metaclass

A metaclass is a class whose instances are classes. In object-oriented programming, a metaclass defines the behavior of classes themselves, including how they are created, what attributes they have, and how they participate in inheritance. Metaclasses provide a way to customize class construction and to impose structure or behavior on a group of classes.

In Python, every class has a metaclass, with the default being type. A class can specify a metaclass by using a metaclass declaration, and that metaclass governs the creation and initialization of the class object. A metaclass can implement methods such as __new__ and __init__ to control how the class object is created and initialized, and __prepare__ to customize the class namespace before it is populated. It can also override __call__ to influence how instances of the class are created, effectively tying instance creation to the metaclass.

Metaclasses are commonly used to enforce coding standards, inject common methods, register classes automatically, or implement cross-cutting behavior such as logging, validation, or access control at class creation time. They can also be used by libraries and frameworks, for example to define base behaviors for data models in an ORM, to automatically map class attributes to fields, or to collect metadata about subclasses.

A key distinction is that a class is a blueprint for objects, while a metaclass is a blueprint for classes. Using metaclasses adds power and flexibility, but also complexity, so they are typically employed when substantial control over class creation or organization is needed. Some languages offer similar capabilities under different names (such as eigenclasses in Ruby), but the explicit metaclass mechanism is most closely associated with Python.