Magic-Dunder Methods

__add__()

This magic method is used to add attributes of two objects i.e, class instances. If we define __add__() method in such a way that it adds two object of same class by overloading + operator, the operation obj1 + obj2  adds attributes of the objects by invoking the method implicitly. 

class number:
  def __init__(self,integer):
    self.integer=integer
  
  def __add__(self,obj):
    return self.integer+obj.integer
     
num1=number(5)
num2=number(10)

num1+num2

Output

15

__repr__()

This magic method is used to represent instance of a class in string. __repr__() method is invoked when we try to print object a class by calling print() method. So, we can overload  __repr__() method to get customize string representation of any object.

class Student:

  #constructor in Python
  def __init__(self,name,enr_id):
    #instance variable
    self.name=name
    self.enr_id=enr_id
  
  def __repr__(self):
      return f"Enrollment id of {self.name} is {self.enr_id}"

student= Student("Dhruv",5105133)
print(student)#print invokes __repr__ method internally

Output

Enrollment id of Dhruv is 5105133

__len__()

This magic method is used to find length of instance attribute of a class. When len() is called on any object, it returns length of container attribute of the object if __len__() method is defined accordingly inside the class.

class Student:

  def __init__(self,name,marks):
    #instance variable
    self.name=name
    self.marks=marks
  
  def __len__(self):
    return len(self.marks)

student= Student("Dhruv",[75,80,78,89,65])  
len(student) #invokes __len__() method

Output

5

Must Read, python ord

FAQs

1. What are the magic methods used for comparing object’s attributes?
__eq__() for checking equality, __ne__() for not equality, __gt__() for greater than, __lt__() for checking less than.
 

2. What is __call__() method?
This magic method is invoked when an object of a class is invoked. We can define this function according to our objective without defining another function as we can call the method by just using the name of the class object.

class Addition:
    def __init__(self, val):
        self.val = val
    def __call__(self, a):
        return self.val + a
obj = Addition(5)
print(obj(10)) #calling by object name

Output

15

You can compile it with online python compiler.

Key Takeaways

This article covered Magic-Dunder Methods, special type of methods in Python.

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