Validate before crossing a boundary

15-minute session: read for 3 minutes, trace for 3, experiment for 7, and complete the check for 2. Spend longer whenever you need.

Treat external values as untrusted. Define valid type, size, range, and structure before use. Reject an invalid value with a clear error instead of silently turning it into a different request. Test both endpoints and the values immediately outside them. A validated value still needs authorization for the requested action.

Practice and evidence: Predict the example or draw its state transitions. Change one assumption, test the boundary, and record the actual result. Explain what this small model does not establish about a production system.

def valid_score(value):
    return type(value) is int and 0<=value<=100
print([valid_score(x) for x in [-1,0,100,101,True]])

Expected result: [False, True, True, False, False]

Practice and evidence

Keep three notes: your prediction, the actual result, and why they match or differ. The completion check grades understanding; the practice task builds skill.