NCERT Textbook: Brief Overview of Python | Informatics Practices for Class 11 - Humanities/Arts PDF Download

 Page 1


3.1 Introduct Ion to Python An ordered set of instructions or commands to be 
executed by a computer is called a program. The 
language used to specify those set of instructions 
to the computer is called a programming language 
for example Python, C, C++, Java, etc.
This chapter gives a brief overview of Python 
programming language. Python is a very popular 
and easy to learn programming language, created 
by Guido van Rossum in 1991. It is used in a 
variety of fields, including software development, 
web development, scientific computing, big  data 
Brief Overview 
of Python
Chapter 
3 
In this chapter
 » Introduction to Python
 » Python Keywords
 » Identifiers 
 » Variables
 » Data Types
 » Operators
 » Expressions
 » Input and Output
 » Debugging
 » Functions
 » if..else Statements
 » for Loop
 » Nested Loops
“Don't you hate code that's not properly 
indented? Making it [indenting] part of 
the syntax guarantees that all code is 
properly indented.” 
—  G. van Rossum
Chap 3.indd   31 19-Jul-19   3:16:31 PM
2024-25
Page 2


3.1 Introduct Ion to Python An ordered set of instructions or commands to be 
executed by a computer is called a program. The 
language used to specify those set of instructions 
to the computer is called a programming language 
for example Python, C, C++, Java, etc.
This chapter gives a brief overview of Python 
programming language. Python is a very popular 
and easy to learn programming language, created 
by Guido van Rossum in 1991. It is used in a 
variety of fields, including software development, 
web development, scientific computing, big  data 
Brief Overview 
of Python
Chapter 
3 
In this chapter
 » Introduction to Python
 » Python Keywords
 » Identifiers 
 » Variables
 » Data Types
 » Operators
 » Expressions
 » Input and Output
 » Debugging
 » Functions
 » if..else Statements
 » for Loop
 » Nested Loops
“Don't you hate code that's not properly 
indented? Making it [indenting] part of 
the syntax guarantees that all code is 
properly indented.” 
—  G. van Rossum
Chap 3.indd   31 19-Jul-19   3:16:31 PM
2024-25
32
Informat Ics Pract Ices – c lass XI
3.1.2 Execution Modes 
There are two ways to run a program using the Python 
interpreter:
a) Interactive mode 
b) Script mode
(A) Interactive Mode
In the interactive mode, we can type a Python statement 
on the >>> prompt directly. As soon as we press enter, 
the interpreter executes the statement and displays the 
result(s), as shown in Figure 3.2.
Working in the interactive mode is convenient for 
testing a single line code for instant execution. But in 
the interactive mode, we cannot save the statements for 
Figure 3.2: Python Interpreter in Interactive Mode
Download Python
The latest version of 
Python is available on the 
official website: 
https://www.python.
org/
and Artificial Intelligence. The programs given in this book 
are written using Python 3.7.0. However, one can install 
any version of Python 3 to follow the programs given.
3.1.1 Working with Python 
To write and run (execute) a Python program, we need 
to have a Python interpreter installed on our computer 
or we can use any online Python interpreter. The 
interpreter is also called Python shell. A sample screen 
of Python interpreter is shown in Figure 3.1. Here, the 
symbol >>> is called Python prompt, which indicates 
that the interpreter is ready to receive instructions. 
We can type commands or statements on this prompt  
for execution.
Figure 3.1: Python Interpreter or Shell
Chap 3.indd   32 19-Jul-19   3:16:32 PM
2024-25
Page 3


3.1 Introduct Ion to Python An ordered set of instructions or commands to be 
executed by a computer is called a program. The 
language used to specify those set of instructions 
to the computer is called a programming language 
for example Python, C, C++, Java, etc.
This chapter gives a brief overview of Python 
programming language. Python is a very popular 
and easy to learn programming language, created 
by Guido van Rossum in 1991. It is used in a 
variety of fields, including software development, 
web development, scientific computing, big  data 
Brief Overview 
of Python
Chapter 
3 
In this chapter
 » Introduction to Python
 » Python Keywords
 » Identifiers 
 » Variables
 » Data Types
 » Operators
 » Expressions
 » Input and Output
 » Debugging
 » Functions
 » if..else Statements
 » for Loop
 » Nested Loops
“Don't you hate code that's not properly 
indented? Making it [indenting] part of 
the syntax guarantees that all code is 
properly indented.” 
—  G. van Rossum
Chap 3.indd   31 19-Jul-19   3:16:31 PM
2024-25
32
Informat Ics Pract Ices – c lass XI
3.1.2 Execution Modes 
There are two ways to run a program using the Python 
interpreter:
a) Interactive mode 
b) Script mode
(A) Interactive Mode
In the interactive mode, we can type a Python statement 
on the >>> prompt directly. As soon as we press enter, 
the interpreter executes the statement and displays the 
result(s), as shown in Figure 3.2.
Working in the interactive mode is convenient for 
testing a single line code for instant execution. But in 
the interactive mode, we cannot save the statements for 
Figure 3.2: Python Interpreter in Interactive Mode
Download Python
The latest version of 
Python is available on the 
official website: 
https://www.python.
org/
and Artificial Intelligence. The programs given in this book 
are written using Python 3.7.0. However, one can install 
any version of Python 3 to follow the programs given.
3.1.1 Working with Python 
To write and run (execute) a Python program, we need 
to have a Python interpreter installed on our computer 
or we can use any online Python interpreter. The 
interpreter is also called Python shell. A sample screen 
of Python interpreter is shown in Figure 3.1. Here, the 
symbol >>> is called Python prompt, which indicates 
that the interpreter is ready to receive instructions. 
We can type commands or statements on this prompt  
for execution.
Figure 3.1: Python Interpreter or Shell
Chap 3.indd   32 19-Jul-19   3:16:32 PM
2024-25
Brief Overview Of Pyth On 
33
future use and we have to retype the statements to run 
them again.
(B) Script Mode
In the script mode, we can write a Python program in 
a file, save it and then use the interpreter to execute 
the program from the file. Such program files have 
a .py extension and they are also known as scripts. 
Usually, beginners learn Python in interactive mode, 
but for programs having more than a few lines, we 
should always save the code in files for future use. 
Python scripts can be created using any editor. Python 
has a built-in editor called IDLE which can be used 
to create programs. After opening the IDLE, we can 
click File>New File to create a new file, then write our 
program on that file and save it with a desired name. 
By default, the Python scripts are saved in the Python 
installation folder. 
Figure 3.3: Python Code in Script Mode (prog3-1.py)
Figure 3.4: Execution of Python in Script mode using IDLE
To execute a Python program in script mode,  
a) Open the program using an editor, for example 
IDLE as shown in Figure 3.3.
b) In IDLE, go to [Run]->[Run Module] to execute the 
prog3-1.py as shown in Figure 3.4.
c) The output appears on shell as shown in Figure 
3.5. 
Figure 3.5: Output of a Program prog 3-1.py executed in Script Mode
IDLE : Integrated  
Development and  
Learning Environment 
Chap 3.indd   33 19-Jul-19   3:16:32 PM
2024-25
Page 4


3.1 Introduct Ion to Python An ordered set of instructions or commands to be 
executed by a computer is called a program. The 
language used to specify those set of instructions 
to the computer is called a programming language 
for example Python, C, C++, Java, etc.
This chapter gives a brief overview of Python 
programming language. Python is a very popular 
and easy to learn programming language, created 
by Guido van Rossum in 1991. It is used in a 
variety of fields, including software development, 
web development, scientific computing, big  data 
Brief Overview 
of Python
Chapter 
3 
In this chapter
 » Introduction to Python
 » Python Keywords
 » Identifiers 
 » Variables
 » Data Types
 » Operators
 » Expressions
 » Input and Output
 » Debugging
 » Functions
 » if..else Statements
 » for Loop
 » Nested Loops
“Don't you hate code that's not properly 
indented? Making it [indenting] part of 
the syntax guarantees that all code is 
properly indented.” 
—  G. van Rossum
Chap 3.indd   31 19-Jul-19   3:16:31 PM
2024-25
32
Informat Ics Pract Ices – c lass XI
3.1.2 Execution Modes 
There are two ways to run a program using the Python 
interpreter:
a) Interactive mode 
b) Script mode
(A) Interactive Mode
In the interactive mode, we can type a Python statement 
on the >>> prompt directly. As soon as we press enter, 
the interpreter executes the statement and displays the 
result(s), as shown in Figure 3.2.
Working in the interactive mode is convenient for 
testing a single line code for instant execution. But in 
the interactive mode, we cannot save the statements for 
Figure 3.2: Python Interpreter in Interactive Mode
Download Python
The latest version of 
Python is available on the 
official website: 
https://www.python.
org/
and Artificial Intelligence. The programs given in this book 
are written using Python 3.7.0. However, one can install 
any version of Python 3 to follow the programs given.
3.1.1 Working with Python 
To write and run (execute) a Python program, we need 
to have a Python interpreter installed on our computer 
or we can use any online Python interpreter. The 
interpreter is also called Python shell. A sample screen 
of Python interpreter is shown in Figure 3.1. Here, the 
symbol >>> is called Python prompt, which indicates 
that the interpreter is ready to receive instructions. 
We can type commands or statements on this prompt  
for execution.
Figure 3.1: Python Interpreter or Shell
Chap 3.indd   32 19-Jul-19   3:16:32 PM
2024-25
Brief Overview Of Pyth On 
33
future use and we have to retype the statements to run 
them again.
(B) Script Mode
In the script mode, we can write a Python program in 
a file, save it and then use the interpreter to execute 
the program from the file. Such program files have 
a .py extension and they are also known as scripts. 
Usually, beginners learn Python in interactive mode, 
but for programs having more than a few lines, we 
should always save the code in files for future use. 
Python scripts can be created using any editor. Python 
has a built-in editor called IDLE which can be used 
to create programs. After opening the IDLE, we can 
click File>New File to create a new file, then write our 
program on that file and save it with a desired name. 
By default, the Python scripts are saved in the Python 
installation folder. 
Figure 3.3: Python Code in Script Mode (prog3-1.py)
Figure 3.4: Execution of Python in Script mode using IDLE
To execute a Python program in script mode,  
a) Open the program using an editor, for example 
IDLE as shown in Figure 3.3.
b) In IDLE, go to [Run]->[Run Module] to execute the 
prog3-1.py as shown in Figure 3.4.
c) The output appears on shell as shown in Figure 
3.5. 
Figure 3.5: Output of a Program prog 3-1.py executed in Script Mode
IDLE : Integrated  
Development and  
Learning Environment 
Chap 3.indd   33 19-Jul-19   3:16:32 PM
2024-25
34
Informat Ics Pract Ices – c lass XI
3.2 Python Keywords Keywords are reserved words. Each keyword has a 
specific meaning to the Python interpreter. As Python 
is case sensitive, keywords must be written exactly as 
given in Table 3.1.
Table 3.1 Python keywords
False class finally is return
None
continue for lambda
try
True
def from nonlocal
while
and
del global not
with
as
elif if or
yield
assert
else import pass
 
break
except in raise
 
3.3 Ident If Iers In programming languages, identifiers are names used 
to identify a variable, function, or other entities in a 
program. The rules for naming an identifier in Python 
are as follows: 
• The name should begin with an uppercase or a 
lowercase alphabet or an underscore sign (_). This 
may be followed by any combination of characters 
a-z, A-Z, 0-9 or underscore (_). Thus, an identifier 
cannot start with a digit.
• It can be of any length. (However, it is preferred to 
keep it short and meaningful). 
• It should not be a keyword or reserved word given in 
Table 3.1.
• We cannot use special symbols like !, @, #, $, %, etc. 
in identifiers.
For example, to find the average of marks obtained 
by a student in three subjects namely Maths, English, 
Informatics Practices (IP), we can choose the identifiers 
as marksMaths, marksEnglish, marksIP and avg 
rather than a, b, c, or A, B, C, as such alphabets do not 
give any clue about the data that variable refers to.
avg = (marksMaths + marksEnglish + marksIP)/3
3.4 Var Iables Variable is an identifier whose value can change. For 
example  variable age can have  different  value for 
different person. Variable name should be unique in a 
program. Value of a variable can be string (for example, 
n otes Chap 3.indd   34 19-Jul-19   3:16:32 PM
2024-25
Page 5


3.1 Introduct Ion to Python An ordered set of instructions or commands to be 
executed by a computer is called a program. The 
language used to specify those set of instructions 
to the computer is called a programming language 
for example Python, C, C++, Java, etc.
This chapter gives a brief overview of Python 
programming language. Python is a very popular 
and easy to learn programming language, created 
by Guido van Rossum in 1991. It is used in a 
variety of fields, including software development, 
web development, scientific computing, big  data 
Brief Overview 
of Python
Chapter 
3 
In this chapter
 » Introduction to Python
 » Python Keywords
 » Identifiers 
 » Variables
 » Data Types
 » Operators
 » Expressions
 » Input and Output
 » Debugging
 » Functions
 » if..else Statements
 » for Loop
 » Nested Loops
“Don't you hate code that's not properly 
indented? Making it [indenting] part of 
the syntax guarantees that all code is 
properly indented.” 
—  G. van Rossum
Chap 3.indd   31 19-Jul-19   3:16:31 PM
2024-25
32
Informat Ics Pract Ices – c lass XI
3.1.2 Execution Modes 
There are two ways to run a program using the Python 
interpreter:
a) Interactive mode 
b) Script mode
(A) Interactive Mode
In the interactive mode, we can type a Python statement 
on the >>> prompt directly. As soon as we press enter, 
the interpreter executes the statement and displays the 
result(s), as shown in Figure 3.2.
Working in the interactive mode is convenient for 
testing a single line code for instant execution. But in 
the interactive mode, we cannot save the statements for 
Figure 3.2: Python Interpreter in Interactive Mode
Download Python
The latest version of 
Python is available on the 
official website: 
https://www.python.
org/
and Artificial Intelligence. The programs given in this book 
are written using Python 3.7.0. However, one can install 
any version of Python 3 to follow the programs given.
3.1.1 Working with Python 
To write and run (execute) a Python program, we need 
to have a Python interpreter installed on our computer 
or we can use any online Python interpreter. The 
interpreter is also called Python shell. A sample screen 
of Python interpreter is shown in Figure 3.1. Here, the 
symbol >>> is called Python prompt, which indicates 
that the interpreter is ready to receive instructions. 
We can type commands or statements on this prompt  
for execution.
Figure 3.1: Python Interpreter or Shell
Chap 3.indd   32 19-Jul-19   3:16:32 PM
2024-25
Brief Overview Of Pyth On 
33
future use and we have to retype the statements to run 
them again.
(B) Script Mode
In the script mode, we can write a Python program in 
a file, save it and then use the interpreter to execute 
the program from the file. Such program files have 
a .py extension and they are also known as scripts. 
Usually, beginners learn Python in interactive mode, 
but for programs having more than a few lines, we 
should always save the code in files for future use. 
Python scripts can be created using any editor. Python 
has a built-in editor called IDLE which can be used 
to create programs. After opening the IDLE, we can 
click File>New File to create a new file, then write our 
program on that file and save it with a desired name. 
By default, the Python scripts are saved in the Python 
installation folder. 
Figure 3.3: Python Code in Script Mode (prog3-1.py)
Figure 3.4: Execution of Python in Script mode using IDLE
To execute a Python program in script mode,  
a) Open the program using an editor, for example 
IDLE as shown in Figure 3.3.
b) In IDLE, go to [Run]->[Run Module] to execute the 
prog3-1.py as shown in Figure 3.4.
c) The output appears on shell as shown in Figure 
3.5. 
Figure 3.5: Output of a Program prog 3-1.py executed in Script Mode
IDLE : Integrated  
Development and  
Learning Environment 
Chap 3.indd   33 19-Jul-19   3:16:32 PM
2024-25
34
Informat Ics Pract Ices – c lass XI
3.2 Python Keywords Keywords are reserved words. Each keyword has a 
specific meaning to the Python interpreter. As Python 
is case sensitive, keywords must be written exactly as 
given in Table 3.1.
Table 3.1 Python keywords
False class finally is return
None
continue for lambda
try
True
def from nonlocal
while
and
del global not
with
as
elif if or
yield
assert
else import pass
 
break
except in raise
 
3.3 Ident If Iers In programming languages, identifiers are names used 
to identify a variable, function, or other entities in a 
program. The rules for naming an identifier in Python 
are as follows: 
• The name should begin with an uppercase or a 
lowercase alphabet or an underscore sign (_). This 
may be followed by any combination of characters 
a-z, A-Z, 0-9 or underscore (_). Thus, an identifier 
cannot start with a digit.
• It can be of any length. (However, it is preferred to 
keep it short and meaningful). 
• It should not be a keyword or reserved word given in 
Table 3.1.
• We cannot use special symbols like !, @, #, $, %, etc. 
in identifiers.
For example, to find the average of marks obtained 
by a student in three subjects namely Maths, English, 
Informatics Practices (IP), we can choose the identifiers 
as marksMaths, marksEnglish, marksIP and avg 
rather than a, b, c, or A, B, C, as such alphabets do not 
give any clue about the data that variable refers to.
avg = (marksMaths + marksEnglish + marksIP)/3
3.4 Var Iables Variable is an identifier whose value can change. For 
example  variable age can have  different  value for 
different person. Variable name should be unique in a 
program. Value of a variable can be string (for example, 
n otes Chap 3.indd   34 19-Jul-19   3:16:32 PM
2024-25
Brief Overview Of Pyth On 
35
‘b’, ‘Global Citizen’), number (for example 10,71,80.52) 
or any combination of alphanumeric (alphabets and 
numbers for example ‘b10’) characters. In Python, we 
can use an assignment statement to create new variables 
and assign specific values to them. 
gender   = 'M'
message  = "Keep Smiling"
price    = 987.9
Variables must always be assigned values before 
they are used in the program, otherwise it will lead 
to an error. Wherever a variable name occurs in the 
program, the interpreter replaces it with the value of 
that particular variable.
Program 3-2 Write a Python program to find the sum 
of two numbers.
#Program 3-2
#To find the sum of two given numbers
num1 = 10
num2 = 20
result = num1 + num2
print(result) 
#print function in python displays the output
Output:
30
Program 3-3 Write a Python program to find the area 
of a rectangle given that its length is 10 
units and breadth is 20 units.
#Program 3-3 
#To find the area of a rectangle
length = 10
breadth = 20
area = length * breadth
print(area)
Output:
200
3.5 d ata t y Pes 
Every value belongs to a specific data type in Python. 
Data type identifies the type of data which a variable 
can hold and the operations that can be performed on 
those data. Figure 3.6 enlists the data types available 
in Python.
Comments are used 
to add a remark or 
a note in the source 
code. Comments 
are not executed by 
interpreter. They 
are added with the 
purpose of making 
the source code 
easier for humans 
to understand. They 
are used primarily 
to document the 
meaning and purpose 
of source code.
 In Python, a single 
line comment starts 
with # (hash sign). 
Everything following 
the # till the end of 
that line is treated 
as a comment and 
the interpreter 
simply ignores it 
while executing the 
statement.
Chap 3.indd   35 19-Jul-19   3:16:32 PM
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