How to reference a class method in a class attribute?

Question

In a package, I have isolated a set of functions/variables that work altogether in a class. As these functions did not require a specific context, they became class methods. They may use some external functions, here represented by the save_c function. But I can't get the following code to work:

def save_c(v):
    print("c :" + v)


class Writer(object):

    @classmethod
    def save(cls, t, v):
        cls.writers[t](v)


    @classmethod
    def save_a(cls, v):
        print("a :" + v)


    @classmethod
    def save_b(cls, v):
        print("b :" + v)

    writers = [save_a, save_b, save_c]


if __name__ == "__main__":
    Writer.save(1, [1, 2, 3])

Previous code results into the following error:

TypeError: 'classmethod' object is not callable

Does anybody know if what I'm trying to do is reachable? Or am I obliged to use regular methods and instanciate a Writer wherever I need to?

Answer 1

You could also make it work by adding a wrapper class that is smarter about what calling whatever it's dealing with.

Since you mentioned in a comment that you're going to need to make the call to one of thewritersmultiple places, I've updated my answer to address this by adding a nested wrapper class to encapsulate the logic of calling the various callable types in one spot. This is a classic example of using OOP to apply the DRY principle.

def save_c(v):
    print("c :" + v)

class Writer(object):
    @classmethod
    def save(cls, t, v):
        cls.writers[t](cls, v)

    @classmethod
    def save_a(cls, v):
        print("a :" + v)

    @staticmethod
    def save_b(v):
        print("b :" + v)

    def _wrapper(writer):
        if isinstance(writer, classmethod):
            return type('ClassmethodWrapper', (object,), {'__call__':
                        lambda self, cls, v: writer.__func__(cls, str(v))})()
        elif isinstance(writer, staticmethod):
            return type('StaticmethodWrapper', (object,), {'__call__':
                        lambda self, cls, v: writer.__func__(str(v))})()
        else:
            return type('VanillaWrapper', (object,), {'__call__':
                        lambda self, cls, v: writer(str(v))})()

    writers = list(map(_wrapper, (save_a, save_b, save_c)))

if __name__ == "__main__":
    Writer.save(0, [1, 2, 3])  # -> a :[1, 2, 3]
    Writer.save(1, [4, 5, 6])  # -> b :[4, 5, 6]
    Writer.save(2, [7, 8, 9])  # -> c :[7, 8, 9]