Guide to Programming with Python Chapter Nine Inheritance Working with multiple objects

OOP So Far Object-oriented programming is a programming language model organized around "objects” An object is a software bundle of related attributes and behavior (methods) A class is a blueprint or prototype from which objects are created class Player(object): def __init__(self, name = "Enterprise", fuel = 0): self.name = name self.fuel = fuel def status(self): ... myship = Ship("Appolo") myship.status() Object encapsulation & respect privacy

This Week’s Objectives Inheritance makes objects (classes) special Derive new classes from existing ones Extend the definition of existing classes Override method definitions of existing classes Create objects of different classes in the same program Allow objects to communicate with each other Create more complex objects by combining simpler ones The Blackjack Game Guide to Programming with Python

Inheritance Models “is a” Relationship Car Van Sports car Convertible Animals Reptile Mammals Fish Dog Cat Human Guide to Programming with Python

Using Inheritance to Create New Classes Inheritance: An element of OOP that allows a new class to be based on an existing one where the new automatically gets (or inherits) all of the methods and attributes of the existing class The children classes get all the capabilities (methods) and properties (attributes) the parent class has; the children classes are also called derived classes Get the code for free! (code-reuse) – inheritance allows a new class to re-use code which already existed in another class (the parent class) Guide to Programming with Python

Derived Classes are New Classes To create specializations of existing classes or objects by adding new attributes and methods! often called subtyping when applied to classes. In specialization, the new class or object has data or behavior aspects that are not part of the inherited class. Over-ridding (e.g., over-ridding of the + operator, so + has different meaning, addition of two numbers, concatenation of two strings, etc) – the same method that does something different Guide to Programming with Python

Inheritance Example: Animal Class class Animal(object): def __init__(self, name): # Constructor self.name = name def get_name(self): return self.name class Cat(Animal): def talk(self): return 'Meow!' class Dog(Animal): return 'Woof! Woof!' animals = [Cat('Missy'), Cat('Mr. Bojangles'), Dog('Lassie')] for animal in animals: print animal.talk() + ' I am ' + animal.get_name() Base class: A class upon which another is based; it is inherited from by a derived class Derived class: A class that is based upon another class; it inherits from a base class

Altering the Behavior of Inherited Methods: Overriding Override: To redefine how inherited method of base class works in derived class Two choices when overriding Completely new functionality vs. overridden method Incorporate functionality of overridden method, add more Guide to Programming with Python

Overriding to Create a New Version class Animal(object): def __init__(self, name): self.name = name def talk(self): return 'Hello!' class Cat(Animal): return 'Meow!'

Animal A > Animal B? class Animal(object): def __init__(self, name, age): self.name = name self.age = age def get_name(self): return self.name def __gt__(self, other): #override comparison operators return self.age > other.age print Animal('Missy', 4) > Animal('Lassie', 3) Guide to Programming with Python

Overriding to Add More One can incorporate inherited method’s functionality in overridden method Class Card(object): ... class Positionable_Card(Card): def __init__(self, rank, suit, face_up = True): super(Positionable_Card, self).__init__(rank, suit) #invoke parent’s method by calling super() self.is_face_up = face_up Superclass: Another name for a base class Card is the superclass of Positionable_Card

Invoking Base Class Methods Incorporate inherited method’s functionality by calling super() Positionable_Card constructor invokes Card constructor and creates new attribute super() lets you invoke the method of a superclass First argument is the class name, Positionable_Card Second is reference to object itself, self Last is superclass method to call with parameters sent, __init__(rank, suit) Guide to Programming with Python

Understanding Polymorphism Polymorphism: Aspect of object-oriented programming that allows you to send same message to objects of different classes, related by inheritance, and achieve different but appropriate results for each object When you invoke talk() method of Cat object, you get different result than when you invoke the same method of a Animal (or Dog) object Guide to Programming with Python

Summary: What can be Done Through Inheritance New class gets all methods and attributes of existing class New class can also define additional methods and attributes, to create more specialized version of existing class New class can override the old methods When overriding a method, the new definition can have completely different functionality than the original definition or the new definition can incorporate the functionality of the original The super() function allows you to invoke the method of a superclass

A Little More on Inheritance Need to avoid yo-yo problem (caused by the too complicated inheritance hierarchy) Python supports a limited form of multiple inheritance (applies depth-first, left-to-right rule) class DerivedClass(Base1, Base2, Base3): ... Python interprets this by applying the depth-first, left-to right rule: if an attribute is not found in DerivedClass, it is searched in Base1, then (recursively) in the base classes of Base1, and only if it is not found there, it is searched in Base2, and so on)

Working with Multiple Objects We can have multiple objects in a program Message: Communication between objects; one object sends another a message when it invokes a method of the other (We already know that we can exchange information among functions through parameters and return values) Guide to Programming with Python

The Alien Blaster Program Figure 9.3: Visual representation of objects exchanging a message hero, a Player object, sends invader, an Alien object, a message. Guide to Programming with Python

Communications between Objects (Alien and Player) class Player(object): def blast(self, enemy): #enemy refers to the Alien object print "The player blasts an enemy." enemy.die() #invokes the Alien object’s die()method class Alien(object): def die(self): print "Good-bye, cruel universe.” hero = Player() invader = Alien() hero.blast(invader) # here an object is passed to a function of another object Guide to Programming with Python

Combining Objects Real-world objects often made up of other objects Can mimic composition and collection in OOP Drag racer composed of body, tires, and engine Drag_Racer class with attribute engine that references Race_Engine object Zoo is collection of animals Zoo class that has an attribute animals which is a list of different Animal objects Guide to Programming with Python

Blackjack Game: the Card Class class Card(object): """ A playing card. """ RANKS = ["A", "2", "3", "4", "5", "6", "7", "8", "9", "10", "J", "Q", "K"] SUITS = ["c", "d", "h", "s"] #class attributes def __init__(self, rank, suit): self.rank = rank #object attributes self.suit = suit def __str__(self): reply = self.rank + self.suit return reply card1 = Card(”A", ”d") # A Card object with rank "A" and suit "d" is the ace of diamonds print card1

Blackjack Game: the Hand Class class Hand(object): """ A hand of playing cards. """ def __init__(self): self.cards = [] #attribute – a list of Card objects def __str__(self): #special function, returns string for entire hand if self.cards: reply = "" for card in self.cards: reply += str(card) + " " else: reply = "<empty>" return reply def add(self, card): #adds card to list of cards self.cards.append(card) def give(self, card, other_hand): self.cards.remove(card) other_hand.add(card)

Combining Objects my_hand = Hand() card1 = Card("A", "c") card1 = Card("2", "c") my_hand.add(card1) my_hand.add(card2) print my_hand # Ac 2c your_hand = Hand() my_hand.give(card1, your_hand) my_hand.give(card2, your_hand) print your_hand # Ac 2c Guide to Programming with Python

Summary In object-oriented programming, objects can send messages to each other by invoking each other’s methods Objects can be composed of other objects or have collections of objects Guide to Programming with Python