Python class extensions a la Objective-C "categories"

class_extend.py

#class extensions a la Objective-C "categories"
#using metaclasses

excluded_methods = frozenset(["__module__", "__qualname__"])

def class_extend(cls):
    class Meta(type):
        def __new__(self, name, bases, attrs, **kwargs):
            for name, value in attrs.items():
                if name not in excluded_methods:
                    setattr(cls, name, value)
            return cls

    return Meta

# example of class extension follows

# define a base class
class FooBase(object):
    def foo(self):
        return 42

# override method
class Foo(FooBase):
    def foo(self):
        return 43

# make an instance
foo = Foo()

# let's extend Foo. Note that super() will start lookup in FooBase
class Foo(metaclass=class_extend(Foo)):
    def foo(self):
        return super().foo()

# yes we get 42
print(foo.foo())