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

Question

What is the difference between old style and new style classes in Python? When should I use one or the other?

Answer 1

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