Dunder or magic methods in Python

Difficulty Level : Easy
Last Updated : 14 Mar, 2018

Dunder or magic methods in Python are the methods having two prefix and suffix underscores in the method name. Dunder here means “Double Under (Underscores)”. These are commonly used for operator overloading. Few examples for magic methods are: __init__, __add__, __len__, __repr__ etc.

The __init__ method for initialization is invoked without any call, when an instance of a class is created, like constructors in certain other programming languages such as C++, Java, C#, PHP etc. These methods are the reason we can add two strings with ‘+’ operator without any explicit typecasting.

Here’s a simple implementation :

# declare our own string class
class String:
      
    # magic method to initiate object
    def __init__(self, string):
        self.string = string
          
# Driver Code
if __name__ == '__main__':
      
    # object creation
    string1 = String('Hello')
  
    # print object location
    print(string1)

Output :

<__main__.String object at 0x7fe992215390>

The above snippet of code prints only the memory address of the string object. Let’s add a __repr__ method to represent our object.

# declare our own string class
class String:
      
    # magic method to initiate object
    def __init__(self, string):
        self.string = string
          
    # print our string object
    def __repr__(self):
        return 'Object: {}'.format(self.string)
  
# Driver Code
if __name__ == '__main__':
      
    # object creation
    string1 = String('Hello')
  
    # print object location
    print(string1)

Output :

Object: Hello

If we try to add a string to it :

# declare our own string class
class String:
      
    # magic method to initiate object
    def __init__(self, string):
        self.string = string
          
    # print our string object
    def __repr__(self):
        return 'Object: {}'.format(self.string)
  
# Driver Code
if __name__ == '__main__':
      
    # object creation
    string1 = String('Hello')
      
    # concatenate String object and a string
    print(string1 +' world')

Output :

TypeError: unsupported operand type(s) for +: 'String' and 'str'

Now add __add__ method to String class :

# declare our own string class
class String:
      
    # magic method to initiate object
    def __init__(self, string):
        self.string = string 
          
    # print our string object
    def __repr__(self):
        return 'Object: {}'.format(self.string)
          
    def __add__(self, other):
        return self.string + other
  
# Driver Code
if __name__ == '__main__':
      
    # object creation
    string1 = String('Hello')
      
    # concatenate String object and a string
    print(string1 +' Geeks')

Output :

Hello Geeks

Reference : docs.python.org.

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