Dunder Functions

Dunder word is taken by composing “Double” and “Underscore”. So, dunder functions are functions that start and end with double underscores. In python, interpreter treats them specially and these functions are not used directly. However, in some cases we may require to use these functions.

These functions enable us to make user defined data structures compatible with basic data types, apply arithmetic operations on instances, etc. For instance, we can make an instance of class Person addition with another instance of same class (or different class).

class Person():
    def __init__(self, age):
        self.age = age
    def __add__(self, another):
        return Person(self.age + another.age)
 
a = Person(2)
b = Person(2)
 
print((a + b).age)
4

There are several dunder functions available.

Dunder function	Usage
__setitem__ and __getitem__	Handling indexing
__add__, __sub__, __div__ etc	Arithmetic operations
__init__	Initialise object
__lt__, __gt__, __le__ etc	Relational operators.

References

1. https://mathspp.com/blog/pydonts/dunder-methods