<class '__main__.B'>
回答by user2352151
In Python 3.0, user-defined class objects are instances of the object named type, which is itself a class. ? In Python 2.6, new-style classes inherit from object, which is a subclass of type; ? classic classes are instances of type and are not created from a class.
在 Python 3.0 中，用户定义的类对象是名为 type 的对象的实例，它本身就是一个类。? 在 Python 2.6 中，新式类继承自 object，它是 type 的子类；? 经典类是类型的实例，不是从类创建的。
