Track 9 · Lesson 9 of 11

Assertions and invariants

Use assert to state what must be true inside your program, catch your own bugs early, and know why assertions must never validate user input.

You will be able to

• Write an assert statement with a helpful message
• Use assertions for preconditions, postconditions and invariants that only a bug can break
• Explain why assertions are switched off by python -O and must never validate user input
• Spot the always-true assertion caused by parentheses