Class & Static Methods

Methods of objects we've looked at so far are called by an instance of a class, which is then passed to the self parameter of the method. Class methods are different - they are called by a class, which is passed to the cls parameter of the method. A common use of these are factory methods, which instantiate an instance of a class, using different parameters than those usually passed to the class constructor.

Class methods are marked with a classmethod decorator. Technically, the parameters self and cls are just conventions; they could be changed to anything else. However, they are universally followed, so it is wise to stick to using them.

Static methods behave like plain functions, except for the fact that you can call them from an instance of the class.

class Rectangle:
  def __init__(self,w,h):
    self.w=w
    self.h=h
  def area(self):
    return self.w*self.h
  @classmethod
  def new_square(cls,side_length):
    return cls(side_length,side_length)
  @staticmethod
  def spam(x):
    print("Spam and eggs"+x)

square=Rectangle.new_square(5)
print(square.area())
Rectangle.spam(" and Ham")

25
Spam and eggs and Ham

Class method vs Static Method

• A class method takes cls as the first parameter while a static method needs no specific parameters.
• A class method can access or modify the class state while a static method can’t access or modify it.
• In general, static methods know nothing about the class state. They are utility-type methods that take some parameters and work upon those parameters. On the other hand class methods must have class as a parameter.
• We use @classmethod decorator in python to create a class method and we use @staticmethod decorator to create a static method in python.

When to use what?

• We generally use class method to create factory methods. Factory methods return class objects ( similar to a constructor ) for different use cases.
• We generally use static methods to create utility functions.

# Python program to demonstrate
# use of class method and static method.
from datetime import date

class Person:
    def __init__(self, name, age):
        self.name = name
        self.age = age

    # a class method to create a Person object by birth year.
    @classmethod
    def fromBirthYear(cls, name, year):
        return cls(name, date.today().year - year)

    # a static method to check if a Person is adult or not.
    @staticmethod
    def isAdult(age):
        return age > 18

person1 = Person('mayank', 21)
person2 = Person.fromBirthYear('mayank', 1996)

print (person1.age)
print (person2.age)

# print the result
print (Person.isAdult(22))

21
25
True