Understand the usage of python classmethods

python

By Vikrant

August 29, 2018

While trying to understand the difference between staticmethod and classmethod I landed on this blog post. I was bit confused after reading the comment “classmethod can be helpful in case of inheritence instead of having one staticmethod calling another staticmethod”.

I used the same example in blog post to extend it bit to understand the practical implementation.

I am overriding the compute_area staticmethod function in inherited class this function was called by another staticmethod compute_volume. NOTE: I need to call compute_area from compute_volume using Pizza.compute_area(radius) where Pizza is the class name, you can’t use self instead of Pizza.

import math
class Pizza():
    
    def __init__(self, radius, height):
        self.radius = radius
        self.height = height
    
    @staticmethod
    def compute_area(radius):
        return math.pi * (radius ** 2)
    
    @staticmethod
    def compute_volume(height, radius):
        return height * Pizza.compute_area(radius)
    
    def get_volume(self):
        return self.compute_volume(self.height, self.radius)

class Italian(Pizza):
    @staticmethod
    def compute_area(radius):
        print("In inherited")
        return math.pi * (radius ** 2)
    
instance1=Pizza(4,5)    
print(instance1.get_volume())
instance2=Italian(5,6)
print(instance2.get_volume())

Result of run are:

251.327412287
471.238898038

It ran fine but it didn’t run the overrided compute_area method which I defined in Italian class instead used compute_area from parent class. It happened because we are calling compute_area using Pizza classname in compute_volume method.

Let’s use classmethod now instead of staticmethod to define compute_volume method in parent class.

import math
class Pizza():
    
    def __init__(self, radius, height):
        self.radius = radius
        self.height = height
    
    @staticmethod
    def compute_area(radius):
        return math.pi * (radius ** 2)
    
    #@staticmethod
    #def compute_volume(height, radius):
    #    return height * Pizza.compute_area(radius)
    
    @classmethod
    def compute_volume(cls, height, radius):
        return height * cls.compute_area(radius)
    
    def get_volume(self):
        return self.compute_volume(self.height, self.radius)

class Italian(Pizza):
    @staticmethod
    def compute_area(radius):
        print("In inherited")
        return math.pi * (radius ** 2)
    
instance1=Pizza(4,5)    
print(instance1.get_volume())
instance2=Italian(5,6)
print(instance2.get_volume())

Results of run are:

251.327412287
In inherited
471.238898038

This gives us expected results because classname was dynamically was replaced with Italian in-place of cls while calling staticmethod from classmethod.