Expert Python Programming - Third Edition

Metaclass pitfalls

Like some other advanced Python features, the metaclasses are very elastic and can be easily abused. While the call signature of the class is rather strict, Python does not enforce the type of the return parameter. It can be anything as long as it accepts incoming arguments on calls and has the required attributes whenever it is needed.

One such object that can be anything-anywhere is the instance of the Mock class that's provided in the unittest.mock module. Mock is not a metaclass and also does not inherit from the type class. It also does not return the class object on instantiating. Still, it can be included as a metaclass keyword argument in the class definition, and this will not raise any syntax errors. Using Mock ...