The @classmethod decorator, is a builtin function decorator that is an expression that gets evaluated after your function is defined. The result of that evaluation shadows your function definition.
A class method receives the class as implicit first argument, just like an instance method receives the instance
class C(object): @classmethod def fun(cls, arg1, arg2, ...): .... fun: function that needs to be converted into a class method returns: a class method for function.
- A class method is a method which is bound to the class and not the object of the class.
- They have the access to the state of the class as it takes a class parameter that points to the class and not the object instance.
- It can modify a class state that would apply across all the instances of the class. For example it can modify a class variable that will be applicable to all the instances.
A static method does not receive an implicit first argument.
class C(object): @staticmethod def fun(arg1, arg2, ...): ... returns: a static method for function fun.
- A static method is also a method which is bound to the class and not the object of the class.
- A static method can’t access or modify class state.
- It is present in a class because it makes sense for the method to be present in class.
Class method vs Static Method
- A class method takes cls as first parameter while a static method needs no specific parameters.
- A class method can access or modify class state while a static method can’t access or modify it.
- 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 object ( similar to a constructor ) for different use cases.
- We generally use static methods to create utility functions.
How to define a class method and a static method?
To define a class method in python, we use @classmethod decorator and to define a static method we use @staticmethod decorator.
Let us look at an example to understand the difference between both of them. Let us say we want to create a class Person. Now, python doesn’t support method overloading like C++ or Java so we use class methods to create factory methods. In the below example we use a class method to create a person object from birth year.
As explained above we use static methods to create utility functions. In the below example we use a static method to check if a person is adult or not.
# 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 21 True
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