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
add_card for Hands as well as Decks.
Hand also inherits
Deck, but it doesn’t really do what we want: instead of populating the hand with 52 new cards, the init method for Hands should initialize
cards with an empty list.
If we provide an init method in the
Hand class, it overrides the one in the
# 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
add_card to deal a card:
>>> 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 called
#inside class Deck: def move_cards(self, hand, num): for i in range(num): hand.add_card(self.pop_card())
move_cards takes two arguments, a Hand object and the number of cards to deal. It modifies both
hand, and returns
In some games, cards are moved from one hand to another, or from a hand back to the deck. You can use
move_cards for any of these operations:
self can be either a Deck or a Hand, and
hand, despite the name, can also be a
Write a Deck method called
deal_hands that takes two parameters, the number of hands and the number of cards per hand, and that creates new Hand objects, deals the appropriate number of cards per hand, and returns a list of Hand objects.
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.