Computer Science 15-112, Fall 2011
Class Notes: Classes

Required Reading
Dictionaries vs Structs vs Classes
Multiple Instances (One Dictionary Per Instance)
Equality Testing (__eq__)
Converting to Strings (__str__ and __repr__)
Using in Sets and Dictionaries (__hash__ and __eq__)
Class Variables (Data Attributes) and Class Methods
Inheritance
1. Specifying a Superclass
2. isinstance vs type()
3. Overriding methods
4. Multiple Inheritance
Examples from class
1. Text Adventure
2. Fraction

Classes

Required Reading
1. The Python Language Reference, Ch 3 (Data Model)
2. The Python Tutorial, Ch 9 (Classes)

Dictionaries vs Structs vs Classes

# Using a dictionary

def dogYears(dog):
    return dog["age"]*7

def printInfo(dog):
    print dog["name"]
    print dogYears(dog)

myPet = dict()
myPet["name"] = "marceau"
myPet["age"] = 10

printInfo(myPet)

# Using a Struct

def dogYears(dog):
    return dog.age*7

def printInfo(dog):
    print dog.name
    print dogYears(dog)

class Struct: pass
myPet = Struct()
myPet.name = "marceau"
myPet.age = 10

printInfo(myPet)

# Using a class

class Dog(object):
    def __init__(self, name, age):
        self.name = name
        self.age = age

    def dogYears(self):
        return self.age*7

    def printInfo(self):
        print self.name
        print self.dogYears()

myPet = Dog("marceau", 10)
myPet.printInfo()

Multiple Instances (One Dictionary Per Instance)

class Dog(object):
    def __init__(self, name, age):
        self.name = name
        self.age = age

    def dogYears(self):
        return self.age*7

    def printInfo(self):
        print self.name
        print self.dogYears()

pet1 = Dog("marceau", 10)
pet2 = Dog("fido", 5)

print "You should not use instance.__dict__ to access"
print "instance attributes, but in theory you sure could...!"

print pet1.__dict__ # prints {'age': 10, 'name': 'marceau'}
print pet2.__dict__ # prints {'age': 5, 'name': 'fido'}

Equality Testing (__eq__)

The problem

class Dog(object):
    def __init__(self, name, age):
        self.name = name
        self.age = age

pet1 = Dog("marceau", 10)
pet2 = Dog("marceau", 10)
print pet1 == pet2  # prints False (but we want True!)

The solution: Use __eq__

class Dog(object):
    def __init__(self, name, age):
        self.name = name
        self.age = age

    def __eq__(self, other):
        return (self.name == other.name) and (self.age == other.age)

pet1 = Dog("marceau", 10)
pet2 = Dog("marceau", 10)
print pet1 == pet2  # prints True (huzzah!)

Converting to Strings (__str__ and __repr__)

The (first) problem

class Dog(object):
    def __init__(self, name, age):
        self.name = name
        self.age = age

    def __eq__(self, other):
        return (self.name == other.name) and (self.age == other.age)

pet1 = Dog("marceau", 10)
print pet1   # prints <__main__.Dog object at 0x0000000002308CC0>

Failed solution #1: Use __str__

class Dog(object):
    def __init__(self, name, age):
        self.name = name
        self.age = age

    def __eq__(self, other):
        return (self.name == other.name) and (self.age == other.age)

    def __str__(self):
        return "Dog(%s, %s)" % (self.name, self.age)

pet1 = Dog("marceau", 10)
print pet1   # prints Dog(marceau, 10)  (huzzah!)
print "That's great, but what about this:"
print [pet1] # prints [<__main__.Dog object at 0x0000000002308CC0>]

Failed Solution #2: Use __repr__

class Dog(object):
    def __init__(self, name, age):
        self.name = name
        self.age = age

    def __eq__(self, other):
        return (self.name == other.name) and (self.age == other.age)

    def __repr__(self):
        return "Dog(%s, %s)" % (self.name, self.age)

pet1 = Dog("marceau", 10)
print pet1   # prints Dog(marceau, 10)
print [pet1] # prints [Dog(marceau, 10)]  (huzzah!)

print "That's great, but what about this (it is preferred"
print "for eval(repr(x)) to return an object equal to x):"
pet2 = eval(repr(pet1))  # NameError: name 'marceau' is not defined
print pet2 == pet1

Working Solution #3: Use __repr__ with %r

class Dog(object):
    def __init__(self, name, age):
        self.name = name
        self.age = age

    def __eq__(self, other):
        return (self.name == other.name) and (self.age == other.age)

    def __repr__(self):
        return "Dog(%r, %r)" % (self.name, self.age)

pet1 = Dog("marceau", 10)
print pet1   # prints Dog('marceau', 10)
print [pet1] # prints [Dog('marceau', 10)]
pet2 = eval(repr(pet1))
print pet2 == pet1  # prints True (huzzah!)

Using in Sets and Dictionaries (__hash__ and __eq__)

The problem

class Dog(object):
    def __init__(self, name, age):
        self.name = name
        self.age = age

    def __eq__(self, other):
        return (self.name == other.name) and (self.age == other.age)

pet1 = Dog("marceau", 10)
s = set()
s.add(pet1)

pet2 = Dog("marceau", 10)
print pet2 == pet1 # prints True
print pet1 in s    # prints True
print pet2 in s    # prints False (should be True!)

The solution (__hash__ and __eq__)

class Dog(object):
    def __init__(self, name, age):
        self.name = name
        self.age = age

    def __eq__(self, other):
        return (self.name == other.name) and (self.age == other.age)
    
    def __hash__(self):
        # replace hashables tuple with instance data attributes for your own class
        hashables = (self.name, self.age)
        # Then just use the code below unmodified.
        # It is based on Bernstein's hash function, which is
        # simple enough but works well enough
        result = 0
        for value in hashables:
            result = 33*result + hash(value)
        return hash(result)

pet1 = Dog("marceau", 10)
s = set()
s.add(pet1)

pet2 = Dog("marceau", 10)
print pet2 == pet1 # prints True
print pet1 in s    # prints True
print pet2 in s    # prints True (huzzah!)

Class Variables (Data Attributes) and Class Methods

class Dog(object):
    dogCount = 0   # Class Variable (Data Attribute)
    
    @classmethod
    def getDogCount(cls):
        return Dog.dogCount

    def __init__(self, name, age):
        self.name = name
        self.age = age
        Dog.dogCount += 1

print Dog.getDogCount()  # prints 0
pet1 = Dog("marceau", 10)
print Dog.getDogCount()  # prints 1
pet2 = Dog("marceau", 10)
print Dog.getDogCount()  # prints 2

Inheritance

Specifying a Superclass

class Monster(object):
    def sayBoo(self):
        print "boo!"

class SeaMonster(Monster):
    def swim(self):
        print "glug glug!"

m1 = SeaMonster()
m2 = Monster()

m1.sayBoo() # prints: boo! 
m2.sayBoo() # prints: boo!

m1.swim()   # prints: glug glug!
m2.swim()   # AttributeError: 'Monster' object has no attribute 'swim' (as expected)

isinstance vs type()

class Monster(object):
    def sayBoo(self):
        print "boo!"

class SeaMonster(Monster):
    def swim(self):
        print "glug glug!"

m1 = SeaMonster()
m2 = Monster()

print type(m1) == Monster     # False
print type(m2) == Monster     # True
print type(m1) == SeaMonster  # True
print type(m2) == SeaMonster  # False

print isinstance(m1, Monster)    # True (differs from (type(m1) == Monster) above!)
print isinstance(m2, Monster)    # True
print isinstance(m1, SeaMonster) # True
print isinstance(m2, SeaMonster) # False

Overriding methods

class Animal(object):
    def __init__(self, name):
        self.name = name
        print "Creating an Animal named", name

    def speak(self):
        print self.name + " says: '%s'" % self.getSpeakingSound()
        
    def getSpeakingSound(self):
        return "<generic animal sound>"

class Dog(Animal):
    # override Animal's __init__, but still call it
    def __init__(self, name):
        # Call superclass's __init__ method
        super(Dog, self).__init__(name)
        # now do dog-specific things
        print "Creating a dog named", self.name

    # override Animal's getSpeakingSound entirely (do not call it)
    def getSpeakingSound(self):
        return "woof!"

class Cat(Animal):
    # override Animal's __init__, but still call it
    def __init__(self, name):
        # Call superclass's __init__ method
        super(Cat, self).__init__(name)
        # now do cat-specific things
        print "Creating a cat named", self.name

    # override Animal's getSpeakingSound entirely (do not call it)
    def getSpeakingSound(self):
        return "meow!"

animal1 = Dog("fred")      # prints: Creating an Animal named fred
                           #         Creating a dog named fred
animal2 = Cat("wilma")     # prints: Creating an Animal named wilma
                           #         Creating a cat named wilma
animal3 = Animal("barney") # prints: Creating an Animal named barney

animal1.speak() # prints: fred says: 'woof!'
animal2.speak() # prints: wilma says: 'meow!'
animal3.speak() # prints: barney says: '<generic animal sound>'

Multiple Inheritance
Note: Python supports multiple inheritance, where a class may have more than one superclass. We will not cover that in 15-112.

Examples from class
1. Text Adventure
  See PartialAdventure.py.
2. Fraction
  See Fractions.py.

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