Fluent Python - Chapter 1. The Python Data Model

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You can think of the data model as a description of Python as a framework. It formalizes the interfaces of the building blocks of the language itself, such as sequences, iterators, functions, classes, context managers, and so on.

A Pythonic Card Deck

Special methods: __getitem__ and __len__. 

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]

nametuple can be used to build classes of objects that are just bundles of attributes with no custom methods. 

beer_card = Card('7', 'diamonds')
beer_card
Card(rank='7', suit='diamonds')

len() function returns the number of cards in it. 

deck = FrenchDeck()
len(deck)
52

index operation, provided by __getitem__ method. 

deck[0]
Card(rank='2', suit='spades')
deck[-1]
Card(rank='A', suit='hearts')

Get a random item from a sequence 

from random import choice
choice(deck)
Card(rank='7', suit='spades')
choice(deck)
Card(rank='10', suit='hearts')

Slicing support, because __getitem__ delegates to the [] oeprator of self._cards 

deck[:3]
[Card(rank='2', suit='spades'), Card(rank='3', suit='spades'), Card(rank='4', suit='spades')]
deck[12::13]
[Card(rank='A', suit='spades'), Card(rank='A', suit='diamonds'), Card(rank='A', suit='clubs'), Card(rank='A', suit='hearts')]

__getitem__ method make the deck iterable 

for card in deck:
    print(card)
Card(rank='2', suit='spades')
Card(rank='3', suit='spades')
Card(rank='4', suit='spades')
...

Iterated in reverse 

for card in reversed(deck):
    print(card)
Card(rank='A', suit='hearts')
Card(rank='K', suit='hearts')
Card(rank='Q', suit='hearts')
...

If a collection has no __contains__, the in oeprator does a sequential scan. 

Card('Q', 'hearts') in deck
True
Card('7', 'beasts') in deck
False

Sorting 

suit_values = dict(spades=3, hearts=2, diamonds=1, clubs=0)

def spades_high(card):
    rank_value = FrenchDeck.ranks.index(card.rank)
    return rank_value * len(suit_values) + suit_values[card.suit]

for card in sorted(deck, key=spades_high):
    print(card)
Card(rank='2', suit='clubs')
Card(rank='2', suit='diamonds')
Card(rank='2', suit='hearts')
Card(rank='2', suit='spades')
...
Card(rank='A', suit='clubs')
Card(rank='A', suit='diamonds')
Card(rank='A', suit='hearts')
Card(rank='A', suit='spades')