Exception Handling in Python

An exception is an error that happens while the program is running, such as dividing by zero or converting text that is not a number. Without handling, the program stops. With try and except you can catch the error and show a friendly message instead.

• try: the code that might fail
• except: runs when an error happens
• else: runs when NO error happens
• finally: always runs (clean-up)
• raise: throw your own exception

Basic Exception Handling

a = 10
b = 0
try:
    c = a / b
    print(f"c = {c}")
except ZeroDivisionError:
    print("Error: Division by zero is not allowed.")

Handling Different Exceptions

Use a separate except block for each kind of error.

try:
    num = int(input("Enter a number: "))
    result = 10 / num
    print(f"Result: {result}")
except ValueError:
    print("Error: Please enter a valid number.")
except ZeroDivisionError:
    print("Error: Division by zero is not allowed.")

Catching Multiple Exceptions Together

try:
    num = int(input("Enter a number: "))
    result = 10 / num
    print(f"Result: {result}")
except (ValueError, ZeroDivisionError) as e:
    print(f"Error: {e}")

Using the Exception Base Class

Exception catches almost every error. Use it as a last safety net, after the specific exceptions.

try:
    num = int(input("Enter a number: "))
    result = 10 / num
    print(f"Result: {result}")
except Exception as e:
    print(f"Something went wrong: {e}")

else and finally

try:
    num = int(input("Enter a number: "))
    result = 10 / num
except ValueError:
    print("Invalid input. Please enter a valid number.")
except ZeroDivisionError:
    print("Division by zero is not allowed.")
else:
    print(f"Result: {result}")       # only when there was no error
finally:
    print("Execution completed.")    # always runs

Raising Your Own Exception

Use raise to stop the program when data breaks a business rule.

age = 15
if age < 18:
    raise Exception("Age must be at least 18 to vote.")
print("You are eligible to vote.")

Example: attendance check

attendance = int(input("Enter your attendance percent: "))
if attendance < 75:
    raise Exception("Students with attendance below 75% are not eligible")
print("Congrats, you are eligible!")