FluentPython - DataModel

Sun, 15 Mar 2026 00:00:00 +0000

Every Python object consists of specific methods called magic methods or dunder methods. These methods allow your custom objects to interact with fundamental language constructs like collections, iteration, and operator overloading.

For example: len

The len function is the result of implementing the len magic method.

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]


deck = FrenchDeck()

print(len(deck)) ## 52

print(deck[2]) ## Card(rank='4', suit='spades')

Those methods are meant to be called by the Python interpreter, not by the programmer. If you implement the len method, you should call len(my_obj) directly — it will automatically invoke len through the interpreter.

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FluentPython - DataModel