Lesson 12: Exception Handling in Python

🎯 Lesson Objective

To understand how to handle errors in Python gracefully using exceptions, preventing program crashes and improving reliability.

🧩 1. What Is an Exception?

An exception is an error that occurs during program execution.
Examples: dividing by zero, file not found, invalid input, etc.

✅ Example:

num = 10
print(num / 0)  # This will raise ZeroDivisionError

Output:

ZeroDivisionError: division by zero

⚙️ 2. Try and Except Block

Use try to test code and except to handle exceptions.

try:
    num = int(input("Enter a number: "))
    print(10 / num)
except ZeroDivisionError:
    print("Cannot divide by zero!")
except ValueError:
    print("Invalid input! Enter an integer.")

Behavior:

If input is 0 → prints “Cannot divide by zero!”
If input is not a number → prints “Invalid input! Enter an integer.”

🔄 3. Handling Multiple Exceptions

You can handle multiple exceptions in one block.

try:
    x = int(input("Enter a number: "))
    y = int(input("Enter another number: "))
    result = x / y
except (ZeroDivisionError, ValueError) as e:
    print("Error occurred:", e)
else:
    print("Result is:", result)
finally:
    print("Execution finished.")

🧰 4. Common Built-in Exceptions

Exception	Cause	Example
`ZeroDivisionError`	Division by zero	`10/0`
`ValueError`	Invalid value	`int("abc")`
`FileNotFoundError`	File doesn’t exist	`open("nofile.txt")`
`TypeError`	Wrong data type	`"5" + 5`
`IndexError`	Invalid index	`arr[10]`
`KeyError`	Key not found in dict	`dict["x"]`

🔁 5. Try, Except, Else, Finally

try:
    num = int(input("Enter a number: "))
    result = 10 / num
except ZeroDivisionError:
    print("Cannot divide by zero!")
else:
    print("Division successful, result:", result)
finally:
    print("End of program")

try: code that may raise an exception
except: code to handle exception
else: runs if no exception occurs
finally: always runs

🧩 6. Raising Exceptions

You can raise exceptions intentionally using raise.

age = int(input("Enter your age: "))
if age < 18:
    raise ValueError("Age must be 18 or above.")

🌍 7. Real-Life Example – Safe Input

while True:
    try:
        num = int(input("Enter a positive number: "))
        if num < 0:
            raise ValueError("Number cannot be negative")
        break
    except ValueError as e:
        print("Error:", e)

print("You entered:", num)