README.md

classmaker

Make complex classes avoiding metaclass conflicts.

Rationale

Python sometimes cops out of building a class when you were intuitively expecting there to be no problems. The classic situation is as follows:

>>> class MetaClassA(type):
...     pass

>>> class MetaClassB(type):
...     pass

>>> class BaseClassA(metaclass=MetaClassA):
...     pass

>>> class BaseClassB(metaclass=MetaClassB):
...     pass

>>> class MyClass(BaseClassA, BaseClassB):
...     pass
Traceback (most recent call last):
TypeError: metaclass conflict: the metaclass of a derived class must be a (non-strict) subclass of the metaclasses of all its bases

Creating the last class raises TypeError: metaclass conflict: the metaclass of a derived class must be a (non-strict) subclass of the metaclasses of all its bases (on python2 and python3), because python refuses to guess exactly what you wanted the metaclass of MyClass to be (should it be class MetaClass(MetaClassA, MetaClassB), or class MetaClass(MetaClassA, MetaClassB), for example).

However, 99% of the time, you don't really care, you just want it to work, generally because the metaclasses are unrelated. classmaker will make the decision for you! This is what it looks like (using the class hierarchy above):

>>> from classmaker import classbuilder

>>> @classbuilder(bases=(BaseClassA, BaseClassB))
... class MyClass:
...    pass

>>> assert isinstance(MyClass, MetaClassA)
>>> assert isinstance(MyClass, MetaClassB)

>>> mc = MyClass()
>>> assert isinstance(mc, BaseClassA)
>>> assert isinstance(mc, BaseClassB)

Credits

The smart bit of this was written by Michele Simionato in his recipe solving the metaclass conflict. David Park packaged this up and add the somewhat more convenient @classbuilder decorator.