Class attributes

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.

class Card:

    """ represents a standard playing card.

        class attributes: suit_names, rank_names

        instance attributes: suit, rank """

    suit_names = ['Clubs','Diamonds','Hearts','Spades']

    rank_names = [None,'Ace','','','','','','',

                  '','','','Jack','Queen','King']

    def __init__(self, suit=0, rank=2):

        self.suit = suit

        self.rank = rank

    def __str__(self):

        return '%s of %s' % (Card.suit_names[self.suit],

                             Card.rank_names[self.rank])

Because suit_names and rank_names are defined outside of any method, they are class attributes; that is, they are associated with the class Card rather than with a particular Card instance. Attributes like suit and rank are more precisely called instance attributes because they are associated with a particular instance. Both kinds of attributes 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.

Finally, 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 index 2 maps to the string ‘2’, and so on.

Here is a diagram that shows the Card class object and one Card instance:

from Thinking in Python

巴特西

Class attributes

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