Python - Class Methods

Methods belongs to an object of a class and used to perform specific operations. We can divide Python methods in three different categories, which are class method, instance method and static method.

A Pythonclass method is a method that is bound to the class and not to the instance of the class. It can be called on the class itself, rather than on an instance of the class.

Most of us often get class methods confused with static methods. Always remember, while both are called on the class,static methods do not have access to the "cls" parameter and therefore it cannot modify the class state.

Unlike class method, theinstance method can access the instancevariables of the an object. It can also access the class variable as it is common to all the objects.

Creating Class Methods in Python

There are two ways to create class methods in Python −

• Using classmethod() Function
• Using @classmethod Decorator

Using classmethod() Function

Python has a built-in functionclassmethod() which transforms an instance method to aclass method which can be called with the reference to the class only and not the object.

Syntax

classmethod(instance_method)

Example

In the Employee class, define a showcount() instance method with the "self" argument (reference to calling object). It prints the value of empCount. Next, transform the method to class method counter() that can be accessed through the class reference.

class Employee:   empCount = 0   def __init__(self, name, age):      self.__name = name      self.__age = age      Employee.empCount += 1   def showcount(self):      print (self.empCount)         counter = classmethod(showcount)e1 = Employee("Bhavana", 24)e2 = Employee("Rajesh", 26)e3 = Employee("John", 27)e1.showcount()Employee.counter()

Output

Call showcount() with object and call count() with class, both show the value of employee count.

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Using @classmethod Decorator

Use of@classmethod() decorator is the prescribed way to define a class method as it is more convenient than first declaring an instance method and then transforming it into a class method.

Syntax

@classmethoddef method_name():   # your code

Example

The class method acts as an alternate constructor. Define anewemployee() class method with arguments required to construct a new object. It returns the constructed object, something that the__init__() method does.

class Employee:    empCount = 0    def __init__(self, name, age):        self.name = name        self.age = age        Employee.empCount += 1    @classmethod    def showcount(cls):        print (cls.empCount)    @classmethod    def newemployee(cls, name, age):        return cls(name, age)e1 = Employee("Bhavana", 24)e2 = Employee("Rajesh", 26)e3 = Employee("John", 27)e4 = Employee.newemployee("Anil", 21)Employee.showcount()

There are four Employee objects now. If we run the above program, it will show the count of Employee object −

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Access Class Attributes in Class Method

Class attributes are those variables that belong to a class and whose value is shared among all the instances of that class.

To access class attributes within a class method, use thecls parameter followed by dot (.) notation and name of the attribute.

Example

In this example, we are accessing a class attribute in class method.

class Cloth:   # Class attribute   price = 4000   @classmethod   def showPrice(cls):      return cls.price# Accessing class attributeprint(Cloth.showPrice())

On running the above code, it will show the following output −

4000

Dynamically Add Class Method to a Class

The Pythonsetattr() function is used to set an attribute dynamically. If you want to add a class method to a class, pass the method name as a parameter value to setattr() function.

Example

The following example shows how to add a class method dynamically to a Python class.

class Cloth:   pass# class method@classmethoddef brandName(cls):   print("Name of the brand is Raymond")# adding dynamicallysetattr(Cloth, "brand_name", brandName)newObj = Cloth()newObj.brand_name()

When we execute the above code, it will show the following output −

Name of the brand is Raymond

Dynamically Delete Class Methods

The Pythondel operator is used to delete a class method dynamically. If you try to access the deleted method, the code will raise AttributeError.

Example

In the below example, we are deleting the class method named "brandName" using del operator.

class Cloth:   # class method   @classmethod   def brandName(cls):      print("Name of the brand is Raymond")# deleting dynamicallydel Cloth.brandNameprint("Method deleted")

On executing the above code, it will show the following output −

Method deleted