Chapter 18 Inheritance
In this chapter we will develop classes to represent playing cards, decks of cards, and poker hands. If you don’t play poker, you can read about it at wikipedia.org/wiki/Poker, but you don’t have to; I’ll tell you what you need to know for the exercises.
If you are not familiar with Anglo-American playing cards, you can read about them at wikipedia.org/wiki/Playing_cards.
18.1 Card objects
There are fifty-two cards in a deck, each of which belongs to one of four suits and one of thirteen ranks. The suits are Spades, Hearts, Diamonds, and Clubs (in descending order in bridge). The ranks are Ace, 2, 3, 4, 5, 6, 7, 8, 9, 10, Jack, Queen, and King. Depending on the game that you are playing, an Ace may be higher than King or lower than 2.
If we want to define a new object to represent a playing card, it is
obvious what the attributes should be: rank and
suit. It is not as obvious what type the attributes
should be. One possibility is to use strings containing words like
An alternative is to use integers to encode the ranks and suits. In this context, “encode” means that we are going to define a mapping between numbers and suits, or between numbers and ranks. This kind of encoding is not meant to be a secret (that would be “encryption”).
For example, this table shows the suits and the corresponding integer codes:
This code makes it easy to compare cards; because higher suits map to higher numbers, we can compare suits by comparing their codes.
The mapping for ranks is fairly obvious; each of the numerical ranks maps to the corresponding integer, and for face cards:
I am using the ↦ symbol to make it clear that these mappings are not part of the Python program. They are part of the program design, but they don’t appear explicitly in the code.
The class definition for Card looks like this:
class Card(object): """represents a standard playing card.""" def __init__(self, suit=0, rank=2): self.suit = suit self.rank = rank
As usual, the init method takes an optional parameter for each attribute. The default card is the 2 of Clubs.
To create a Card, you call Card with the suit and rank of the card you want.
queen_of_diamonds = Card(1, 12)
18.2 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. 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])
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
Every card has its own suit and rank, but there
is only one copy of
Putting it all together, the expression
The first element of
With the methods we have so far, we can create and print cards:
>>> card1 = Card(2, 11) >>> print card1 Jack of Hearts
Here is a diagram that shows 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
18.3 Comparing cards
For built-in types, there are relational operators
(<, >, ==, etc.)
values and determine when one is greater than, less than, or equal to
another. For user-defined types, we can override the behavior of
the built-in operators by providing a method named
The correct ordering for cards is not obvious. For example, which is better, the 3 of Clubs or the 2 of Diamonds? One has a higher rank, but the other has a higher suit. In order to compare cards, you have to decide whether rank or suit is more important.
The answer might depend on what game you are playing, but to keep things simple, we’ll make the arbitrary choice that suit is more important, so all of the Spades outrank all of the Diamonds, and so on.
With that decided, we can write
# inside class Card: def __cmp__(self, other): # check the suits if self.suit > other.suit: return 1 if self.suit < other.suit: return -1 # suits are the same... check ranks if self.rank > other.rank: return 1 if self.rank < other.rank: return -1 # ranks are the same... it's a tie return 0
You can write this more concisely using tuple comparison:
# inside class Card: def __cmp__(self, other): t1 = self.suit, self.rank t2 = other.suit, other.rank return cmp(t1, t2)
The built-in function cmp has the same interface as
Exercise 1 Write a
Now that we have Cards, the next step is to define Decks. Since a deck is made up of cards, it is natural for each Deck to contain a list of cards as an attribute.
The following is a class definition for Deck. The init method creates the attribute cards and generates the standard set of fifty-two cards:
class Deck(object): def __init__(self): self.cards =  for suit in range(4): for rank in range(1, 14): card = Card(suit, rank) self.cards.append(card)
The easiest way to populate the deck is with a nested loop. The outer loop enumerates the suits from 0 to 3. The inner loop enumerates the ranks from 1 to 13. Each iteration creates a new Card with the current suit and rank, and appends it to self.cards.
18.5 Printing the deck
Here is a
#inside class Deck: def __str__(self): res =  for card in self.cards: res.append(str(card)) return '\n'.join(res)
This method demonstrates an efficient way to accumulate a large
string: building a list of strings and then using join.
The built-in function str invokes the
Since we invoke join on a newline character, the cards are separated by newlines. Here’s what the result looks like:
>>> deck = Deck() >>> print deck Ace of Clubs 2 of Clubs 3 of Clubs ... 10 of Spades Jack of Spades Queen of Spades King of Spades
Even though the result appears on 52 lines, it is one long string that contains newlines.
18.6 Add, remove, shuffle and sort
To deal cards, we would like a method that removes a card from the deck and returns it. The list method pop provides a convenient way to do that:
#inside class Deck: def pop_card(self): return self.cards.pop()
Since pop removes the last card in the list, we are dealing from the bottom of the deck. In real life bottom dealing is frowned upon1, but in this context it’s ok.
To add a card, we can use the list method append:
#inside class Deck: def add_card(self, card): self.cards.append(card)
A method like this that uses another function without doing much real work is sometimes called a veneer. The metaphor comes from woodworking, where it is common to glue a thin layer of good quality wood to the surface of a cheaper piece of wood.
In this case we are defining a “thin” method that expresses a list operation in terms that are appropriate for decks.
As another example, we can write a Deck method named shuffle using the function shuffle from the random module:
# inside class Deck: def shuffle(self): random.shuffle(self.cards)
Don’t forget to import random.
Write a Deck method named sort that uses the list method
sort to sort the cards in a Deck. sort uses
The language feature most often associated with object-oriented programming is inheritance. Inheritance is the ability to define a new class that is a modified version of an existing class.
It is called “inheritance” because the new class inherits the methods of the existing class. Extending this metaphor, the existing class is called the parent and the new class is called the child.
As an example, let’s say we want a class to represent a “hand,” that is, the set of cards held by one player. A hand is similar to a deck: both are made up of a set of cards, and both require operations like adding and removing cards.
A hand is also different from a deck; there are operations we want for hands that don’t make sense for a deck. For example, in poker we might compare two hands to see which one wins. In bridge, we might compute a score for a hand in order to make a bid.
This relationship between classes—similar, but different—lends itself to inheritance.
The definition of a child class is like other class definitions, but the name of the parent class appears in parentheses:
class Hand(Deck): """represents a hand of playing cards"""
This definition indicates that Hand inherits from Deck;
that means we can use methods like
Hand also inherits
If we provide an init method in the Hand class, it overrides the one in the Deck class:
# inside class Hand: def __init__(self, label=''): self.cards =  self.label = label
So when you create a Hand, Python invokes this init method:
>>> hand = Hand('new hand') >>> print hand.cards  >>> print hand.label new hand
But the other methods are inherited from Deck, so we can use
>>> deck = Deck() >>> card = deck.pop_card() >>> hand.add_card(card) >>> print hand King of Spades
A natural next step is to encapsulate this code in a method
#inside class Deck: def move_cards(self, hand, num): for i in range(num): hand.add_card(self.pop_card())
In some games, cards are moved from one hand to another,
or from a hand back to the deck. You can use
Exercise 3 Write a Deck method called
Inheritance is a useful feature. Some programs that would be repetitive without inheritance can be written more elegantly with it. Inheritance can facilitate code reuse, since you can customize the behavior of parent classes without having to modify them. In some cases, the inheritance structure reflects the natural structure of the problem, which makes the program easier to understand.
On the other hand, inheritance can make programs difficult to read. When a method is invoked, it is sometimes not clear where to find its definition. The relevant code may be scattered among several modules. Also, many of the things that can be done using inheritance can be done as well or better without it.
18.8 Class diagrams
So far we have seen stack diagrams, which show the state of a program, and object diagrams, which show the attributes of an object and their values. These diagrams represent a snapshot in the execution of a program, so they change as the program runs.
They are also highly detailed; for some purposes, too detailed. A class diagram is a more abstract representation of the structure of a program. Instead of showing individual objects, it shows classes and the relationships between them.
There are several kinds of relationship between classes:
A class diagram is a graphical representation of these relationships2. For example, this diagram shows the relationships between Card, Deck and Hand.
The arrow with a hollow triangle head represents an IS-A relationship; in this case it indicates that Hand inherits from Deck.
The standard arrow head represents a HAS-A relationship; in this case a Deck has references to Card objects.
The star (*) near the arrow head is a multiplicity; it indicates how many Cards a Deck has. A multiplicity can be a simple number, like 52, a range, like 5..7 or a star, which indicates that a Deck can have any number of Cards.
A more detailed diagram might show that a Deck actually contains a list of Cards, but built-in types like list and dict are usually not included in class diagrams.
Exercise 4 Read TurtleWorld.py, World.py and Gui.py and draw a class diagram that shows the relationships among the classes defined there.
Inheritance can make debugging a challenge because when you invoke a method on an object, you might not know which method will be invoked.
Suppose you are writing a function that works with Hand objects. You would like it to work with all kinds of Hands, like PokerHands, BridgeHands, etc. If you invoke a method like shuffle, you might get the one defined in Deck, but if any of the subclasses override this method, you’ll get that version instead.
Any time you are unsure about the flow of execution through your program, the simplest solution is to add print statements at the beginning of the relevant methods. If Deck.shuffle prints a message that says something like Running Deck.shuffle, then as the program runs it traces the flow of execution.
As an alternative, you could use this function, which takes an object and a method name (as a string) and returns the class that provides the definition of the method:
def find_defining_class(obj, meth_name): for ty in type(obj).mro(): if meth_name in ty.__dict__: return ty
Here’s an example:
>>> hand = Hand() >>> print find_defining_class(hand, 'shuffle') <class 'Card.Deck'>
So the shuffle method for this Hand is the one in Deck.
Here’s a program design suggestion: whenever you override a method, the interface of the new method should be the same as the old. It should take the same parameters, return the same type, and obey the same preconditions and postconditions. If you obey this rule, you will find that any function designed to work with an instance of a superclass, like a Deck, will also work with instances of subclasses like a Hand or PokerHand.
If you violate this rule, your code will collapse like (sorry) a house of cards.
The following are the possible hands in poker, in increasing order of value (and decreasing order of probability):
The goal of these exercises is to estimate the probability of drawing these various hands.
This exercise uses TurtleWorld from Chapter 4. You will write code that makes Turtles play tag. If you are not familiar with the rules of tag, see wikipedia.org/wiki/Tag_(game).
You can get my solution from thinkpython.com/code/Tagger.py.
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