🛡️ Learn how to handle exceptions gracefully in Python and prevent your programs from crashing due to unexpected errors.

Python uses a robust exception handling mechanism that allows you to catch and manage runtime errors, making your applications more resilient and user-friendly.

An exception is an event that occurs during the execution of a program that disrupts the normal flow of instructions.

Exceptions can occur due to:

• Invalid user input
• File not found
• Division by zero
• Network or database issues
• Any unexpected condition

Unlike syntax errors, which are caught before execution, exceptions happen at runtime.

Occurs when Python cannot parse the code correctly.

if True
    print("Hello")

🔹 Output:

SyntaxError: expected ':'

Occurs during execution:

x = 10 / 0

🔹 Output:

ZeroDivisionError: division by zero

Use try...except to catch and handle exceptions gracefully.

🔹 Basic Syntax:

try:
    # code that may cause an exception
except ExceptionType:
    # code to handle the exception

try:
    x = 10 / 0
except ZeroDivisionError:
    print("Cannot divide by zero!")

try:
    age = int(input("Enter your age: "))
except ValueError:
    print("Invalid input! Please enter a number.")

You can handle multiple exceptions using multiple except blocks or grouping them together.

try:
    x = int(input("Enter numerator: "))
    y = int(input("Enter denominator: "))
    result = x / y
except ValueError:
    print("Please enter valid numbers.")
except ZeroDivisionError:
    print("Cannot divide by zero.")

try:
    x = int(input("Enter a number: "))
    print(100 / x)
except (ValueError, ZeroDivisionError):
    print("Invalid input or division by zero.")

To catch all types of exceptions, use a generic except clause — but it's best practice to log the error for debugging.

🔹 Example:

try:
    x = int(input("Enter a number: "))
    print(100 / x)
except Exception as e:
    print(f"An error occurred: {e}")

🔸 This is useful for logging or displaying general error messages while still allowing specific handlers for known issues.

The finally block always executes, whether an exception occurs or not. It’s ideal for closing files, network connections, or releasing resources.

🔹 Example:

try:
    file = open("data.txt", "r")
    content = file.read()
except FileNotFoundError:
    print("File not found.")
finally:
    print("Cleaning up...")
    try:
        file.close()
    except:
        pass

🔸 Even if an exception occurs, the finally block runs after the try and except blocks.

Use the else block to run code only if no exception was raised.

🔹 Example:

try:
    num = int(input("Enter a number: "))
    result = 100 / num
except ValueError:
    print("Invalid input.")
except ZeroDivisionError:
    print("Can't divide by zero.")
else:
    print(f"Result: {result}")

🔸 The else block helps separate logic that should only execute on success.

This structure gives you full control over exception handling and resource management.

🔹 Example:

try:
    x = int(input("Enter a number: "))
    result = 100 / x
except ValueError:
    print("Invalid input.")
except ZeroDivisionError:
    print("Cannot divide by zero.")
else:
    print(f"Result: {result}")
finally:
    print("Program finished.")

🔸 Execution Order:

1. try
2. If exception → except → finally
3. If no exception → else → finally

You can raise exceptions manually using the raise keyword.

🔹 Example:

def validate_age(age):
    if age < 0:
        raise ValueError("Age cannot be negative.")
    print("Age is valid.")

try:
    validate_age(-5)
except ValueError as e:
    print(f"Caught error: {e}")

🔸 Useful for enforcing business rules or data validation.

• ❌ Don’t use bare except: without specifying an exception type.
• ✅ Prefer catching specific exceptions to avoid hiding bugs.
• 📝 Use finally for cleanup operations like closing files or database connections.
• 📊 Use else to isolate code that doesn’t need exception handling.
• 📌 Log exceptions in production apps instead of just printing them.
• 🛡️ Validate inputs before risky operations to reduce exceptions.

print('Enter the net sales for')
try:
    previous = float(input('- Prior period: '))
    current = float(input('- Current period: '))
    change = (current - previous) * 100 / previous
    status = 'increase' if change > 0 else 'decrease'
    print(f"Sales {status} by {abs(change):.2f}%")
except ValueError:
    print("Error: Please enter valid numbers.")
except ZeroDivisionError:
    print("Error: Prior period value cannot be zero.")
except Exception as e:
    print(f"An unexpected error occurred: {e}")

🎉 Congratulations! You now have a solid understanding of Python Exception Handling — including how to use try...except, else, and finally blocks, raise custom exceptions, and apply best practices to make your programs more robust.

Next up: 🔁 Section 9: More on Python Loops – learn advanced loop techniques like for...else, while...else, and emulate do-while behavior.