type of class in python

Question

why if I do:

class C(): pass
type(C())

I got: <type 'instance'>, but if I do:

class C(object): pass
type(c())

I got: <class '__main__.c'> ?

The first is not very userfull

Answer 1

Honestly, I don't think a short answer is in order here. I advise you to start reading about Python's type unification / new-style classes here: http://www.python.org/doc/newstyle/

Answer 2

Look up the difference between old-style and new-style classes. The former are the default, and the latter inherit explicitly from object.

All old-style objects were implemented with the built-in type instance. The fact that they are still the default and their type remains 'instance' is a result of retro-compatibility precautions.

This is extracted from the Python docs (http://docs.python.org/reference/datamodel.html)

3.3. New-style and classic classes Classes and instances come in two flavors: old-style (or classic) and new-style.

Up to Python 2.1, old-style classes were the only flavour available to the user. The concept of (old-style) class is unrelated to the concept of type: if x is an instance of an old-style class, then x.class designates the class of x, but type(x) is always . This reflects the fact that all old-style instances, independently of their class, are implemented with a single built-in type, called instance.

New-style classes were introduced in Python 2.2 to unify classes and types. A new-style class is neither more nor less than a user-defined type. If x is an instance of a new-style class, then type(x) is typically the same as x> .class (although this is not guaranteed - a new-style class instance is permitted to override the value returned for x.class).

The major motivation for introducing new-style classes is to provide a unified object model with a full meta-model. It also has a number of practical benefits, like the ability to subclass most built-in types, or the introduction of “descriptors”, which enable computed properties.

For compatibility reasons, classes are still old-style by default. New-style classes are created by specifying another new-style class (i.e. a type) as a parent class, or the “top-level type” object if no other parent is needed. The behaviour of new-style classes differs from that of old-style classes in a number of important details in addition to what type() returns. Some of these changes are fundamental to the new object model, like the way special methods are invoked. Others are “fixes” that could not be implemented before for compatibility concerns, like the method resolution order in case of multiple inheritance.

While this manual aims to provide comprehensive coverage of Python’s class mechanics, it may still be lacking in some areas when it comes to its coverage of new-style classes. Please see http://www.python.org/doc/newstyle/ for sources of additional information.

Old-style classes are removed in Python 3.0, leaving only the semantics of new-style classes.of new-style classes.