Week 8 [Mon, Mar 6th] - Programming Topics

• Object-Oriented Programing - Part I

  • Classes
    
    

• Installing 3rd Party Modules

Classes

You can define your own classes (i.e., object types) in Python, which can then be used alongside the built-in classes such as list, int, str, etc.

The syntax for defining a class:

class ClassName:
  # <statement-1>
  .
  .
  .
  # <statement-N>

    

    


    
    

It is customary to use Upper Camel Case for class names.

The syntax for creating (also called instantiating) an object of the class is ClassName().

class Employee:
  pass # an empty class

john = Employee() # create an Employee object
print(type(john)) # print type of the john object
alice = Employee()
print(type(alice))

    

    


    
    

<class 'Employee'>
<class 'Employee'>

    

    


    
    

You can add methods to the class by defining them inside the class definition. Note that a method of a class always take self as the first parameter. self refers to the object itself. When calling the method, there is no need to supply an argument for the self parameter as the target object is implicitly taken as the argument for that parameter.

class Pen:
  
  def write(self, text):
    print('writing:', text)
    
p = Pen()
p.write('It was a dark night ...')

    

    


    
    

writing: It was a dark night ...

    

    


    
    

You can specify how to initialize an object of a class by defining an __init__() method in the class. Here are the important things to note about the __init__() method:

1. There are two underscores in front and two behind the word init.
   _init_(self)
__init__(self)
2. It will be called every time you create an instance of the class.
3. If it has parameters, you need to provide arguments for those parameters when you instantiate an object of that class.

class Person:
  
  def __init__(self, name):
    print(name, 'initialized!')
    
tom = Person('Tom')

    

    


    
    

Tom initialized!

    

    


    
    

An object can have attributes i.e., values attached to the object, just as an object can have methods. Attributes and methods of an object can be accessed using the objectname. syntax, as you did with objects of built-in classes.

print(x.get_age())
print(x.birthday)

    

    


    
    

12
01/11/2000

    

    


    
    

The code within a class needs to use the self. to refer to its own attributes and methods. Furthermore, the best place to initialize attributes is the __init__() method.

class Book:
  
  def __init__(self, book_title, book_author):
    self.title = book_title # initialize attribute title
    self.author = book_author # initialize attribute author
    self.describe() # call another method of the class
    
  def describe(self):
    # print attributes of the object
    print('Book info:', self.title, '/by', self.author) 
    
book1 = Book('The Jungle Book', 'Leo Tolstoy')
book2 = Book('The Art of War', 'Sun Tzu')

print()  # print a blank line
print('Title:', book1.title) 
print('Author:', book1.author)
book1.describe()

    

    


    
    

Book info: The Jungle Book /by Leo Tolstoy
Book info: The Art of War /by Sun Tzu

Title: The Jungle Book
Author: Leo Tolstoy
Book info: The Jungle Book /by Leo Tolstoy

    

    


    
    

You can get your classes to work with each other.

class ReadingList:
  
  def __init__(self, initial_list):
    self.books = initial_list
    
  def add_book(self, book):
    self.books.append(book)
    
  def show_authors(self):
    for b in self.books:
      print(b.author)
      
my_list = ReadingList([book1]) # book1 defined in a previous example 
my_list.add_book(book2) # book2 defined in a previous example
my_list.show_authors()

    

    


    
    

Leo Tolstoy
Sun Tzu

    

    


    
    

# ADD StockItem class here
# It should have an attribute: name
# It should have a method: info()

a = StockItem('Apple')
print(a.info())
print(a.name)
b = StockItem('Bag of Beans')
print(b.info())
b.name = 'Bananas'
print(b.info())

    

    


    
    

Item info: Apple
Apple
Item info: Bag of Beans
Item info: Bananas

    

    


    
    

# ADD StockItem class here
# It should have attributes: name, price
# It should have methods: info, increase_price

items = []
items.append(StockItem('Pen', 5))  # a pen worth 5 dollars
items.append(StockItem('Pencil', 3))
items.append(StockItem('Cup', 1))
for item in items:
    item.increase_price(1)  # increase price by 1
    print(item.info())

    

    


    
    

The price of Pen is 6
The price of Pencil is 4
The price of Cup is 2

    

    


    
    

Appendix A: Installing 3rd-Party Modules

In addition to built-in modules you get with Python by default, there are many modules written by others that you can make use of. You can install third-party modules using Python’s pip tool. This tool downloads and installs Python modules onto your computer from https://pypi.python.org/ (aka PyPi or Python Package Index).

First, ensure that you have pip installed by running this command in a command shell (not Python interpreter shell), which should print out the version of the pip tool you have.

pip --version

    

    


    
    

After that, to install a third-party module (or packages as they are sometimes called), you can run the pip install {packagename} command.

pip install colorama

    

    


    
    

Collecting colorama
  Downloading colorama-0.3.9-py2.py3-none-any.whl
Installing collected packages: colorama
Successfully installed colorama-0.3.9

    

    


    
    

import colorama

colorama.init()  # one-time initialization

print(colorama.Fore.YELLOW + ' Some yellow text')
print(colorama.Fore.BLUE + colorama.Back.WHITE + ' Some blue text in a white background')
print(colorama.Style.RESET_ALL + ' Back to normal now')

    

    


    
    

Resources:

• More about installing 3rd-party modules