Class and objects

1. MODULE 5
 Chapter 15:
 Classes and
objects
 Chapter 16:
 Classes and
functions

2. What is a Class and Objects in Python?
Class: The class is a user-defined data structure that binds the data members and
methods into a single unit.
Class is a blueprint or code template for object creation. Using a class, you can
create as many objects as you want.
•Object: An object is an instance of a class. It is a collection of attributes
(variables) and methods. We use the object of a class to perform actions.
Objects have two characteristics: They have states and behaviors (object has
attributes and methods attached to it) Attributes represent its state, and methods
represent its behavior. Using its methods, we can modify its state.
An object is a real-life entity. It is the collection of various data and functions
that operate on those data.

3. Every object has the following property.
Identity: Every object must be uniquely identified.
State: An object has an attribute that represents a state of an object,
and it also reflects the property of an object.
Behavior: An object has methods that represent its behavior.

6. What are classes and objects in
Python?
• Python is an object oriented programming language.
• Class is a abstract data type which can be defined
as a template or blueprint that describes the
behavior / state that the object of its type support.
• An object is an instance of class . Objects have
states and behaviors. Example: A dog has states -
color, name, breed as well as behaviors – wagging
the tail, barking, eating
• Object is simply a collection of data (variables)
and methods (functions) that act on those
data.
• The process of creating this object is
called instantiation

7. Defining a Class in Python
• We define a class using the keyword
class.
• The first string is called docstring and has
a brief description about the class.
Although not mandatory, this is
recommended.
• A class attributes are data
members( variables) and
functions.

8. Syntax
The class statement creates a new class
definition.
The name of the class immediately follows
the keyword class followed by a colon as
follows −
classClassName:
“””Optional class documentation string' class_suite”””
class_suite
• The class has a documentation string, which can be accessed via
ClassName. doc .
• The class_suite consists of all the component statements
defining class members, data attributes and functions.

9. Example
The header indicates that the new class is called Point.
The body is a docstring that explains what the class is for. You can define
variables and
methods inside a class definition.

10. Creating an Object in Python
• Class can be used to create new
object
instances (instantiation) of that
class.
• The procedure to create an object is
similar to a function call.
• Example: blank = Point()
The return value is a reference to a Point object, which we
assign to blank.

11. Example 1 1e1

12. Example 2

13. • The variable empCount is a class variable whose
value is shared among all instances of a this
class. This can be accessed as
Employee.empCount from inside the class or
outside the class.
• The first method init () is a special method, which
is called class constructor or initialization
method that Python calls when you create a
new instance of this class.
• We can declare other class methods like normal
functions with the exception that the first
argument to each method is self.
• Python adds the self argument to the list for
you; you do not need to include it when you call
the methods.

14. Creating Instance Objects
• To create instances of a class, you call the
class using class name and pass in whatever
arguments itsinit method accepts
emp1 = Employee(“Raju",
2000) emp2 =
Employee(“Swamy", 5000)

15. Accessing Attributes
• We can access the object's attributes
using the dot operator with object.
• Class variable would be accessed using
class
name as follows −
emp1.displayEmploy
ee()
emp2.displayEmploy
ee()
print ("Total Employee %d" %
Employee.empCount)

18. Example2 : Rectangle
• Rectangle can be created using following
two possibilities:
– You could specify one corner of the
rectangle (or the center), the width, and the
height.
– You could specify two opposing corners.

21. Instances as return values
• Functions can return
instances.
• For example, find_center
takes a Rectangle as an
argument and returns a Point
that contains the coordinates
of the center of the
Rectangle:

23. Objects are mutable
• You can change the state of an object by
making an assignment to one of its
attributes. For example, to change the
size of a rectangle without changing its
position, you can modify the values of
width and height:
• box.width = box.width + 50
• box.height = box.height + 100

24. Modifying through functions
• You can also write functions that
modify objects.
• For example, grow_rectangle takes a
Rectangle object and two numbers,
dwidth and dheight, and adds the
numbers to the width and height of the
rectangle:
def grow_rectangle(rect, dwidth,
dheight): rect.width +=
dwidth rect.height +=
dheight

25. Copying
• Copying an object is often an alternative
to aliasing. The copy module contains a
function called copy that can duplicate
any object:

27. Exercise:
1. Write a definition for a class named Circle with attributes center and
radius, where center is a Point object and radius is a number.
2. Write a function called draw_rect that takes a Turtle object and a
Rectangle and uses the Turtle to draw the Rectangle