Understanding Python's Special Methods __len__ and __getitem__ with Example Code

Python uses special methods whose names start and end with double underscores to implement fundamental object behavior. Two common examples are __len__ and __getitem__.

import collections

Card = collections.namedtuple('Card', ['rank', 'suit'])

class FrenchDeck:

ranks = [str(n) for n in range(2, 11)] + list('JQKA')

suits = 'spades diamonds clubs hearts'.split()

def __init__(self):

self._cards = [Card(rank, suit) for suit in self.suits for rank in self.ranks]

def __len__(self):

return len(self._cards)

def __getitem__(self, position):

return self._cards[position]

This code defines a FrenchDeck class that implements __len__ and __getitem__. Implementing __len__ allows the built‑in len() function to return the number of cards, while __getitem__ enables indexing and slicing syntax (e.g., deck[0] or deck[2:5]).

Example usage:

deck = FrenchDeck()

len(deck)          # → 52

deck[0]            # → Card(rank='2', suit='spades')

The __getitem__ method forwards the [] operation to the internal list self._cards, so the FrenchDeck object automatically supports slicing.

from math import hypot
class Vector:
    def __init__(self, x=0, y=0):
        self.x = x
        self.y = y
    def __repr__(self):
        return 'Vector(%r, %r)' % (self.x, self.y)
    def __abs__(self):
        return hypot(self.x, self.y)
    def __bool__(self):
        return bool(abs(self))
    def __add__(self, other):
        x = self.x + other.x
        y = self.y + other.y
        return Vector(x, y)
    def __mul__(self, scalar):
        return Vector(self.x * scalar, self.y * scalar)

The Vector class demonstrates additional special methods: __repr__ provides an unambiguous string representation (used by the interpreter), differing from __str__ which is used by print(); __abs__ defines the magnitude; __bool__ determines truthiness based on the magnitude; __add__ and __mul__ implement addition and scalar multiplication, respectively.