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

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

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.

Also, MRO (Method Resolution Order) changed:

• Classic classes do a depth first search from left to right. Stop on first match. They do not have the mro attribute.
• New-style classes MRO is more complicated to synthesize in a single English sentence. 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.

Some other notes:

• New style class objects cannot be raised unless derived from Exception.
• Old style classes are still marginally faster for attribute lookup.