Difference between type and class

[Steven D'Aprano]

On Thu, 31 Jul 2008 13:32:39 +0200, Thomas Troeger wrote:

>> Can someone explain to me the difference between a type and a class?
> 
> If your confusion is of a more general nature I suggest reading the
> introduction of `Design Patterns' (ISBN-10: 0201633612), under
> `Specifying Object Interfaces'.
> 
> In short: A type denotes a certain interface, i.e. a set of signatures,
> whereas a class tells us how an object is implemented (like a
> blueprint). A class can have many types if it implements all their
> interfaces, and different classes can have the same type if they share a
> common interface. 

I fear you're introducing a rather complicated answer for a simple 
question. In Python, the simple answer is that built-in objects like int, 
float, str etc. are referred to as "types", and custom objects created 
using the class keyword are referred to as "classes". This is a 
historical distinction that will disappear in time.

We can see that types and classes already have the same type in Python:

>>> class Parrot(object):
...     pass
...
>>> type(Parrot)
<type 'type'>
>>> type(str)
<type 'type'>



> The following example should clarify matters:
> 
> class A:
> 	def bar(self):
> 		print "A"


Alas, you've chosen the worst-possible example to "clarify" matters, 
because old-style classic classes are *not* unified with types, and will 
disappear in the future:

>>> class Old:  # don't inherit from object
...     pass
...
>>> type(Old)
<type 'classobj'>


So, to the Original Poster:

In Python, new-style classes and types are the same, but it is 
traditional to refer to customer objects as "class" and built-in objects 
as "types". Old-style classes are different, but you are discouraged from 
using old-style classes unless you have a specific reason for needing 
them (e.g. backwards compatibility).


-- 
Steven