What is the difference between old style and new style classes in Python?

From New-style and classic classes:

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 <type 'instance'>.

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 the concepts of class and type. A new-style class is simply a user-defined type, no more, no less.

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 immediate 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.

Python 3 only has new-style classes.

No matter if you subclass from object or not, classes are new-style in Python 3.

Declaration-wise:

New-style classes inherit from object, or from another new-style class.

class NewStyleClass(object):
    pass

class AnotherNewStyleClass(NewStyleClass):
    pass

Old-style classes don't.

class OldStyleClass():
    pass

Python 3 Note:

Python 3 doesn't support old style classes, so either form noted above results in a new-style class.

Important behavior changes between old and new style classes

• super added
• MRO changed (explained below)
• descriptors added
• new style class objects cannot be raised unless derived from Exception (example below)
• __slots__ added

MRO (Method Resolution Order) changed

It was mentioned in other answers, but here goes a concrete example of the difference between classic MRO and C3 MRO (used in new style classes).

The question is the order in which attributes (which include methods and member variables) are searched for in multiple inheritance.

Classic classes do a depth-first search from left to right. Stop on the first match. They do not have the __mro__ attribute.

class C: i = 0
class C1(C): pass
class C2(C): i = 2
class C12(C1, C2): pass
class C21(C2, C1): pass

assert C12().i == 0
assert C21().i == 2

try:
    C12.__mro__
except AttributeError:
    pass
else:
    assert False

New-style classes MRO is more complicated to synthesize in a single English sentence. It is explained in detail here. One of its properties is that a base class is only searched for once all its derived classes have been. They have the __mro__ attribute which shows the search order.

class C(object): i = 0
class C1(C): pass
class C2(C): i = 2
class C12(C1, C2): pass
class C21(C2, C1): pass

assert C12().i == 2
assert C21().i == 2

assert C12.__mro__ == (C12, C1, C2, C, object)
assert C21.__mro__ == (C21, C2, C1, C, object)

New style class objects cannot be raised unless derived from Exception

Around Python 2.5 many classes could be raised, and around Python 2.6 this was removed. On Python 2.7.3:

# OK, old:
class Old: pass
try:
    raise Old()
except Old:
    pass
else:
    assert False

# TypeError, new not derived from `Exception`.
class New(object): pass
try:
    raise New()
except TypeError:
    pass
else:
    assert False

# OK, derived from `Exception`.
class New(Exception): pass
try:
    raise New()
except New:
    pass
else:
    assert False

# `'str'` is a new style object, so you can't raise it:
try:
    raise 'str'
except TypeError:
    pass
else:
    assert False