Introduce oop in python

1. Introduce OOP in Python
Tuan Vo
Jan 2018

2. Remind the method upper() of a String
‘abc’ is data of s
upper() is a method of s
Note: all String can use
method upper()

3. String is a Built-in Data Type
String is called a Built-
in Data Type
String provides many
methods (capitalize,
center,count,decode,… )

4. The question:
Does Python allow us to create a data
type which can contains methods like
String ?

5. The answer:
The answer is “Yes”:
Python allow us to create our data types which
is called user-defined types. User- defined types
might contains data and methods like String.

6. Introduce User- defined types in Python
class Rectangle(object):
width = None
height = None
def CalculateArea(self):
return self.width * self.height
box = Rectangle()
box.width = 100.0
box.height = 200.0
print(box.CalculateArea())
Not worry about the syntax such as class or def CalculateArea(self) , or self
Just concentrate on how it works, I’ll mention the syntax later.

7. Introduce User- defined types in Python
A new type has been created with name: Rectangle
Type Rectangle contains two data: width and height
Type Rectangle contains a method : CalculateArea
The main idea is to manage data and methods in one place.
In this example , width and height of a Rectangle is
managed along with method CalculateArea inside user-
defined type Rectangle
User-defined type such as Rectangle is called class in Python.
Python provides much more functions for a class, will be mentioned in the
next slides.

8. Introduce Class in Python
Define a class
Instance Objects

9. Define a class in Python
Define the name for a class with key word: class Example class Rectangle:
Define the data (properties or fields or attributes): Example width = None
height = None
Define the method: The first parameter of
a method MUST be self.
All properties used inside a method must
be accessed via self. , such as self.width, self.height
def CalculateArea(self):
return self.width * self.height

10. Instance Objects
What is object?
=> Object is an instance of a class or it’s simple a variable with type is the class
class Rectangle:
width = None
height = None
def CalculateArea(self):
return self.width * self.height
box = Rectangle()#box is an instance of class Rectangle , box is called an object
box.width = 100.0
box.height = 200.0
print(box.CalculateArea())
box2 = Rectangle() #box2 is an instance of class Rectangle , box2 is called an object
box2.width = 250
box2.height = 200.0
print(box2.CalculateArea())

11. Set of get value for attributes of an Object
class Rectangle:
width = None
height = None
def CalculateArea(self):
return self.width * self.height
box = Rectangle()
box.width = 100.0 #set value of attribute width
box.height = 200.0 #set value of attribute height
print(box.width) #get value of attribute width
print(box.height) #get value of attribute height
The attributes of an object are sometime called properties or fields.
They handle data for an object.
Set value for an attribute: [NameOfObject].[NameOfAttribute] = [Value]
Get value of an attribute: [NameOfObject].[NameOfAttribute]
Example:

12. Call a method of an Object
Note: Do not need to include self when calling the
method of an instance of a class.
Although the methods of a class always require the first parameter is self, but it’s not necessary to pass
the parameter self to the method (this is syntax of Python )
box = Rectangle()
box.width = 100.0
box.height = 200.0
print(box.CalculateArea())
It’s simple to call the method after the name of an
instance and a dot.
[NameOfObject].[NameOfMethod]([Parameters])
Example box.CalculateArea as below:

13. Summary: Class vs Object
From now on, you can identify terms Class and Object.
A class is a definition of a user-defined type
A class defines its data (properties or fields, or
attributes) and methods
An object is an instance of a class.
An object is a variable that can work in a
program.
A class can have many objects ( many instances )
The ability to put data and methods into a class is called Encapsulation.
Encapsulation is the first characteristic of OOP.
OOP stands for: Object-oriented programming
The term object has been introduced.
The idea of OOP is try to create User-defined types (or class ) to reflect the
objects in real life when programming.
Example if an programming is written to manage a school, then some classes might be created
base on real life
class Student:
Name
EnrollYear
StudentCode
class Course:
StartDate
EndDate
class Subject:
SubjectName

14. Characteristic of OOP
Encapsulation is the first characteristic of OOP.
There are two more characteristic: Inheritance and Polymorphism.

15. Introduce Inheritance in Python
Inheritance is the ability to define a new class base on an existing class.
The existing class is called parent class (or supper ), and the new class is called child class.
See the example below:
class Rectangle:
width = None
height = None
def CalculateArea(self):
return self.width * self.height
class Cuboid(Rectangle):
length = None
def CalculateArea(self):
#reuse attributes width and height from Rectangle
return 2*self.width * self.height + 2*self.width* self.length + 2*self.height*self.length
c = Cuboid()
c.width = 10 #c reuses attribute width from Rectangle
c.height = 20 #c reuses attribute height from Rectangle
c.length = 5
print(c.CalculateArea())#the result will be 700

16. Introduce Inheritance in Python
1. The syntax to create new class: class [NameOfNewClass]([NameOfExistingClass]):
Example: class Cuboid(Rectangle):
2. An object of the new class (child class) can use all public attributes of the existing class
(supper/ parent class )
Example c = Cuboid(), c is an object of Cuboid and Cuboid is the child class of Rectangle, so c
can use attributes width and height of Rectangle.
3. The child class can define a new method has the same name, same parameter(s) as the parent,
this ability is called Overriding Methods. If an object of the child class calls the overriding
method, then the method of the child class will execute not the method of the parent class.
Example class Cuboid defines a new method CalculateArea which is defined in Rectangle,
when calling c.CalculateArea() then the method CalculateArea of Cuboid is called not
CalculateArea of Rectangle

17. Introduce Inheritance in Python
4. A class can be inherited from more than one class or a class might have more than one child
classes.
And a class can only inherit from one class.
Example: Class Sharp has two child classes Rectangle and Circle
class Sharp:
def Draw(self):
print('A Sharp is drawing')
class Rectangle:
width = None
height = None
def Draw(self):
print('A Rectangle is drawing')
def CalculateArea(self):
return self.width * self.height
class Circle:
radius = None
def Draw(self):
print('A Circle is drawing')
def CalculateArea(self):
return self.radius * 3.14

18. Introduce Polymorphism in Python
See the example below:
class Sharp:
def Draw(self):
print('A Sharp is drawing')
class Rectangle:
width = None
height = None
def Draw(self):
print('A Rectangle is drawing')
def CalculateArea(self):
return self.width * self.height
class Circle:
radius = None
def Draw(self):
print('A Circle is drawing')
def CalculateArea(self):
return self.radius * 3.14
s1 = Sharp() #s1 is declare as Sharp
s1.Draw() #will print 'A Sharp is drawing'
s1 = Rectangle() #s1 is now a Rectangle
s1.Draw() #will print 'A Rectangle is drawing'
s1 = Circle() #s1 is now a Circle
s1.Draw() #will print 'A Circle is drawing'

19. Introduce Polymorphism in Python
In the above example:
- Rectangle and Circle inherit from Sharp
- object s1 is first declared as an instance of Sharp
- Then s1 become an instance of Rectangle
- Then s1 become an instance of Circle
The ability s1 can become Rectangle or Circle is called Polymorphism.
In general:
Polymorphism is an ability of an object of an parent class which
can become instance of any of its child classes

20. Summary the characteristic of OOP in Python
Encapsulation
Inheritance
Polymorphism
You can see the most important
characteristic is encapsulation:
No encapsulation > No Inheritance
No Inheritance > No Polymorphism
They are all main characteristic in OOP. The next slides will
introduce some more special functionalities supported in
Python for OOP

21. Introduce Constructor in Class
A constructor is a special method in class which is called when creating an object.
A constructor method MUST be named __init__
A constructor method as the normal methods MUST contain the first parameter self
A class can have only one constructor method
class Rectangle:
width = None
height = None
def __init__(self):#a constructor to set width and height to 0
self.width = 0
self.height = 0
def CalculateArea(self):
return self.width * self.height
box = Rectangle()#width and height will be set to 0 in constructor
print(box.CalculateArea())#the result will be 0

22. Introduce Constructor in Class
Example constructor method can receive parameter(s).
class Rectangle:
width = None
height = None
def __init__(self,w,h):#a constructor to set width and height when creating an object
self.width = w
self.height = h
def CalculateArea(self):
return self.width * self.height
box = Rectangle(10,20)#set width to 10 and height to 20 when creating object
print(box.CalculateArea())#the result will be 200

23. Introduce Destructor in Class
A destructor is a special method in class which is called when destroying an object.
A constructor method MUST be named __del__( self )
class Rectangle:
width = None
height = None
def __del__(self):#a destructor
print('A rectangle is being destroyed')
def CalculateArea(self):
return self.width * self.height
box = Rectangle()
box.width = 10
box.height = 20
print(box.CalculateArea())#the result will be 200
# string 'A rectangle is being destroyed' will be printed
Content of file rectangle-4.py

24. Read more references to know more aspect of
OOP in Python
https://docs.python.org/3/tutorial/classes.html
https://www.tutorialspoint.com/python3/python_classes_objects.htm
https://www.python-course.eu/python3_object_oriented_programming.php
http://greenteapress.com/thinkpython/html/index.html
http://greenteapress.com/thinkpython/html/thinkpython016.html
http://greenteapress.com/thinkpython/html/thinkpython017.html
http://greenteapress.com/thinkpython/html/thinkpython018.html

25. Thank You
Author email: tuanv2t@gmail.com