Exception Handling in Python

What Is an Exception?

An exception is an error that occurs during program execution and, if unhandled, crashes your program. Python provides try/except blocks to handle these errors gracefully instead of letting the program stop.

print(10 / 0)   # ❌ ZeroDivisionError: division by zero — program crashes

Basic try / except

try:
    result = 10 / 0
except ZeroDivisionError:
    print("You can't divide by zero!")

Catching Multiple Exceptions

try:
    num = int(input("Enter a number: "))
    result = 10 / num
except ValueError:
    print("That's not a valid number!")
except ZeroDivisionError:
    print("You can't divide by zero!")

Catching Any Exception

try:
    risky_code()
except Exception as e:
    print("Something went wrong:", e)

Tip: catching the generic Exception is fine for logging, but prefer catching specific exceptions whenever you know what could go wrong.

else and finally

try:
    result = 10 / 2
except ZeroDivisionError:
    print("Error occurred")
else:
    print("No errors — result is", result)   # runs only if no exception
finally:
    print("This always runs")                  # runs no matter what

Raising Your Own Exceptions

def enroll(age):
    if age < 16:
        raise ValueError("Minimum enrollment age is 16")
    print("Enrollment successful!")

try:
    enroll(12)
except ValueError as e:
    print("Enrollment failed:", e)

Common Built-in Exceptions

Custom Exceptions

class InsufficientFundsError(Exception):
    pass

def withdraw(balance, amount):
    if amount > balance:
        raise InsufficientFundsError("Not enough balance!")
    return balance - amount

try:
    withdraw(1000, 5000)
except InsufficientFundsError as e:
    print(e)

Key Takeaways

• Use try/except to handle errors without crashing your program
• else runs only if no exception occurred; finally always runs
• Use raise to trigger your own exceptions, including custom exception classes
• Catch specific exceptions whenever possible instead of a generic Exception