Python in a Nutshell

Metaclasses

Any object, even a class object, has a type. In Python, types and classes are also first-class objects. The type of a class object is also known as the class’s metaclass.^[2] An object’s behavior is determined largely by the type of the object. This also holds for classes: a class’s behavior is determined largely by the class’s metaclass. Metaclasses are an advanced subject, and you may want to skip the rest of this chapter on first reading. However, fully grasping metaclasses can help you obtain a deeper understanding of Python, and sometimes it can even be useful to define your own custom metaclasses.

The distinction between classic and new-style classes relies on the fact that each class’s behavior is determined by its metaclass. In other words, the reason classic classes behave differently from new-style classes is that classic and new-style classes are object of different types (metaclasses):

class Classic: pass
class Newstyle(object): pass
print type(Classic)                  # prints: <type 'class'>
print type(Newstyle)                 # prints: <type 'type'>

The type of Classic is object types.ClassType from standard module types, while the type of Newstyle is built-in object type. type is also the metaclass of all Python built-in types, including itself (i.e., print type(type) also prints <type 'type'>).

How Python Determines a Class’s Metaclass

To execute a class statement, Python first collects the base classes into a tuple t (an empty one, if there are no base classes) and executes the class ...