Errors are a part of programming — everyone makes mistakes! But Python gives us a way to handle errors gracefully so that our programs don’t just crash when something goes wrong. This is done using Exceptions and the try-except block.

✅ What are Exceptions?

An Exception is an error that occurs while a program is running.
For example:

print(10 / 0)   # Division by zero 

👉 This will give an error: ZeroDivisionError.

If not handled, the program will stop. That’s where exception handling comes in.

🔹 Using try-except

We can use try-except to handle errors without stopping the program:

try:
   number = int(input("Enter a number: "))
   print(10 / number)
except ZeroDivisionError:
   print("Oops! You can’t divide by zero.")
except ValueError:
   print("Invalid input. Please enter a number.")

Output Example:

Enter a number: 0
Oops! You can’t divide by zero.

🔹 The else Block

You can add an else block that runs if no exception occurs:

try:
   num = int(input("Enter a positive number: "))
except ValueError:
   print("Invalid input.")
else:
   print(f"Great! You entered {num}.")

🔹 The finally Block

The finally block always runs, whether an exception happened or not.
This is useful for cleaning up resources (like closing files or database connections):

try:
   file = open("example.txt", "r")
   print(file.read())
except FileNotFoundError:
   print("File not found.")
finally:
   print("Closing file (if opened).")

🔹 Raising Exceptions Manually

You can raise your own exceptions when something isn’t right:

age = int(input("Enter your age: "))

if age < 0:
   raise ValueError("Age cannot be negative!")
else:
   print("Valid age entered.")

📝 Quick Recap

• try-except → Handle errors without crashing the program.
• else → Runs only if no errors happen.
• finally → Always runs (cleanup).
• raise → Create custom exceptions.

🎯 Practice Exercises

1. Write a program that asks for two numbers and divides them, but handles:
   • ZeroDivisionError
   • ValueError
2. Open a file (like data.txt):
   • If it exists → print contents
   • If it doesn’t → show a friendly message
   • Use finally to close the file
3. Create a function check_age(age) that:
   • Raises an exception if age < 18
   • Otherwise, prints “You are eligible!”

Next up: Lesson 15: Object-Oriented Programming (OOP) in Python