CBSE Class 12 - Computer Science(083) Python Chapter 05 classes and objects

1. CHAPTER - 05
CLASSES AND OBJECTS

2. Unit I
Programming and Computational
Thinking (PCT-2)
(80 Theory + 70 Practical)
DCSc & Engg, PGDCA,ADCA,MCA.MSc(IT),Mtech(IT),MPhil (Comp. Sci)
Department of Computer Science, Sainik School Amaravathinagar
Cell No: 9431453730
Praveen M Jigajinni
Prepared by
Courtesy CBSE
Class XII

3. INTRODUCTION

4. INTRODUCTION
This chapter deals with classes in
Python. As Python is fully object-oriented,
you can define your own Classes, inherit
from your own or built-in classes, and
instantiate the classes that you've defined.
But before we start with our study on
classes let us understand about namespaces
and scope rules in Python.

5. NAMESPACES

6. These consist of functions such as max() ,
min() and built-in exception names. This
namespace is created when the Python
interpreter starts up, and is never deleted.
The built-in names actually also live in a
module called__ builtin__
1. BULIT IN NAMESPACE

7. The global namespace for a module is
created when the module definition is read in
and normally lasts until the interpreter quits.
The statements executed by the top-level
invocation of the interpreter, either read
from a script file or interactively, are
considered to be part of a module
called__main__ and they have their own
global namespace.
2. GLOBAL NAMES IN A MODULE

8. The global namespace for a module is
created when the module definition is read in
and normally lasts until the interpreter quits.
The statements executed by the top-level
invocation of the interpreter, either read
from a script file or interactively, are
considered to be part of a module
called__main__ and they have their own
global namespace.
3. LOCAL NAMES IN A FUNCTION INVOCATION

9. The local namespace for a function is
created when the function is called, and
deleted when the function returns or raises
an exception that is not handled within the
function. Even each recursive invocation has
its own local namespace.
3. LOCAL NAMES IN A FUNCTION INVOCATION

10. The local namespace for a function is
created when the function is called, and
deleted when the function returns or raises
an exception that is not handled within the
function. Even each recursive invocation has
its own local namespace.
If we talk about classes and objects, the set
of attributes of an object also form a
namespace.
contd…
3. LOCAL NAMES IN A FUNCTION INVOCATION

11. It must be noted that there is absolutely
no relation between names in different
namespaces. Two different modules may
both define same function without any
confusion because the functions are prefixed
with the module name. That means
module1.cmp() has no relation with
module2.cmp().
3. LOCAL NAMES IN A FUNCTION INVOCATION

12. SCOPE RULES

13. A scope is a region of a Python program
where a namespace is directly accessible. The
location where the names are assigned in the
code determines the scope of visibility of the
name in that code.
SCOPE RULES

14. LEGB RULE

15. According to this rule, when a name is
encountered during the execution of the
program, it searches for that name in the
following order:
L. Local
E. Enclosing functions
G. Global (module)
B. Built-in (Python)
LEGB RULE

16. L. Local - It first makes a local search i.e. in
current def statement. The import
statements and function definitions bind
the module or function name in the local
scope. In fact, all operations that introduce
new names use the local scope.
LEGB RULE – L Local

17. E. Enclosing functions - It searches in all
enclosing functions, form inner to outer.
LEGB RULE – E. Enclosing functions

18. G. Global (module) - It searches for global
modules or for names declared global in a
def within the file.
LEGB RULE – G. Global (module)

19. B. Built-in (Python) - Finally it checks for any
built in functions in Python.
LEGB RULE – B. Built-in (Python)

20. CLASSES AND OBJECTS
DEFINING CLASSES AND OBJECTS

21. Python is an object-oriented programming
language.
Object-oriented programming (OOP)
focuses on creating reusable patterns of
code.
A blueprint or prototypes created by a
programmer for an object is called class.
Class is a container of attributes and
methods. Its an Abstract Data Type (ADT)
Contd…
WHAT IS CLASS?

22. A USER-DEFINED PROTOTYPE for an
object that defines a set of attributes that
characterize any object of the class. The
attributes are data members (class variables
and instance variables) and methods,
accessed via dot notation.
Contd..
WHAT IS CLASS?

23. One of the principles of software
development is the DRY principle, which
stands for DON’T REPEAT YOURSELF. This
principle is geared towards limiting
repetition within code, and object-oriented
programming adheres to the DRY principle
as it reduces redundancy.
DRY PRINCIPLE

24. WHAT IS AN OBJECT?
Object is simply a collection of data
(variables) and methods (functions) that act
on those data.
OR
A unique instance of a data structure
that's defined by its class. An object
comprises both data members (class
variables and instance variables) and
methods.
Contd…

25. WHAT IS AN OBJECT?
OR
An instance of a class. This is the
realized version of the class, where the class
is manifested in the program.
NOTE:
In python everything is an object.

26. CLASS EXAMPLE

27. A Python class starts with the reserved
word 'class', followed by the class name and
a colon(:). The simplest form of class
definition looks like this:
class Test:
var=50
marks=10
def display(self):
print (self.var,self.marks)
DEFINING CLASS

28. DEFINING CLASS

29. DEFINING AN OBJECT AND FUNCTION CALL
After the class creation one can create
an object. The syntax is,
objectname=classname()
t1=Test()
t1.display()
OBJECT DEFINING
Calling member
function using object

30. CLASS VARIABLES or ATTRIBUTES or
DATA MEMBERS
A variable that is shared by all instances
of a class. Class variables are defined within a
class and are called ATTRIBUTES in python. In
other languages it is called as DATA MEMBER.

31. METHODS OR MEMBER FUNCTIONS
A Python method is like a Python function,
but it must be called on an object. And to create
it, you must put it inside a class
In other languages it is called as MEMBER
FUNCTION
Contd..
Python method is like a function, except it is
attached to an object. OR
An function defined in a class is called a
"method". Methods have access to all the data
contained on the instance of the object

32. METHODS OR MEMBER FUNCTIONS
METHOD DEFINING
Python method is a label that you can call
on an object; it is a piece of code to
execute on that object.
Contd..

33. self PARAMETER

34. self PARAMETER
self represents the instance of the class.
By using the "self" keyword we can access the
attributes and methods of the class in python.
NOTE:
self is not keyword. You can
use it as an identifier.

35. There is not strict rule that you have to
use self. It is the convention, one can use any
thing for example
Instead of
self me is used.
One can use
thisob or any
name.
YOU CAN USE ANYTHING INSTEAD OF self

36. DIFFERENCE BETWEEN METHODS AND
FUNCTIONS
There is a thin line between the functions
and the methods
The only major difference is that we
call Python method on an object, but it’s not
the same with functions. Also, methods may
modify an object; Python functions don’t.

37. pass STATEMENT

38. pass STATEMENT
class Test:
pass
Python pass statement is used when a
statement is required syntactically but you
do not want any command or code to
execute. The pass statement is a null
operation; nothing happens when it
executes. For example:

39. pass STATEMENT
class Test:
pass
display(self)
print(‘I am method’)
A Class can have method after pass
statement. For example:

40. del ATTRIBUTE

41. del ATTRIBUTE
The del() method is used to delete the
named attribute from the object, with the
prior permission of the object.
Syntax is
del objectname, attributename
For example
del ob1,name
Contd… in next slide

42. del ATTRIBUTE – EXAMPLE (CODE)

43. del ATTRIBUTE - EXAMPLE (O/P)

44. SCOPE OF VARIABLES IN CLASS

45. SCOPE OF VARIABLES IN CLASS
Scope mean measure of access. It is the life
time of the variable.
In the class the scope of variable is
classified into 3 categories
1. Public
2. Private
3. Protected

46. SCOPE OF VARIABLES IN CLASS
1. Public
By default in class all attributes and
methods are public.
Public members (generally methods
declared in a class) are accessible from
outside the class. The object of the same
class is required to invoke a public method.
The public members are accessed with
dot (.) operator

47. 1. Public
Name and salary
are the attributes
by default public
Object also
accessing the
attribute salary

48. 2. Private
By declaring your data member
private you mean, that nobody should be
able to access it from outside the class, i.e.
strong you can’t touch this policy. Python
supports a technique called name mangling.
Salary is private
member not
accessible by
object or outside

49. 2. Private
Salary is private member
not accessible by object
or outside so attribute
error

50. 2. Private
Python performs name mangling of
private variables. Every member with double
underscore will be changed to
_object._class__variable. If so required, it
can still be accessed from outside the class,
but the practice should be refrained. (Refrain
means stop oneself from doing something)
For example:
Contd..

51. 2. Private
Now salary is made
accessible

52. 2. Private
Output now private member
made accessible, though it is
private variable of class

53. 3. Protected
Protected member is (in C++ and Java)
accessible only from within the class and it’s
subclasses.
How to accomplish this in Python?
The answer is – by convention. By prefixing the
name of your member with a single underscore.
For Example:
Contd..

54. 3. Protected
Making protected variable by
introducing single underscore

55. 3. Protected
Now salary is made
accessible. It is in
protected mode.

56. SOLVIING CBSE QUESTION PAPER 2(C) 4 Marks
PROGRAMS ON CLASSES

57. CBSE QUESTION PAPER - 2015 DELHI

58. (c) Write a class PICTURE in Python with following
specifications : 4
Instance Attributes
– Pno # Numeric value
– Category # String value
– Location # Exhibition Location with
# String value
Methods :
– FixLocation() # A method to assign
# Exhibition Location as per
#Category
# as shown in the following table
CBSE QUESTION PAPER - 2015 DELHI

59. – Enter() # A function to allow user to
enter values
# Pno, Category and call
FixLocation() method
– SeeAll() # A function to display all the
data members
Category Location
Classic Amina
Modern Jim Plaq
Antique Ustad Khan
CBSE QUESTION PAPER - 2015 DELHI

60. CBSE QUESTION PAPER - 2015 OUTSIDE DELHI

61. (c) Write a class PHOTO in Python with following
specifications : 4
Instance Attributes
– Pno # Numeric value
– Category # String Value
– Exhibit # Exhibition Gallery with String value
Methods:
– FixExhibit() #A method to assign
#Exhibition Gallery as per
#Category
#as shown in the following table
CBSE QUESTION PAPER - 2015 OUTSIDE DELHI

62. Category Exhibit
Antique Zaveri
Modern Johnsen
Classic Terenida
-Register() #A function to allow user to
enter values #Pno, Category
and call FixExhibit() method
– ViewAll() #A function to display all the
data members
CBSE QUESTION PAPER - 2015 OUTSIDE DELHI

63. CBSE QUESTION PAPER - 2016 DELHI

64. CBSE QUESTION PAPER - 2016 DELHI
(c) Write a class CITY in Python with following
specifications : 4
Instance Attributes
– Code # Numeric value
– Name # String value
– Pop # Numeric value for Population
– KM # Numeric value
– Density # Numeric value for Population
Density than 12000.

65. CBSE QUESTION PAPER - 2016 DELHI
Methods:
– CalDen() # Method to calculate Density
as Pop/KM
– Record() # Method to allow user to
enter values Code, Name, Pop,
KM and call CalDen() method
– See() # Method to display all the
data members also display a
message “Highly Populated
Area” if the Density is more
than 12000.

66. CLASS TEST
1. Write a program to illustrate class.
2.What is object? How it is created explain in
detail with suitable examples.
3.What is visibility mode? What are its types?
Explain in detail the various types of visibility
modes.
4. Explain in detail LEGB Rule
Class : XII Time: 40 Min
Topic: Classes & Objects Max Marks: 40
Each Question carries 5 Marks

67. Thank You