This document summarizes an introductory session on object-oriented programming in Python. It introduces key concepts of OOP like classes, objects, attributes, methods, encapsulation, inheritance and provides examples of defining classes for a Person and Bank Account. It also demonstrates how to define a class, create objects, access attributes and methods in Python. The benefits of OOP like code reusability, abstraction and inheritance are discussed.

1. CLTL Python Course
Object Oriented Programming in Python
1st session
April 24th 2013

2. What is Object Oriented Programming ?
● Change of paradigm Procedural vs. Data oriented
● Procedural programming
– Focus on functions or procedures
– Usually process and data mixed in the code
– Python basic datatypes (lists, dicts, tuples...)
● Data Oriented programming
– Focus is the data
– Process and data well defined and separated
– Datatypes as complex as we need

3. Main points of the Object Oriented Programming
● Objects, classes, attributes, methods ...
● Reusability
● Data abstraction
● Encapsulation / data hiding
● Natural way of thinking
● Inheritance

4. Classes + Objects
● What is a Class ?
– Concept
– Data type
– Template
– ....
● Objects are instances of a class
CLASS = Person OBJECT = Ruben
● We can define our own classes !

5. Examples of Classes
PERSON
● Attributes
● First name
● Last name
● Mail address
● Birth date
● Methods
● Send a mail
● Ask if the person is
over 18
BANK ACCOUNT
● Attributes
● PERSON
● Number
● Balance
● Methods
● Withdrawal
● Transfer
● Deposit

6. Examples of Objects
PERSON
● John Smith
● Laura Ingells
● Marc van Gaal
BANK ACCOUNT
● Account 12341234
● Account 73763234
● Account 87388234
All these objects have the ATTRIBUTES and METHODS
defined by the class
There is no any SEMANTIC in this, we defined the CLASS

7. Python: Classes and objects
● Define a class in python:
● Define a property for the class (indentation)
● Create an objet of this class
● Access to a property or method of the object

8. Python: Class Methods
● Are basically like a function but defined within a class
● They can be called through objects of the class
● ALL the class methods ALWAYS have as first parameter
the object itself, and it's called self

9. Python: Manipulating data
● The class methods manipulate attributes of the objects
● These methods access to the data through the self parameter

10. Python: Type of Methods
● 'Getter' methods
– To get some information (attribute) from the object
● 'Setter' methods
– To set some information (attribute) to the object
● Processing methods
– To do some stuff
● Special methods
– Overloading methods
– The __init__ method

11. Python: __init__ method
● Always with that name _ _ init _ _ it's the constructor
of the class
● Used for define and initialize attributes
● It's called AUTOMATICALLY when we create the object

12. Python: __init__ method II
● Useful ! --> __init__ can get as many extra parameters as
we need
● Exercise --> Create a method for the function call
print_pet, that prints the data of the pet

13. Classes and objects. Advantages
● Code more clean, easy to reuse and debug and well structured
● Data encapsulation
– You can make parts of your data “private”
– Variables starting with __ are private
● Data abstraction
– Data access through the methods, no matter how you store the
data internally
– Class Person:
● Attribute age as INTEGER or DATE
● Method is_over_18(...) --> True/False
● Reusability of code
– Classes can be stored in one file and used used in N different files
(import statement)

14. Classes and objects
● Highly recommended when you deal with complex data (and even
with simple data)
– You could represent the 'pet' as a tuple
– Now you decide to include more attributes for your pet
– Now you want to store also the information of the father,
which is another PET
– Now ... impossible to manage anymore :)

15. Classes and objects

16. Inheritance
● One class can inherit from another class (more than one)
– General class --> parent / base class
– Specialized class --> subclass
● Similar to hyperonym relation in WordNet :)

17. Inheritance II
● Subclasses can:
– Reuse data/methods from base classes
– Extend the functionality of base classes
– Modify the behaviour of base classes
● Advantages of inheritance:
– Reuse of code
– Logic way of structure our classes
– Code more clean and easy to debug
– ....

18. Inheritance III
● How can we use inheritance in python:
● Our example:
● Both dog and pet are classes
● Dog is a subclass of pet
● All that a pet can do, also a dog
can do it
● NOT all that a dog can do, a pet
can do it

19. Inheritance IV. Exercise :)
● Extend the pet class with a method to print the name of the
dog
● Create one object of 'pet' and one of 'dog'
● Which methods and data has any of the objects available?