simple module import time constraint

constraint.py

class BaseMeta(type):
    def __new__(cls, name, bases, dict):
        if 'foo' not in dict:
            raise RuntimeError('class must have its own foo method!')
        return type.__new__(cls, name, bases, dict)

class Base(metaclass=BaseMeta):
    def foo(self): pass

class Derived(Base):
    def foo(self): pass