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Object oriented versus procedural languages using Python - a gentle introduction

Introduction
With procedural languages, we typically break a problem down into functions. (In some languages like Python, we only have functions but in other languages like Pascal, there are functions and procedures, each doing a similar but different job). Each function in a typical solution does just one task and we call this 'modular programming'. To run this program, our main program is simply a series of calls to the different functions. We can use many programming languages either in a procedural way, or using an object oriented approach. Let's see an example of both approaches using Python 3.

Here is a program that calculates an electricty using a procedural approach. We have four functions. The actual program is started by calling the first function using main() at the bottom of the code. This calls the function called main() which in turn asks the user to enter the number of kWh units they've used, and then calls three functions. The first one multiplies the units by the cost per unit. The second one adds a standing charge. The third one is in charge of displaying the total correctly. Each function carries out one and only one job (modular programming) and the main program simply consists of a function call. Note, that if you are unsure of any of the code e.g. the formatting code for the total, then you should refer to the Python course on out website.

Type in this code and get it working.

oo1

What happens when you enter in a postitive whole number? How about a real number? What about a letter instead of a number? How about a negative number? Try and make the code fail. You should find, for example, that the code accepts negative numbers. Is this correct? What might you do about it if you think it is incorrect?

Now let's look at an OO approach. 

oo2

You should write the code under the first program you wrote and then get them both working together, so you can confirm the answers are the same for both programs when you run it (although we haven't added an exception for our OO program using try - except).

Classes, attributes and methods
The OO program starts by making a 'class'. This is a blueprint, a template for the calculations for a bill. In other programs, we might create a class for a student, for example. This wouldn't be for an actual student, but it is a template for a student, showing what data (called attributes) is being kept about the student and what calculations and operations needs to happen on the data (called 'methods'). Inside our definition of the calculateBill class, you can see the data we need to use and some of the methods that can be called.

Objects
Once you have a class, you can then use it to create actual real 'instances' of the class. An instance of a class is called an object. You might have a class defining what a student is, but you must create an instance of the class 'Mary Smith' to get an actual, real student. When you create an object, they have all of the attributes and all of the methods defined in their class. We created an object in our code with this line:

myBill = calculateBill()

This tells the program to create a real object called myBill using the class called calculateBill(). Now myBill has the methods costPower and addStanding. The other method, that starts with the line def __init__(self) is a special method. When you 

Dot notation
Once I have a real object, I can use any of the methods in the object using dot notation. For example, to use the method costPower, I used this line:

total1 = myBill.costPower(unitsUsed)

MyBill.costPower(unitsUsed) says to the program, 'go to the object called myBill. Then call the method costPower using the data in unitsUsed'. 

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