sasuke sasuke - 1 year ago 47
Python Question

Types and classes in Python

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__'

  1. Why is the type and class for inbuilt things (e.g. list here) the same but different for user classes/types?

  2. Isn't every class an instance of some other class (like Class in Java)? Why no
    for user defined classes?

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


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.