🧩 Python Exception Handling

Overview

In Python, exceptions are errors that occur during program execution.
Instead of crashing the entire program, Python allows you to handle these errors gracefully using Exception Handling.

For example, dividing a number by zero or accessing an invalid index will raise an exception.

Example (without handling):

a = 10 b = 0 print(a / b)  # Error: ZeroDivisionError 

➡️ Output:

ZeroDivisionError: division by zero

To prevent the program from crashing, we use try-except blocks.

⚙️ Why Use Exception Handling?

To avoid program crashes due to runtime errors

To provide user-friendly messages instead of technical errors

To ensure program continues even after errors

To debug and log errors effectively

🧱 Try and Except Block

The try block contains the code that might cause an error,
and the except block handles the error when it occurs.

Syntax:

try:    # code that may raise an error except:    # code to handle the error 

Example:

try:    a = int(input("Enter a number: "))    print(10 / a) except:    print("Something went wrong!")

🔹 Catching Specific Exceptions

You can handle specific types of exceptions like ZeroDivisionError, ValueError, TypeError, etc.

Example:

try:    a = int(input("Enter a number: "))    print(10 / a) except ZeroDivisionError:    print("Cannot divide by zero!") except ValueError:    print("Please enter a valid number!")

🔹 Using else Block

The else block executes only if no error occurs in the try block.

Example:

try:    num = int(input("Enter a number: "))    print("Square =", num ** 2) except ValueError:    print("Invalid input! Please enter a number.") else:    print("Execution successful!")

🔹 Using finally Block

The finally block always executes — whether an error occurs or not.
It’s often used for cleanup actions (like closing files or releasing resources).

Example:

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

🔹 Raising Exceptions Manually

You can manually raise an exception using the raise keyword.

Syntax:

raise ExceptionType("Custom error message")

Example:

age = int(input("Enter your age: ")) if age < 18:    raise ValueError("You must be 18 or older!") else:    print("Access granted.")

🔹 Nested Try-Except

You can also use a try-except block inside another try block.

Example:

try:    a = int(input("Enter number: "))    try:        result = 10 / a        print("Result:", result)    except ZeroDivisionError:        print("Division by zero not allowed.") except ValueError:    print("Invalid input.")

🧱 Common Built-in Exceptions in Python

💡 Practical Examples

✅ Example 1: Safe Division

def safe_divide(a, b):    try:        result = a / b        print("Result:", result)    except ZeroDivisionError:        print("Error: Cannot divide by zero!")    except TypeError:        print("Error: Invalid data type!") safe_divide(10, 2) safe_divide(5, 0) safe_divide("a", 3)

✅ Example 2: File Handling with Exception

try:    file = open("sample.txt", "r")    print(file.read()) except FileNotFoundError:    print("File not found! Please check the filename.") finally:    print("File operation complete.")

✅ Example 3: Custom Exception

class InvalidAgeError(Exception):    pass age = int(input("Enter your age: ")) try:    if age < 18:        raise InvalidAgeError("You are not eligible!")    else:        print("Eligible for registration.") except InvalidAgeError as e:    print(e)

🧠 Quick Practice Tasks

Division Program:
Input two numbers and handle ZeroDivisionError and ValueError.

File Reader:
Try to open a file — handle FileNotFoundError.

Custom Exception:
Create a custom exception if a user enters a negative number.

Nested Try Block:
Handle invalid input and division error in nested try-except.

Use finally:
Write a program that always prints “Program ended” even if an error occurs.