Python 3 Metaclass in 3 Minutes

Python metaclass is a mechanism for customizing class creation.

The default class creation is by calling:

type(name, bases, namespace)

You may a customize a class creation by:

class SomeClass(metaclass=metaclass, **kwds): pass

…or by inheriting from a metaclass-created class.

Basically this is how a class is created:

1. An appropriate metaclass is determined. (If it is not unique, a TypeError is thrown.)

2. If you implement __prepare__, a dict-like object (usually called namespace) is created by:

   metaclass.__prepare__(name, bases, **kwds)
   

   Note: __prepare__ is usually a class method (less often a static method).

3. The class body is executed approximately as:

   exec(body, globals(), namespace)
   

4. The class object is created by calling:

   metaclass(name, bases, namespace, **kwds)
   

   • The metaclass is usually inherited from type, and you usually implement __new__ (less often __init__).
   • (As a matter of fact, metaclass can be any callable object.)

5. The class object is passed to any class decorators.

By the way, __new__() is a static method (special-cased so that you need not to declare it as such).