Issues of choosing classmethod over inheritance in Python

Question

What are the main issues I can come across if I choose to use classmethod over Inheritance in Python? A (silly) example would be:

class Pizza(object):
    def __init__(self, ingredients):
        self.ingredients = ingredients
        self.cooked = False

    def cook(self):        
        print "cooking"
        self.cooked = True

    # A pepperoni pizza using a factory method
    @classmethod
    def Pepperoni(cls):
        return cls("pepperoni")

# An onion pizza inheriting from Pizza base class
class Onion(Pizza):
    def __init__(self):
        ingredient = "onion"
        super(Onion, self).__init__(ingredient)

I know that

I cannot (easily) add custom methods to Pepperoni
I cannot make Pizza an abstract base class

anything else?

Isn't pizza just some ingredients on top of it, why would it have to be *abstract* in the first place? What would a Pepperoni pizza do differently from an Onion pizza or an Onion pizza with added pepperoni?! — Antti Haapala -- Слава Україні, Oct 23 '17 at 14:35
i.e. classmethods vs inheritance serve *completely* different purposes. — Antti Haapala -- Слава Україні, Oct 23 '17 at 14:37
Ok, if they serve *completely* different purposes, could you point out which are the main differences? I could make a `Pizza.Pepperoni()` and also a `Onion()` and both objects would have same methods and properties — Rodrigo E. Principe, Oct 23 '17 at 14:41
This is totally wrong why the class method have to return an instance/class of same class. what do u acheive here ? — Siva Cn, Oct 23 '17 at 15:47
unless Julien Danjou is wrong, one purpose of classmethods in python is to make *factory methods*. See the *from_fridge()* method in https://julien.danjou.info/blog/2013/guide-python-static-class-abstract-methods — Rodrigo E. Principe, Oct 23 '17 at 16:14

score 1 · Accepted Answer · answered Oct 23 '17 at 14:44

1

You seem to think of Pepperoni as a class in its own right, just like Onion. Whereas Onion is a class, Pepperoni is just a function living on Pizza. Sure one should not think of Pepperoni as a normal method, but really it is. The classmethod decorator effectively turns the Pepperoni method into a factory function, producing a new instance. The code is completely equivalent to just writing the function outside of the class:

def Pepperoni():
    return Pizza("pepperoni")

You wouldn't talk about "adding custom methods to Pepperoni" now, but really nothing has changed.

answered Oct 23 '17 at 14:44

jmd_dk

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Yes, I understand the difference between them, but I'm having a hard time trying to decide whether I should use inheritance or just make classmethods.. Imagine you have to code *Pizza* for a store which has a set of 30 different kinds, what would you do? – Rodrigo E. Principe Oct 23 '17 at 14:57
I would simply utilize the `ingredients` argument of `Pizza`: `pizza_with_onion = Pizza('onion')`. The factory function should be used if you somehow need to instantiate a pizza from something other than its ingredients. A whole new class would be needed if you decided also to make kebabs, which is different from a pizza but might still share quite a few methods. – jmd_dk Oct 23 '17 at 15:01
Thanks mate! I guess making a factory class although I don't need to *instantiate a pizza from something other than its ingredients* doesn't make any harm =) – Rodrigo E. Principe Oct 23 '17 at 15:20

Issues of choosing classmethod over inheritance in Python

1 Answers1