By default, new-style classes are constructed using type().
A class definition is read into a separate namespace and the value
of class name is bound to the result of
type(name, bases, dict).
When the class definition is read, if __metaclass__ is defined then the callable assigned to it will be called instead of type(). The allows classes or functions to be written which monitor or alter the class creation process:
dict. Upon class creation, the callable is used instead of the built-in type(). New in version 2.2.
The appropriate metaclass is determined by the following precedence rules:
dict['__metaclass__']exists, it is used.
The potential uses for metaclasses are boundless. Some ideas that have been explored including logging, interface checking, automatic delegation, automatic property creation, proxies, frameworks, and automatic resource locking/synchronization.