NCERT Textbook: Getting Started with Python | Computer Science for Class 11 - Humanities/Arts PDF Download

 Page 1


5.1 Introduct Ion to Python We have written algorithms for different problems in 
Chapter 4. Let us now move a step further and create 
programs using any version of Python 3. But before 
learning about Python programming language, let us 
understand what is a programming language and how 
it works.
An ordered set of instructions to be executed by a 
computer to carry out a specific task is called a program, 
and the language used to specify this set of instructions 
to the computer is called a programming language.
As we know that computers understand the language 
of 0s and 1s which is called machine language or low 
level language. However, it is difficult for humans to 
write or comprehend instructions using 0s and 1s. This 
led to the advent of high-level programming languages 
like Python, C++, Visual Basic, PHP, Java that are easier 
to manage by humans but are not directly understood 
by the computer. 
A program written in a high-level language is called 
source code. Recall from Chapter 1 that language 
translators like compilers and interpreters are needed 
to translate the source code into machine language. 
Python uses an interpreter to convert its instructions 
into machine language, so that it can be understood 
by the computer. An interpreter processes the program 
statements one by one, first translating and then 
executing. This process is continued until an error 
is encountered or the whole program is executed 
successfully. In both the cases, program execution 
will stop. On the contrary, a compiler translates the 
entire source code, as a whole, into the object code. 
After scanning the whole program, it generates error 
messages, if any.
“Computer programming 
is an art, because it applies 
accumulated knowledge 
to the world, because it 
requires skill and ingenuity, 
and especially because 
it produces objects of 
beauty. A programmer who 
subconsciously views himself 
as an artist will enjoy what he 
does and will do it better.”
– Donald Knuth
Chapter 5
Getting Started with 
Python
In this chapter
 » Introduction to 
Python 
 » Python Keywords
 » Identifiers
 » Comments
 » Data Types
 » Operators
 » Expressions
 » Statement
 » Input and Output
 » Type Conversion
 » Debugging
Ch 5.indd   87 08-Apr-19   12:35:10 PM
2024-25
Page 2


5.1 Introduct Ion to Python We have written algorithms for different problems in 
Chapter 4. Let us now move a step further and create 
programs using any version of Python 3. But before 
learning about Python programming language, let us 
understand what is a programming language and how 
it works.
An ordered set of instructions to be executed by a 
computer to carry out a specific task is called a program, 
and the language used to specify this set of instructions 
to the computer is called a programming language.
As we know that computers understand the language 
of 0s and 1s which is called machine language or low 
level language. However, it is difficult for humans to 
write or comprehend instructions using 0s and 1s. This 
led to the advent of high-level programming languages 
like Python, C++, Visual Basic, PHP, Java that are easier 
to manage by humans but are not directly understood 
by the computer. 
A program written in a high-level language is called 
source code. Recall from Chapter 1 that language 
translators like compilers and interpreters are needed 
to translate the source code into machine language. 
Python uses an interpreter to convert its instructions 
into machine language, so that it can be understood 
by the computer. An interpreter processes the program 
statements one by one, first translating and then 
executing. This process is continued until an error 
is encountered or the whole program is executed 
successfully. In both the cases, program execution 
will stop. On the contrary, a compiler translates the 
entire source code, as a whole, into the object code. 
After scanning the whole program, it generates error 
messages, if any.
“Computer programming 
is an art, because it applies 
accumulated knowledge 
to the world, because it 
requires skill and ingenuity, 
and especially because 
it produces objects of 
beauty. A programmer who 
subconsciously views himself 
as an artist will enjoy what he 
does and will do it better.”
– Donald Knuth
Chapter 5
Getting Started with 
Python
In this chapter
 » Introduction to 
Python 
 » Python Keywords
 » Identifiers
 » Comments
 » Data Types
 » Operators
 » Expressions
 » Statement
 » Input and Output
 » Type Conversion
 » Debugging
Ch 5.indd   87 08-Apr-19   12:35:10 PM
2024-25
Computer SCien Ce – Cla SS xi 88
5.1.1 Features of Python 
• Python is a high level language. It is a free and 
open source language.
• It is an interpreted language, as Python programs 
are executed by an interpreter. 
• Python programs are easy to understand as 
they have a clearly defined syntax and relatively 
simple structure.
• Python is case-sensitive. For example, NUMBER 
and number are not same in Python.
• Python is portable and platform independent, 
means it can run on various operating systems and 
hardware platforms.
• Python has a rich library of predefined functions.
• Python is also helpful in web development. Many 
popular web services and applications are built 
using Python.
• Python uses indentation for blocks and 
nested blocks.
5.1.2 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 5.1:
Downloading Python
The latest version of 
Python 3 is available on 
the official website: 
https://www.python.org/
Figure 5.1: Python interpreter or shell
In the above screen, the symbol >>> is the Python 
prompt, which indicates that the interpreter is ready 
to take instructions. We can type commands or 
statements on this prompt to execute them using a 
Python interpreter.
Ch 5.indd   88 08-Apr-19   12:35:10 PM
2024-25
Page 3


5.1 Introduct Ion to Python We have written algorithms for different problems in 
Chapter 4. Let us now move a step further and create 
programs using any version of Python 3. But before 
learning about Python programming language, let us 
understand what is a programming language and how 
it works.
An ordered set of instructions to be executed by a 
computer to carry out a specific task is called a program, 
and the language used to specify this set of instructions 
to the computer is called a programming language.
As we know that computers understand the language 
of 0s and 1s which is called machine language or low 
level language. However, it is difficult for humans to 
write or comprehend instructions using 0s and 1s. This 
led to the advent of high-level programming languages 
like Python, C++, Visual Basic, PHP, Java that are easier 
to manage by humans but are not directly understood 
by the computer. 
A program written in a high-level language is called 
source code. Recall from Chapter 1 that language 
translators like compilers and interpreters are needed 
to translate the source code into machine language. 
Python uses an interpreter to convert its instructions 
into machine language, so that it can be understood 
by the computer. An interpreter processes the program 
statements one by one, first translating and then 
executing. This process is continued until an error 
is encountered or the whole program is executed 
successfully. In both the cases, program execution 
will stop. On the contrary, a compiler translates the 
entire source code, as a whole, into the object code. 
After scanning the whole program, it generates error 
messages, if any.
“Computer programming 
is an art, because it applies 
accumulated knowledge 
to the world, because it 
requires skill and ingenuity, 
and especially because 
it produces objects of 
beauty. A programmer who 
subconsciously views himself 
as an artist will enjoy what he 
does and will do it better.”
– Donald Knuth
Chapter 5
Getting Started with 
Python
In this chapter
 » Introduction to 
Python 
 » Python Keywords
 » Identifiers
 » Comments
 » Data Types
 » Operators
 » Expressions
 » Statement
 » Input and Output
 » Type Conversion
 » Debugging
Ch 5.indd   87 08-Apr-19   12:35:10 PM
2024-25
Computer SCien Ce – Cla SS xi 88
5.1.1 Features of Python 
• Python is a high level language. It is a free and 
open source language.
• It is an interpreted language, as Python programs 
are executed by an interpreter. 
• Python programs are easy to understand as 
they have a clearly defined syntax and relatively 
simple structure.
• Python is case-sensitive. For example, NUMBER 
and number are not same in Python.
• Python is portable and platform independent, 
means it can run on various operating systems and 
hardware platforms.
• Python has a rich library of predefined functions.
• Python is also helpful in web development. Many 
popular web services and applications are built 
using Python.
• Python uses indentation for blocks and 
nested blocks.
5.1.2 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 5.1:
Downloading Python
The latest version of 
Python 3 is available on 
the official website: 
https://www.python.org/
Figure 5.1: Python interpreter or shell
In the above screen, the symbol >>> is the Python 
prompt, which indicates that the interpreter is ready 
to take instructions. We can type commands or 
statements on this prompt to execute them using a 
Python interpreter.
Ch 5.indd   88 08-Apr-19   12:35:10 PM
2024-25
Gettin G Started with Python 89
5.1.3 Execution Modes 
There are two ways to use the Python interpreter:
a) Interactive mode 
b) Script mode
Interactive mode allows execution of individual 
statement instantaneously. Whereas, Script mode 
allows us to write more than one instruction in a file 
called Python source code file that can be executed. 
(A) Interactive Mode
To work in the interactive mode, we can simply 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 5.2.
Figure 5.2: Python interpreter in interactive mode
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 
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 it. 
Python scripts are saved as files where file name has 
extension “.py”. By default, the Python scripts are saved 
in the Python installation folder. To execute a script, we 
can either: 
a) Type the file name along with the path at the 
prompt. For example, if the name of the file is 
prog5-1.py, we type prog5-1.py. We can otherwise 
open the program directly from IDLE as shown in 
Figure 5.3. 
b) While working in the script mode, after saving the 
file, click [Run]->[Run Module] from the menu as 
shown in Figure 5.4.
Ch 5.indd   89 08-Apr-19   12:35:11 PM
2024-25
Page 4


5.1 Introduct Ion to Python We have written algorithms for different problems in 
Chapter 4. Let us now move a step further and create 
programs using any version of Python 3. But before 
learning about Python programming language, let us 
understand what is a programming language and how 
it works.
An ordered set of instructions to be executed by a 
computer to carry out a specific task is called a program, 
and the language used to specify this set of instructions 
to the computer is called a programming language.
As we know that computers understand the language 
of 0s and 1s which is called machine language or low 
level language. However, it is difficult for humans to 
write or comprehend instructions using 0s and 1s. This 
led to the advent of high-level programming languages 
like Python, C++, Visual Basic, PHP, Java that are easier 
to manage by humans but are not directly understood 
by the computer. 
A program written in a high-level language is called 
source code. Recall from Chapter 1 that language 
translators like compilers and interpreters are needed 
to translate the source code into machine language. 
Python uses an interpreter to convert its instructions 
into machine language, so that it can be understood 
by the computer. An interpreter processes the program 
statements one by one, first translating and then 
executing. This process is continued until an error 
is encountered or the whole program is executed 
successfully. In both the cases, program execution 
will stop. On the contrary, a compiler translates the 
entire source code, as a whole, into the object code. 
After scanning the whole program, it generates error 
messages, if any.
“Computer programming 
is an art, because it applies 
accumulated knowledge 
to the world, because it 
requires skill and ingenuity, 
and especially because 
it produces objects of 
beauty. A programmer who 
subconsciously views himself 
as an artist will enjoy what he 
does and will do it better.”
– Donald Knuth
Chapter 5
Getting Started with 
Python
In this chapter
 » Introduction to 
Python 
 » Python Keywords
 » Identifiers
 » Comments
 » Data Types
 » Operators
 » Expressions
 » Statement
 » Input and Output
 » Type Conversion
 » Debugging
Ch 5.indd   87 08-Apr-19   12:35:10 PM
2024-25
Computer SCien Ce – Cla SS xi 88
5.1.1 Features of Python 
• Python is a high level language. It is a free and 
open source language.
• It is an interpreted language, as Python programs 
are executed by an interpreter. 
• Python programs are easy to understand as 
they have a clearly defined syntax and relatively 
simple structure.
• Python is case-sensitive. For example, NUMBER 
and number are not same in Python.
• Python is portable and platform independent, 
means it can run on various operating systems and 
hardware platforms.
• Python has a rich library of predefined functions.
• Python is also helpful in web development. Many 
popular web services and applications are built 
using Python.
• Python uses indentation for blocks and 
nested blocks.
5.1.2 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 5.1:
Downloading Python
The latest version of 
Python 3 is available on 
the official website: 
https://www.python.org/
Figure 5.1: Python interpreter or shell
In the above screen, the symbol >>> is the Python 
prompt, which indicates that the interpreter is ready 
to take instructions. We can type commands or 
statements on this prompt to execute them using a 
Python interpreter.
Ch 5.indd   88 08-Apr-19   12:35:10 PM
2024-25
Gettin G Started with Python 89
5.1.3 Execution Modes 
There are two ways to use the Python interpreter:
a) Interactive mode 
b) Script mode
Interactive mode allows execution of individual 
statement instantaneously. Whereas, Script mode 
allows us to write more than one instruction in a file 
called Python source code file that can be executed. 
(A) Interactive Mode
To work in the interactive mode, we can simply 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 5.2.
Figure 5.2: Python interpreter in interactive mode
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 
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 it. 
Python scripts are saved as files where file name has 
extension “.py”. By default, the Python scripts are saved 
in the Python installation folder. To execute a script, we 
can either: 
a) Type the file name along with the path at the 
prompt. For example, if the name of the file is 
prog5-1.py, we type prog5-1.py. We can otherwise 
open the program directly from IDLE as shown in 
Figure 5.3. 
b) While working in the script mode, after saving the 
file, click [Run]->[Run Module] from the menu as 
shown in Figure 5.4.
Ch 5.indd   89 08-Apr-19   12:35:11 PM
2024-25
Computer SCien Ce – Cla SS xi 90
Program 5-1 Write a program to show print statement 
in script mode.
Figure 5.3: Python source code file (prog5-1.py)
5.2 Python Keywords Keywords are reserved words. Each keyword has a 
specific meaning to the Python interpreter, and we can 
use a keyword in our program only for the purpose for 
which it has been defined. As Python is case sensitive, 
keywords must be written exactly as given in Table 5.1.
Table 5.1 Python keywords
False class finally is return
None continue for lambda try
c) The output appears on shell as shown in  
Figure 5.5.
Figure 5.4: Execution of Python in Script mode using IDLE
Figure 5.5: Output of a program executed in script mode
Ch 5.indd   90 21-May-19   11:57:33 AM
2024-25
Page 5


5.1 Introduct Ion to Python We have written algorithms for different problems in 
Chapter 4. Let us now move a step further and create 
programs using any version of Python 3. But before 
learning about Python programming language, let us 
understand what is a programming language and how 
it works.
An ordered set of instructions to be executed by a 
computer to carry out a specific task is called a program, 
and the language used to specify this set of instructions 
to the computer is called a programming language.
As we know that computers understand the language 
of 0s and 1s which is called machine language or low 
level language. However, it is difficult for humans to 
write or comprehend instructions using 0s and 1s. This 
led to the advent of high-level programming languages 
like Python, C++, Visual Basic, PHP, Java that are easier 
to manage by humans but are not directly understood 
by the computer. 
A program written in a high-level language is called 
source code. Recall from Chapter 1 that language 
translators like compilers and interpreters are needed 
to translate the source code into machine language. 
Python uses an interpreter to convert its instructions 
into machine language, so that it can be understood 
by the computer. An interpreter processes the program 
statements one by one, first translating and then 
executing. This process is continued until an error 
is encountered or the whole program is executed 
successfully. In both the cases, program execution 
will stop. On the contrary, a compiler translates the 
entire source code, as a whole, into the object code. 
After scanning the whole program, it generates error 
messages, if any.
“Computer programming 
is an art, because it applies 
accumulated knowledge 
to the world, because it 
requires skill and ingenuity, 
and especially because 
it produces objects of 
beauty. A programmer who 
subconsciously views himself 
as an artist will enjoy what he 
does and will do it better.”
– Donald Knuth
Chapter 5
Getting Started with 
Python
In this chapter
 » Introduction to 
Python 
 » Python Keywords
 » Identifiers
 » Comments
 » Data Types
 » Operators
 » Expressions
 » Statement
 » Input and Output
 » Type Conversion
 » Debugging
Ch 5.indd   87 08-Apr-19   12:35:10 PM
2024-25
Computer SCien Ce – Cla SS xi 88
5.1.1 Features of Python 
• Python is a high level language. It is a free and 
open source language.
• It is an interpreted language, as Python programs 
are executed by an interpreter. 
• Python programs are easy to understand as 
they have a clearly defined syntax and relatively 
simple structure.
• Python is case-sensitive. For example, NUMBER 
and number are not same in Python.
• Python is portable and platform independent, 
means it can run on various operating systems and 
hardware platforms.
• Python has a rich library of predefined functions.
• Python is also helpful in web development. Many 
popular web services and applications are built 
using Python.
• Python uses indentation for blocks and 
nested blocks.
5.1.2 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 5.1:
Downloading Python
The latest version of 
Python 3 is available on 
the official website: 
https://www.python.org/
Figure 5.1: Python interpreter or shell
In the above screen, the symbol >>> is the Python 
prompt, which indicates that the interpreter is ready 
to take instructions. We can type commands or 
statements on this prompt to execute them using a 
Python interpreter.
Ch 5.indd   88 08-Apr-19   12:35:10 PM
2024-25
Gettin G Started with Python 89
5.1.3 Execution Modes 
There are two ways to use the Python interpreter:
a) Interactive mode 
b) Script mode
Interactive mode allows execution of individual 
statement instantaneously. Whereas, Script mode 
allows us to write more than one instruction in a file 
called Python source code file that can be executed. 
(A) Interactive Mode
To work in the interactive mode, we can simply 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 5.2.
Figure 5.2: Python interpreter in interactive mode
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 
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 it. 
Python scripts are saved as files where file name has 
extension “.py”. By default, the Python scripts are saved 
in the Python installation folder. To execute a script, we 
can either: 
a) Type the file name along with the path at the 
prompt. For example, if the name of the file is 
prog5-1.py, we type prog5-1.py. We can otherwise 
open the program directly from IDLE as shown in 
Figure 5.3. 
b) While working in the script mode, after saving the 
file, click [Run]->[Run Module] from the menu as 
shown in Figure 5.4.
Ch 5.indd   89 08-Apr-19   12:35:11 PM
2024-25
Computer SCien Ce – Cla SS xi 90
Program 5-1 Write a program to show print statement 
in script mode.
Figure 5.3: Python source code file (prog5-1.py)
5.2 Python Keywords Keywords are reserved words. Each keyword has a 
specific meaning to the Python interpreter, and we can 
use a keyword in our program only for the purpose for 
which it has been defined. As Python is case sensitive, 
keywords must be written exactly as given in Table 5.1.
Table 5.1 Python keywords
False class finally is return
None continue for lambda try
c) The output appears on shell as shown in  
Figure 5.5.
Figure 5.4: Execution of Python in Script mode using IDLE
Figure 5.5: Output of a program executed in script mode
Ch 5.indd   90 21-May-19   11:57:33 AM
2024-25
Gettin G Started with Python 91
True def from nonlocal while
and del global not with
as elif if or yield
assert else import pass 
break except in raise 
5.3  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 5.1.
• We cannot use special symbols like !, @, #, $, %, 
etc., in identifiers.
For example, to find the average of marks obtained 
by a student in three subjects, we can choose the 
identifiers as marks1, marks2, marks3 and avg rather 
than a, b, c, or A, B, C.
avg = (marks1 + marks2 + marks3)/3
Similarly, to calculate the area of a rectangle, we can 
use identifier names, such as area, length, breadth 
instead of single alphabets as identifiers for clarity and 
more readability.
area = length * breadth
5.4 Var Iables A variable in a program is uniquely identified by a name 
(identifier). Variable in Python refers to an object — an 
item or element that is stored in the memory. Value 
of a variable can be a string (e.g., ‘b’, ‘Global Citizen’), 
numeric (e.g., 345) or any combination of alphanumeric 
characters (CD67). In Python we can use an assignment 
statement to create new variables and assign specific 
values to them. 
n otes Ch 5.indd   91 08-Apr-19   12:35:11 PM
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