Assertions and Invariants

An assertion states an assumption directly in code.

assert len(scores) > 0

If the condition is true, execution continues. If it is false, Python raises an AssertionError.

Assertions are useful while learning because they make hidden assumptions visible.

What is an invariant?

An invariant is something that should remain true at a certain point in the program.

Inside a loop that sums scores, a useful invariant is:

total is the sum of the scores processed so far

In code, a simpler check may be:

assert total >= 0

if scores are known to be non-negative.

Assert before a risky operation

Division by length assumes the length is not zero:

average = total / len(scores)

Say that explicitly:

Without the assertion, the program would fail at division. With the assertion, the reason is named before the risky operation.

Assertions are not user input handling

Assertions are mainly for programmer assumptions. They are not a full way to handle every possible bad input from a user.

For public input, code should usually check and respond:

But while building and testing learning code, assertions are excellent small guards.

Write helpful messages

Assertions can include a message:

assert len(scores) > 0, "scores must not be empty"

The message should name the assumption, not blame the reader.