Good real-world uses of metaclasses (e.g. in Python)

Question

I'm learning about metaclasses in Python. I think it is a very powerful technique, and I'm looking for good uses for them. I'd like some feedback of good useful real-world examples of using metaclasses. I'm not looking for example code on how to write a metaclass (there are plenty examples of useless metaclasses out there), but real examples where you have applied the technique and it was really the appropriate solution. The rule is: no theoretical possibilities, but metaclasses at work in a real application.

I'll start with the one example I know:

• Django models, for declarative programming, where the base class Model uses a metaclass to fill the model objects of useful ORM functionality from the attribute definitions.

Looking forward to your contributions.

Answer 1

In Python 2.6 and 3.1, the Python standard library provides an abc.ABCMeta, a meta-class for Abstract Base Classes ("ABCs"). Classes that use the meta-class can use @abstractmethod and @abstractproperty to define abstract methods and properties. The meta-class will ensure that derived classes override the abstract methods and properties.

Also, classes that implement the ABC without actually inheriting from it can register as implementing the interface, so that issubclass and isinstance will work.

For example, the collections module defines the Sequence ABC. It also calls Sequence.register(tuple) to register the built-in tuple type as a Sequence, even though tuple does not actually inherit from Sequence.