A metaclass is a class used to create a class. Normal classes are instances of the type class by default. Metaclasses are usually subclasses of the type class, which redefines class creation protocol methods in order to customize the class creation call issued at the end of a class statement; they typically redefine the methods __new__ and __init__ to tap into the class creation protocol. Metaclasses can also be coded other ways—as simple functions, for example—but they are responsible for making and returning an object for the new class.

In Python 3.0 and later, use a keyword argument in the class header line: class C(metaclass=M). In Python 2.X, use a class attribute instead: __metaclass__ = M. In 3.0, the class header line can also name normal superclasses (a.k.a. base classes) before the metaclass keyword argument.