Python: pass a class-function as a reference to an external function

September 26, 2022

I have a class with several functions.
From outside that class, I want to specify with a reference which function to call – but I’m not sure how.

For example, I have an Animal class with two functions sound and food. I want to write a Zoo class which receives one of the Animal‘s functions as input and applies that function to every animal instance it holds (the function all_animals_features).

class Animal:
    def __init__(self, sound, food):
        self.my_sound = sound
        self.my_food = food

    def sound(self):
        # Do some complicated stuff....
        return self.my_sound

    def food(self):
        return self.my_food


class Zoo():
    def __init__(self, animals):
        self.animals = animals

    def all_animals_features(self, f):
        return [animal.f() for animal in self.animals]

dog = Animal('Woof', 'Bone')
cat = Animal('Meow', 'Cream')
zoo = Zoo([cat, dog])
zoo.all_animals_features(Animal.sound)

But of course, 'Animal' object has no attribute 'f'.

Any idea how can this be implemented?

Clarification: if all that is needed, as demonstrated by this silly example, is just getting an attribute then it may be simpler to use getattr().

>Solution :

In your case you just need to adjust the way the method is called:

class Zoo():
    def __init__(self, animals):
        self.animals = animals

    def all_animals_features(self, f):
        return [f(animal) for animal in self.animals]

dog = Animal('Woof', 'Bone')
cat = Animal('Meow', 'Cream')
zoo = Zoo([cat, dog])
print(zoo.all_animals_features(Animal.sound))

Output: