Types and classes in Python

Question

I'm a bit confused about types and classes in Python. For e.g. the following REPL conversation confuses me:

>>> 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?

Any explanation/further reading which can clarify this behaviour would be much appreciated. TIA.

You should define your classes with class A(object). Then you will get <class '__main__.A'> for type(a). — Felix Kling
– Felix Kling, Commented Dec 18, 2010 at 19:50
This question is similar to: Class vs. Type in Python. If you believe it’s different, please edit the question, make it clear how it’s different and/or how the answers on that question are not helpful for your problem. — shea
– shea, Commented Dec 16, 2024 at 21:28

Zero Piraeus · Accepted Answer · 2013-06-22 14:10:29Z

16

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.

Try typing the following into your REPL:

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:

> 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 list as well:

> type(list)
<type "type">

same as our class B(object): pass result. And also

> c = []
> type(c)
<type "list">

which is telling you about the instance of the object and not it's definition.

edited Jun 22, 2013 at 14:10

Zero Piraeus

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answered Dec 18, 2010 at 19:51

wheaties

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3 Comments

sasuke Over a year ago

It seems that my wrong was with not using new typeclasses which ended up giving me weird results (surely for backward compatibility). The link looks interesting. Thanks.

Smart Manoj Over a year ago

In Python 3.11.9, type(A) returns <class 'type'>

shea Dec 16, 2024 at 21:14

@SmartManoj this answer is out of date, see the other responses

Lennart Regebro · Accepted Answer · 2010-12-18 20:02:20Z

6

It's "Hystorical reasons". Or possible "Histerical". It's all fixed in Python 3:

>>> 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'>

answered Dec 18, 2010 at 20:02

Lennart Regebro

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2 Comments

wheaties Over a year ago

That's nice to know. Didn't realize Python 3 had fixed the issue all together. I assume then, that all classes regardless of explicitly deriving from object derive from object then?

shea Dec 16, 2024 at 21:22

Beginner's note that type(class) is invaild, but type(type) is <class 'type'>. the former might be less suprising if you compare to type(def).

user2352151 · Accepted Answer · 2016-05-22 08:23:04Z

3

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.

answered May 22, 2016 at 8:23

user2352151

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