Itse1359-1410-overview of python classes  (Page 3/9)

Three important OOP concepts

OOP is an abbreviation for Object-Oriented Programming. Most books on OOP will tell you that in order to understand OOP, you need to understand the following three concepts :

• Encapsulation
• Inheritance
• Polymorphism

I will discuss the first two concepts in more detail in this and future modules. As near as I can tell, unlike C++ and Java, Python does not support polymorphism, atleast not in any significant way.

C++ and Java support two forms of polymorphism:

• Compile-time polymorphism
• Runtime polymorphism

Both of these depend on the "strongly-typed" nature of C++ and Java. Because Python is a "weakly-typed" (if typed at all) programming language, I don't know how to implement either form of polymorphism using Python. (However, if I am wrong on this, please let me know and I will be happy to learn how to implement polymorphism in Python.)

Generally, speaking, the concepts in the above list increase in difficulty going down the list from top to bottom. Therefore, I will begin with encapsulation and work my way down the list in successive modules.

What is an Object-Oriented Program ?

Many authors would answer this question something like the following:

An Object-Oriented Program consists of a group of cooperating objects, exchanging messages, for the purpose of achieving a common objective.

What is an object ?

An object is a software construct that encapsulates data, along with the ability to use or modify that data.

What is encapsulation ?

An interesting description of encapsulation was provided in an article by Rocky Lhotka regarding VB.NET. That description reads as follows:

"Encapsulation is the concept that an object should totally separate its interface from its implementation. All the data and implementation code for an object should be entirely hidden behind its interface. The idea is that we can create an interface (Public methods in a class) and, as long as that interface remains consistent, the application can interact with our objects. This remains true even if we entirely rewrite the code within a given method thus the interface is independent of the implementation."

I like this description, so I won't try to improve on it. I do need to point out that according to The Python Tutorial ,

"In fact, nothing in Python makes it possible to enforce data hiding -- it is all based upon convention."

Within that restriction, it is still possible to approximate encapsulation as described by Lhotka using Pythonprovided that appropriate conventions are adhered to.

Discussion and sample code

A real-world analogy -- a car radio

Abstract concepts, such as the concept of an object or encapsulation, can often be best understood by comparing them to real-world analogies. One imperfect, but fairly good analogy to a software object is the radio in your car.

The ability to store data

Your car radio probably has the ability to store data, and to allow you to use and modify that data at will. (However, you can only use and modify that data through use of the human interface that is provided by the manufacturer of the radio.) The data that can be stored in your car radio probably includes a list of five or more frequencies that correspond to your favorite radio stations.