Python Basics · Error Handling · Lesson 21 of 36
Raising Your Own Exceptions
Concept
Raising Your Own Exceptions
The raise keyword lets your own functions signal a problem, using a built-in exception type like ValueError or a custom message.
Raising stops the function immediately, the same way return does, but signals failure instead of a normal result.
By the end of this lesson
- Explain the core idea behind Raising Your Own Exceptions.
- Predict output before running a change.
- Test one realistic and one unusual input.
- Use the result to make the next decision.
How to study this page
- 1. Read one concept.
- 2. Change the example.
- 3. Run, compare, and explain.
- 4. Complete the challenge below.
Syntax
raise ExceptionType("message")ExampleRunnable
def withdraw(balance, amount):
if amount > balance:
raise ValueError("Insufficient funds")
return balance - amount
try:
withdraw(50, 100)
except ValueError as error:
print(error)Self-check before continuing
Without looking at the example, describe what changes when you modify one input in Raising Your Own Exceptions. Then reopen the editor and prove your explanation with a small test.
You are ready to continue when you can predict, test, and explain the result.
Your turn
Write a function divide(a, b) that raises a ValueError when b is 0, then call it inside a try/except.
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Console