Exception Handling

What is an Exception?

An exception is an error that happens while a program runs (like dividing by zero). Without handling, the program crashes. Exception handling lets you catch the error and respond gracefully.

try / except

try:
    # risky code
    risky_operation()
except SomeError:
    # runs if that error happens
    handle_it()

Python tries the try block. If an error occurs, it jumps to the matching except instead of crashing.

Common Errors

Program 1: Safe Division

a = int(input("Numerator: "))
b = int(input("Denominator: "))
try:
    print("Result:", a / b)
except ZeroDivisionError:
    print("Error: cannot divide by zero")

Without the try, dividing by 0 would crash the program. Now it prints a friendly message.

Program 2: Safe Number Input

try:
    age = int(input("Enter your age: "))
    print("Next year you will be", age + 1)
except ValueError:
    print("Please enter a valid number")

If the user types text instead of a number, int() raises ValueError, which we catch.

Program 3: Multiple except + finally

try:
    nums = [10, 20, 30]
    i = int(input("Index: "))
    print(nums[i])
except ValueError:
    print("Index must be a number")
except IndexError:
    print("Index out of range")
finally:
    print("Done checking")

• Each except handles a different error type.
• finally always runs — error or not — good for cleanup.

Raising Your Own Error

Use raise to signal an error yourself when input is invalid.

def set_age(age):
    if age < 0:
        raise ValueError("Age cannot be negative")
    print("Age set to", age)

try:
    set_age(-5)
except ValueError as e:
    print("Caught:", e)

Common Mistakes

• Catching every error with a bare except: — hides real bugs.
• Putting too much code inside one try block.
• Forgetting that finally runs no matter what.

Practice Tasks

1. Ask for two numbers and divide them safely.
2. Keep asking for a number until the user enters a valid one.
3. Open a file and handle FileNotFoundError nicely.
4. Write a function that raises an error for a negative marks value.

Summary

• Exceptions are runtime errors; handling stops crashes.
• try runs risky code; except catches specific errors.
• finally always runs; raise creates your own error.