Modules and tests that challenge a contract

The idea

A Python module executes its top-level statements when first imported in an interpreter. Put demonstration or command-line behavior behind if __name__ == "__main__" so another module can reuse the functions without launching that behavior. Importing still executes definitions and other unguarded statements; the guard does not make the whole module lazy.

A useful test distinguishes correct code from a plausible defect. Include empty input, exact threshold values, repeated values, and checks that promised input preservation actually holds. For a function expected to reject malformed data, assert the specific exception instead of accepting any failure. Tests document selected behavior; a passing suite is evidence about those cases, not proof that every possible input is correct.

def above(values, limit):
    return [n for n in values if n > limit]

if __name__ == "__main__":
    source = [2, 3, 3]
    assert above(source, 3) == []
    assert source == [2, 3, 3]
    print("checks passed")

checks passed

The exact-boundary test detects an accidental >=, and the second assertion verifies input preservation.