10.6.2 Class Card—Introducing Class Attributes

Class Attributes FACES and SUITS

• Each object of a class has its own copies of the class’s data attributes
• A class attribute (also called a class variable) represents class-wide information
  • Belongs to the class, not to a specific object of that class
• Two class attributes (lines 5–7):
  • FACES is a list of the card face names
  • SUITS is a list of the card suit names

# card.py
"""Card class that represents a playing card and its image file name."""

class Card:
    FACES = ['Ace', '2', '3', '4', '5', '6',
             '7', '8', '9', '10', 'Jack', 'Queen', 'King']
    SUITS = ['Hearts', 'Diamonds', 'Clubs', 'Spades']

Class Attributes FACES and SUITS (cont.)

• Define a class attribute by assigning a value to it inside the class’s definition, but not inside any of the class’s methods or properties (in which case, they’d be local variables)
• FACES and SUITS are constants that are not meant to be modified
• Style Guide for Python Code recommends naming your constants with all capital letters.

Class Attributes FACES and SUITS (cont.)

• We’ll use FACES and SUITS to initialize each Card we create
• Do not need a separate copy of each list in every Card object
• Class attributes are typically accessed through the class’s name (as in, Card.FACES or Card.SUITS)
• Class attributes exist as soon as you import their class’s definition

Card Method __init__

• Method __init__ defines a Card’s _face and _suit data attributes

def __init__(self, face, suit):
        """Initialize a Card with a face and suit."""
        self._face = face
        self._suit = suit

Read-Only Properties face, suit and image_name

• Once a Card is created, its face, suit and image_name do not change, so these are read-only properties
• A property is not required to have a corresponding data attribute
• Card property image_name’s value is created dynamically by getting the Card object’s string representation with str(self), replacing any spaces with underscores and appending the '.png' filename extension

@property
    def face(self):
        """Return the Card's self._face value."""
        return self._face

    @property
    def suit(self):
        """Return the Card's self._suit value."""
        return self._suit

    @property
    def image_name(self):
        """Return the Card's image file name."""
        return str(self).replace(' ', '_') + '.png'

Methods That Return String Representations of a Card

• Card provides three special methods that return string representations
• Method __repr__ returns a string representation that looks like a constructor expression

def __repr__(self):
        """Return string representation for repr()."""
        return f"Card(face='{self.face}', suit='{self.suit}')"

Methods That Return String Representations of a Card (cont.)

• Method __str__ returns a string of the format 'face of suit'

def __str__(self):
        """Return string representation for str()."""
        return f'{self.face} of {self.suit}'

Methods That Return String Representations of a Card (cont.)

• In the __str__ method of class DeckOfCards, we use f-strings to format the Cards in fields of 19 characters each
• Class Card’s special method __format__ is called when a Card object is formatted as a string

def __format__(self, format):
        """Return formatted string representation for str()."""
        return f'{str(self):{format}}'

• Second argument is the format string used to format the object
• To use the format parameter’s value as a format specifier, enclose the parameter name in braces to the right of the colon

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