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Class attributes

20 January, 2016 - 15:50

In order to print Card objects in a way that people can easily read, we need a mapping from the integer codes to the corresponding ranks and suits. A natural way to do that is with lists of strings. We assign these lists to class attributes:

# inside class Card:
suit_names = ['Clubs', 'Diamonds', 'Hearts', 'Spades']
rank_names = [None, 'Ace', '2', '3', '4', '5', '6', '7','8', '9', '10', 'Jack', 'Queen', 'King']
def __str__(self):
return '%s of %s' % (Card.rank_names[self.rank],Card.suit_names[self.suit])

Variables like suit_names and rank_names, which are defined inside a class but outside of any method, are called class attributes because they are associated with the class object Card.

This term distinguishes them from variables like suit and rank, which are called instance attributes because they are associated with a particular instance.

Both kinds of attribute are accessed using dot notation. For example, in __str__, self is a Card object, and self.rank is its rank. Similarly, Card is a class object, and Card.rank_names is a list of strings associated with the class.

Every card has its own suit and rank, but there is only one copy of suit_names and rank_names.

Figure 18.1 Figure 18.1: Object diagram. 

Putting it all together, the expression Card.rank_names[self.rank] means “use the attribute rank from the object self as an index into the list rank_names from the class Card, and select the appropriate string.”

The first element of rank_names is None because there is no card with rank zero. By including None as a place-keeper, we get a mapping with the nice property that the index 2 maps to the string '2', and so on. To avoid this tweak, we could have used a dictionary instead of a list.

With the methods we have so far, we can create and print cards:

>>> card1 = Card(2, 11)>>> print card1Jack of Hearts

Figure 18.1 is a diagram of the Card class object and one Card instance. Card is a class object, so it has type type. card1 has type Card. (To save space, I didn’t draw the contents of suit_names and rank_names).