Chapter 01 - Exception Handling

CBSE Class 12 Computer Science

1.1 Introduction

While executing a Python program, the program may:

  • Not execute at all
  • Execute partially
  • Terminate abruptly
  • Produce unexpected output

These problems occur due to errors in the program.

Types of Errors

  1. Syntax Errors – detected before execution
  2. Runtime Errors (Exceptions) – occur during execution
  3. Logical Errors – program runs but gives wrong output

👉 Exception Handling is the mechanism used to handle runtime errors, so that the program does not crash abruptly.

📌 In Python, runtime errors are represented using exception objects.

1.2 Syntax Errors

What are Syntax Errors?

Syntax errors occur when Python grammar rules are violated.

📌 Detected by the Python interpreter before execution.

Examples of Syntax Errors

print("Hello World"

➡ Missing closing bracket

if x == 10
    print(x)

➡ Missing colon

for i in range(5)
print(i)

➡ Incorrect indentation

Important Points

  • Program does not execute until syntax errors are corrected
  • Also called parsing errors

  • Python displays:

    • Error name
    • Line number
    • Error description

📌 SyntaxError is also an exception, but it is detected earlier than other exceptions.

1.3 Exceptions

What is an Exception?

An exception is a runtime error that occurs while the program is executing.

📌 Even if syntax is correct, execution may fail.

Common Situations Causing Exceptions

  • Division by zero
  • Accessing a non-existing file
  • Invalid type conversion
  • Using an undefined variable

Example

print(10 / 0)

➡ ZeroDivisionError

print(x)

➡ NameError

int("abc")

➡ ValueError

Key Concept

  • When an error occurs, Python raises an exception
  • If not handled, the program terminates abnormally

1.4 Built-in Exceptions

Python provides many predefined exceptions for common runtime errors.

Common Built-in Exceptions (NCERT Important)

Exception Raised When
SyntaxError Invalid syntax
ValueError Wrong value
TypeError Wrong data type
ZeroDivisionError Division by zero
IndexError Index out of range
NameError Variable not defined
ImportError Module not found
EOFError End of input
IndentationError Incorrect indentation

Example Demonstrations

a = [1, 2, 3]
print(a[5])     # IndexError

import maths     # ImportError

print(5 + "A")  # TypeError

1.5 Raising Exceptions

Python allows programmers to explicitly raise exceptions.

🔸 1.5.1 raise Statement

Used to forcefully trigger an exception.

Syntax

raise ExceptionName("Error message")

Example

age = -10
if age < 0:
    raise ValueError("Age cannot be negative")

📌 Once raise is executed:

  • Normal flow stops
  • Control jumps to exception handler
  • Remaining code is skipped

🔸 1.5.2 assert Statement

Used to test conditions during program execution.

Syntax

assert condition, "Error message"

Example

def square(n):
    assert n >= 0, "Negative number not allowed"
    return n * n

print(square(5))
print(square(-3))

➡ Raises AssertionError for invalid input

📌 Mostly used for:

  • Debugging
  • Input validation

1.6 Handling Exceptions

Exception handling prevents abrupt program termination.

Why Exception Handling is Needed?

  • Improves program reliability
  • Prevents crashes
  • Provides meaningful error messages
  • Separates error handling code from main logic

🔸 1.6.1 try – except Block

Syntax

try:
    # risky code
except ExceptionName:
    # handling code

Example

try:
    x = int(input("Enter number: "))
    print(10 / x)
except ZeroDivisionError:
    print("Division by zero not allowed")

🔸 Multiple except Blocks

try:
    x = int(input("Enter number: "))
    print(10 / x)
except ZeroDivisionError:
    print("Cannot divide by zero")
except ValueError:
    print("Only integers allowed")

📌 Python checks except blocks top to bottom

🔸 Generic except

except:
    print("Some error occurred")

📌 Must always be last except block

🔸 try – except – else

  • else executes only if no exception occurs

    try:
x = int(input())
y = int(input())
print(x / y)
except ZeroDivisionError:
print("Zero not allowed")
else:
print("Division successful")

1.7 Finally Clause

What is finally?

The finally block always executes, whether:

  • Exception occurs
  • Exception does not occur
  • Exception is handled or not

Syntax

try:
    # code
except:
    # handler
finally:
    # always executed

Example

try:
    f = open("data.txt")
    print(f.read())
except FileNotFoundError:
    print("File not found")
finally:
    print("Program ended")

📌 Used mainly for:

  • Closing files
  • Releasing resources
  • Cleanup operations

📝 NCERT EXAM SUMMARY (Very Important)

  • Syntax errors → detected before execution
  • Exceptions → runtime errors
  • raise → manually throws exception
  • assert → tests condition
  • else → executes only if no exception
  • finally → executes always
  • Unhandled exception → program crashes

Board-Level Solved Programs (CBSE Standard)

Program 1: Handle Division by Zero

Question: Write a program to divide two numbers and handle division by zero.

try:
    a = int(input("Enter first number: "))
    b = int(input("Enter second number: "))
    result = a / b
    print("Result:", result)
except ZeroDivisionError:
    print("Error: Division by zero is not allowed")

📌 Concept tested: try–except, ZeroDivisionError

Program 2: Handle Invalid Input (ValueError)

try:
    num = int(input("Enter an integer: "))
    print("Square:", num * num)
except ValueError:
    print("Error: Please enter a valid integer")

📌 Concept: ValueError

Program 3: Handle Multiple Exceptions

try:
    x = int(input("Enter numerator: "))
    y = int(input("Enter denominator: "))
    print(x / y)
except ZeroDivisionError:
    print("Denominator cannot be zero")
except ValueError:
    print("Only integers are allowed")

📌 Very common board question

Program 4: Generic Exception Handling

try:
    print(a)
except:
    print("Some error occurred")

📌 Concept: Generic except

Program 5: Use of else Block

try:
    a = int(input())
    b = int(input())
    print(a / b)
except ZeroDivisionError:
    print("Division by zero")
else:
    print("Division successful")

📌 Key point: else runs only if no exception

Program 6: Use of finally Block

try:
    f = open("test.txt", "r")
    print(f.read())
except FileNotFoundError:
    print("File not found")
finally:
    print("Program execution completed")

📌 Exam favourite

Program 7: Raise an Exception Manually

age = int(input("Enter age: "))
if age < 0:
    raise ValueError("Age cannot be negative")
else:
    print("Valid age")

Program 8: Use of assert

def check_positive(n):
    assert n > 0, "Number must be positive"
    print("Valid number")

check_positive(5)
check_positive(-2)

📌 Concept: AssertionError

Program 9: Handle NameError

try:
    print(x)
except NameError:
    print("Variable is not defined")

Program 10: Handle TypeError

try:
    print(5 + "A")
except TypeError:
    print("Invalid operation between incompatible types")

Program 11: File Handling with Exception

try:
    f = open("data.txt", "r")
    print(f.read())
except FileNotFoundError:
    print("File does not exist")

Program 12: Input Until Valid Integer

while True:
    try:
        num = int(input("Enter integer: "))
        break
    except ValueError:
        print("Invalid input, try again")
print("You entered:", num)

Program 13: Nested try–except

try:
    try:
        x = int(input())
        print(10 / x)
    except ZeroDivisionError:
        print("Inner: Zero division")
except ValueError:
    print("Outer: Invalid input")

Program 14: EOFError Handling

try:
    text = input("Enter text: ")
    print(text)
except EOFError:
    print("End of file reached")

Program 15: Exception Without Handler (Conceptual)

print(10 / 0)

CASE-STUDY QUESTIONS (CBSE 2025 Pattern)

Case Study 1: Online Examination System

An online exam system accepts marks entered by teachers. Sometimes, teachers enter non-numeric values or zero by mistake.

Questions:

  1. Which exception occurs if a teacher enters "abc" instead of marks? ✔ ValueError

  2. Which exception occurs if marks are divided by zero? ✔ ZeroDivisionError

  3. Write a Python code to handle both errors.

    try:
marks = int(input("Enter marks: "))
average = 100 / marks
print(average)
except ValueError:
print("Marks must be numeric")
except ZeroDivisionError:
print("Marks cannot be zero")

Case Study 2: Banking Application

A banking app must ensure withdrawal amount is positive.

Question: How can this be enforced using Python?

amount = int(input("Enter withdrawal amount: "))
assert amount > 0, "Invalid withdrawal amount"
print("Transaction successful")

Case Study 3: File Upload System

A program reads a file uploaded by a user.

  1. What exception occurs if file does not exist? ✔ FileNotFoundError

  2. Why should finally be used?

✔ To ensure cleanup code always runs.

try:
    f = open("upload.txt")
except FileNotFoundError:
    print("Upload failed")
finally:
    print("Process ended")
Page last updated: 17-Mar-2026 18:38 IST
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