In Python, a metaclass is a class whose instances are classes. Just as an ordinary class defines the behavior of certain objects, a metaclass allows for the customization of class instantiation.

The functionality of the metaclass partly coincides with that of class decorators, but metaclasses act in a different way than decorators:

• decorators bind the names of decorated functions or classes to new callable objects. Class decorators are applied when classes are instantiated;
• metaclasses redirect class instantiations to dedicated logic, contained in metaclasses. Metaclasses are applied when class definitions are read to create classes, well before classes are instantiated.

Metaclasses usually enter the game when we program advanced modules or frameworks, where a lot of precise automation must be provided.

The typical use cases for metaclasses:

• logging;
• registering classes at creation time;
• interface checking;
• automatically adding new methods;
• automatically adding new variables.