Class 04 readings: Classes and Objects

Python is an object oriented programming language.

Almost everything in Python is an object, with its properties and methods.

A Class is like an object constructor, or a “blueprint” for creating objects.

Create a Class

To create a class, use the keyword class:

Example Create a class named MyClass, with a property named x:

class MyClass:
  x = 5

Create Object

Now we can use the class named MyClass to create objects:

Example Create an object named p1, and print the value of x:

p1 = MyClass()
print(p1.x)

The `init()` Function

The examples above are classes and objects in their simplest form, and are not really useful in real life applications.

To understand the meaning of classes we have to understand the built-in __init__() function.

All classes have a function called __init__(), which is always executed when the class is being initiated.

Use the __init__() function to assign values to object properties, or other operations that are necessary to do when the object is being created: Example

Create a class named Person, use the __init__() function to assign values for name and age:

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

p1 = Person("John", 36)

print(p1.name)
print(p1.age)

Object Methods

Objects can also contain methods. Methods in objects are functions that belong to the object.

Let us create a method in the Person class:

Example Insert a function that prints a greeting, and execute it on the p1 object:

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

  def myfunc(self):
    print("Hello my name is " + self.name)

p1 = Person("John", 36)
p1.myfunc()

The self Parameter

The self parameter is a reference to the current instance of the class, and is used to access variables that belongs to the class.

It does not have to be named self , you can call it whatever you like, but it has to be the first parameter of any function in the class.

Modify Object Properties

You can modify properties on objects like this:

Example Set the age of p1 to 40:

p1.age = 40

Delete Object Properties

You can delete properties on objects by using the del keyword:

Example Delete the age property from the p1 object:

del p1.age

Thinking Recursively in Python

Let’s take Christmas presents delivery for example:

Santa Claus has a list of houses he loops through. He goes to a house, drops off the presents, eats the cookies and milk, and moves on to the next house on the list. Since this algorithm for delivering presents is based on an explicit loop construction, it is called an iterative algorithm.

The algorithm for iterative present delivery implemented in Python:

houses = ["Eric's house", "Kenny's house", "Kyle's house", "Stan's house"]

def deliver_presents_iteratively():
    for house in houses:
        print("Delivering presents to", house)

But at Santa’s age, he shouldn’t have to deliver all the presents by himself. I propose an algorithm with which he can divide the work of delivering presents among his elves:

Appoint an elf and give all the work to him
Assign titles and responsibilities to the elves based on the number of houses for which they are responsible:
- `> 1 He is a manager and can appoint two elves and divide his work among them
- = 1 He is a worker and has to deliver the presents to the house assigned to him

This is the typical structure of a recursive algorithm. If the current problem represents a simple case, solve it. If not, divide it into subproblems and apply the same strategy to them.

The algorithm for recursive present delivery implemented in Python:

houses = ["Eric's house", "Kenny's house", "Kyle's house", "Stan's house"]

# Each function call represents an elf doing his work 
def deliver_presents_recursively(houses):
    # Worker elf doing his work
    if len(houses) == 1:
        house = houses[0]
        print("Delivering presents to", house)

    # Manager elf doing his work
    else:
        mid = len(houses) // 2
        first_half = houses[:mid]
        second_half = houses[mid:]

        # Divides his work among two elves
        deliver_presents_recursively(first_half)
        deliver_presents_recursively(second_half)