NCERT Solutions for Class 12 Computer Science Chapter 1: Exception Handling in Python

A syntax error is a specific type of exception that is detected when we have not followed the rules of the particular programming language while writing a program. On the other hand, an exception is a Python object that represents any type of error or exceptional condition encountered during program execution. This includes not only syntax errors but also runtime errors and logical errors. Therefore, every syntax error is an exception but every exception cannot be a syntax error.

 

numerator = float(input("Enter the numerator: "))

denominator = float(input("Enter the denominator: "))

assert denominator != 0, "Error: Denominator cannot be zero."

quotient = numerator / denominator

print("Quotient:", quotient)

Output

Enter the numerator: 12

Enter the denominator: 3

Quotient: 4.0

Enter the numerator: 5

Enter the denominator: 0

Traceback (most recent call last):

  File "c:\PythonPlayground\q3.py", line 3, in <module>

    assert denominator != 0, "Error: Denominator cannot be zero."

AssertionError: Error: Denominator cannot be zero.

i) ImportError — It is raised when the requested module definition is not found.

Example :

import module

Output

ModuleNotFoundError: No module named 'module'

ii) IOError — It is raised when the file specified in a program statement cannot be opened.

Example :

file = open("file1.txt", "r")

Output

FileNotFoundError: [Errno 2] No such file or directory: 'file1.txt'

iii) NameError — It is raised when a local or global variable name is not defined.

Example :

print(var+40)

Output

NameError: name 'var' is not defined.

iv) ZeroDivisionError — It is raised when the denominator in a division operation is zero.

Example :

print(50/0)

Output

ZeroDivisionError: division by zero

 

The raise statement is used to throw an exception during the execution of a program.

numerator = float(input("Enter the numerator: "))

denominator = float(input("Enter the denominator: "))

if denominator == 0:

    raise ZeroDivisionError("Error: Denominator cannot be zero.")

else: 

    quotient = numerator / denominator

    print("Quotient:", quotient)

Output

Enter the numerator: 25

Enter the denominator: 5

Quotient: 5.0

 

Enter the numerator: 2

Enter the denominator: 0

Traceback (most recent call last):

  File "c:\PythonPlayground\q3.py", line 4, in <module>

    raise ZeroDivisionError("Error: Denominator cannot be zero.")

ZeroDivisionError: Error: Denominator cannot be zero.

 

1. Exception Handling — The process of writing additional code in a program to give proper messages or instructions to the user upon encountering an exception is known as exception handling.

2. Throwing an exception — Throwing an exception refers to the process of creating an exception object and passing it to the runtime system or the appropriate exception handler.

3. Catching an exception — Catching an exception refers to the process of executing a suitable handler or block of code specifically designed to handle that particular exception when it occurs during program execution.

 

An exception is said to be caught when a code that is designed to handle a particular exception is executed. Exceptions, if any, are caught in the try block and handled in the except block. While writing or debugging a program, a user might doubt an exception to occur in a particular part of the code. Such suspicious lines of codes are put inside a try block. Every try block is followed by an except block. The appropriate code to handle each of the possible exceptions (in the code inside the try block) are written inside the except clause. While executing the program, if an exception is encountered, further execution of the code inside the try block is stopped and the control is transferred to the except block. The syntax of try … except clause is as follows:

try:

    [ program statements where exceptions might occur]

except [exception-name]:

    [ code for exception handling if the exception-name error is encountered]

 

print("Learning Exceptions...")

try:

    num1 = int(input("Enter the first number"))  

    num2 = int(input("Enter the second number"))  

    quotient = (num1 / num2)

    print("Both numbers entered were correct")

except ValueError:  # 1 : to enter only integers

    print("Please enter only numbers")

except ZeroDivisionError:  # 2 : Denominator should not be zero

    print("Number 2 should not be zero")

else:

    print("Great.. you are a good programmer")

finally: #  3 : to be executed at the end

    print("JOB OVER... GO GET SOME REST")

Explanation

1. When using int(input("Enter the first number")) or int(input("Enter the second number")), the user is expected to input an integer. If the user enters a non-integer value (like a string or a floating-point number), a ValueError will be raised during the conversion to an integer. The except ValueError: block is used to handle this situation by displaying a message asking the user to enter only numbers.

2. In the line quotient = (num1 / num2), if num2 is entered as zero, it will lead to a ZeroDivisionError during the division operation (num1 / num2). The except ZeroDivisionError: block is used to handle this scenario by displaying a message informing the user that the second number should not be zero.

The finally: block is used to define code that should be executed regardless of whether an exception occurs or not.

Note — The TypeError occurs when an incorrect number of arguments is provided for a function, while the ValueError occurs when the number of arguments are correct but they contain inappropriate values. Hence, in the following code TypeError is raised due to providing an incorrect number of arguments to the math.sqrt() and math.pow() function and it is handled using except.

import math

 

try:

    result = math.pow(2, 3, 4, 5)  # pow() expects 2 arguments,

                                   # but 4 are provided

except TypeError:

    print("TypeError occurred with math.pow()")

else:

    print("Result:", result)

 

try:

    result = math.sqrt(9, 2)  # sqrt() expects 1 argument,

                              # but 2 are provided

except TypeError:

    print("TypeError occurred with math.sqrt()")

else:

    print("Result:", result)

Output

TypeError occurred with math.pow()

TypeError occurred with math.sqrt()

 

The statements inside the finally block are always executed, regardless of whether an exception has occurred in the try block or not. It is a common practice to use the finally clause while working with files to ensure that the file object is closed.

print("Learning Exceptions...")

try:

    num1 = int(input("Enter the first number: "))  

    num2 = int(input("Enter the second number: "))  

    quotient = (num1 / num2)

    print(quotient)

    print("Both numbers entered were correct")

except ValueError:  

    print("Please enter only numbers")

except ZeroDivisionError:  

    print("Number 2 should not be zero")

else:

    print("Great.. you are a good programmer")

finally: 

    print("JOB OVER... GO GET SOME REST")

Output

Learning Exceptions...

Enter the first number: 12

Enter the second number: 4

3.0

Both numbers entered were correct

Great.. you are a good programmer

JOB OVER... GO GET SOME REST

 

Learning Exceptions...

Enter the first number: var

Please enter only numbers

JOB OVER... GO GET SOME REST

 

Learning Exceptions...

Enter the first number: 33

Enter the second number: 0

Number 2 should not be zero

JOB OVER... GO GET SOME REST