Property-based testing with generated inputs

Goal

Find inputs that break an invariant which example-based tests would not have thought of, and obtain a minimal reproducing input automatically.

Prerequisites

A function with a stateable property: round-trips (decode(encode(x)) == x), idempotence (f(f(x)) == f(x)), invariants after an operation (sorted output, preserved length), or agreement with a simpler reference implementation.

Steps

1. Write the property as a test function that takes generated arguments; with Hypothesis, decorate it with @given and strategies such as st.text() or st.lists(st.integers()).
2. Start with broad strategies; narrow them only when the property genuinely does not apply (document why).
3. Run the test; when it fails, the library shrinks the input to a minimal counterexample and replays it on later runs.
4. Turn each counterexample into an explicit example test so the regression stays visible even if strategies change.
5. Keep generation bounded (sizes, time) so the suite stays fast in CI.

Expected result

Properties hold for thousands of inputs; failures arrive as small, readable counterexamples such as an empty string or a surrogate code point.

Limits and test basis

Properties are harder to state than examples and can be vacuous if strategies are too narrow. Generated inputs do not replace tests of specific business rules. The mechanics described follow the cited documentation.