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Classes in Python

Dr. Huidae Cho
Institute for Environmental and Spatial Analysis...University of North Georgia

1   How are classes different from functions?

Classes create a new object type that provides both data and functionality while functions only provide the latter.

Classes can be instantiated to create a new object (an instance).

Classes create a new namespace.

2   Defining a class

A simple class:

class Accumulator:
    # attribute variable
    total = 0
    # attribute function requires self (Accumulator in this case) as the first argument
    # Accumulator.add() is a function object
    def add(self, x):
        self.total += x # access the total attribute
    def print(self):
        print(self.total)
accum = Accumulator()
# accum.add() is a method object
accum.add(1) # the first self argument is missing
accum.add(2)
accum.print()
# what about this?
Accumulator.add(1)
Accumulator.add(2)
Accumulator.print()

3   Class with arguments

Define a class with arguments.

class Accumulator2:
    def __init__(self, start):  # automatically invoked when an instance is created
        self.total = start
    def add(self, x):
        self.total += x
    def print(self):
        print(self.total)
accum = Accumulator2(3)
accum.add(1)
accum.add(2)
accum.print()

4   Class variables

All objects instantiated from the same class share class variables.

class Dog:
    kind = 'canine'             # class variable shared by instances
    def __init__(self, name):
        self.name = name        # instance variable unique to each instance

fido = Dog('Fido')
print(fido.kind, fido.name)

Dog.kind = 'feline'
buddy = Dog('Buddy')            # inherits feline
print(buddy.kind, buddy.name)

Dog.kind = 'bovine'
print(buddy.kind, buddy.name)   # kind from the class

buddy.kind = 'canine'           # creates its own kind attribute
Dog.kind = 'bovine'
print(buddy.kind, buddy.name)   # no more from the class

print(Dog.kind)
print(Dog.name) # Oops!         # classes don't have instance variables

5   Derived classes

We can derive a class from a base class.

class Mammal:
    hair = True
    def __init__(self, name):
        self.name = name
    def walk(self):
        print('{name} walks'.format(name=self.name))
class Dog(Mammal):
    def bark(self):
        print('{name} barks'.format(name=self.name))

dog = Mammal('A dog')
print(dog.hair)
dog.walk()

buddy = Dog('Buddy')
buddy.walk()
buddy.bark()

6   Defining a regular function using a class

Recall the __call__ attribute? We can do the same

class Accumulator:
    def __init__(self):
        self.sum = 0
    def __call__(self, x):
        self.sum += x
    def print(self):
        print(self.sum)
accum = Accumulator()
accum(1)          # just like a regular function
accum(2)
accum.__call__(3) # alternative way
accum.print()

7   What about class pointers?

Classes also have their pointers.

class Class:
	def __init__(self, val):
		self.x = val
	def print_x(self):
		print(self.x)
def Return(c):
	return c

a = Return(Class)(10) # indirect instantiation
a.print_x()

b = Class(20)
b.print_x()

8   Exercises

8.1   Exercise: TupleLister

Write a class that takes a tuple when creating an instance and provides a print method that lists all elements in the tuple one per line.

class TupleLister:
    # your code here

tupls = TupleLister(('apple', 'orange', 'pear'))
tupls.print()
# apple
# orange
# pear

8.2   Exercise: FactSum

Write a class that takes a positive integer and provides two methods for factorial (fact) and summation (sum).

class FactSum:
    # your code here

factsum = FactSum(5)
print(factsum.fact())
print(factsum.sum())
# 120
# 15

8.3   Exercise: SportsCar

Write a Car class with the name, has_engine, and max_passengers variables and the start function.

Write a SportsCar class derived from the Car class that provides an additional has_turbo variable and overrides the start function.

class Car:
    # your code here
class SportsCar:
    # your code here

boring_car = Car('My Car')
print(boring_car.name)           # My Car
print(boring_car.has_engine)     # True
print(boring_car.max_passengers) # 5
boring_car.start()               # vroom...

dream_car = SportsCar('Dream Car')
print(dream_car.name)            # Dream Car
print(dream_car.has_engine)      # True
print(dream_car.has_turbo)       # True
print(dream_car.max_passengers)  # 2
dream_car.start()                # Vrooooom!!!