Writing a class that accepts a callback in Python?

Question

I need to write a class that allows a subclass to set an attribute with the name of a function. That function must then be callable from instances of the class.

For example, I say I need to write a Fruit class where the subclass can pass in a welcome message. The Fruit class must expose an attribute print_callback that can be set.

class Fruit(object):
    print_callback = None

    def __init__(self, *args, **kwargs):
        super(Fruit, self).__init__(*args, **kwargs)
        self.print_callback("Message from Fruit: ")

I need to expose an API that is can be consumed by this code (to be clear, this code cannot change, say it is 3rd party code):

def apple_print(f):
    print "%sI am an Apple!" % f

class Apple(Fruit):
    print_callback = apple_print

If I run:

mac = Apple()

I want to get:

Message from Fruit: I am an Apple!

Instead I get:

TypeError: apple_print() takes exactly 1 argument (2 given)

I think this is because self is passed in as the first argument.

So how do I write the Fruit class? Thanks!

Answer 1

You can use directly:

class Apple(Fruit):
    print_callback = staticmethod(apple_print)

or:

class Apple(Fruit):
    print_callback = classmethod(apple_print)

In the first case, you'll get only one parameter (the original). In the second, you'll receive two parameters where the first will be the class on which it was called.

Hope this helps, and is shorter and less complex.