Py Dunder Methods — Free Python Tutorial

TL;DR: Learn Py Dunder Methods in Python with a free, beginner-friendly tutorial, examples and practice for Indian students on Syllab.in.

Py Dunder Methods in Python

Dunder methods (double underscore methods) let you define how your objects behave with Python operators and built-in functions. They are the heart of Python's data model.

__str__ and __repr__: control how your object is printed. __str__ is human-friendly, __repr__ is developer-friendly (for debugging).

__len__, __getitem__, __setitem__: make your object behave like a list or dict. __iter__ and __next__ make it iterable.

__add__, __sub__, __mul__: define arithmetic. __eq__, __lt__, __gt__: define comparisons. This is called operator overloading.

class MyClass:
    def __init__(self, value):
        self.value = value
    def __str__(self):      # print(obj) → human readable
        return f"MyClass({self.value})"
    def __repr__(self):     # repr(obj) → developer view
        return f"MyClass(value={self.value!r})"
    def __len__(self):      # len(obj)
        return self.value
    def __add__(self, other):  # obj1 + obj2
        return MyClass(self.value + other.value)
    def __eq__(self, other):   # obj1 == obj2
        return self.value == other.value

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