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()         


        

Answer 1

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.

Answer 2

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'>

Answer 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.