sasuke sasuke - 1 year ago 55
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.

Answer Source

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.