property-based-testing

Use the Hypothesis library with the @given decorator and strategies from hypothesis.strategies to generate inputs. Define properties like roundtrip (decode(encode(x)) == x) or idempotence (f(f(x)) == f(x)), and add @example decorators for edge cases like empty lists and zero values.

Use Hypothesis for Python, fast-check for JavaScript/TypeScript, proptest for Rust, rapid for Go, jqwik for Java, and ScalaCheck for Scala. For Solidity smart contracts, use Echidna or Medusa fuzzers to test state invariants.

Use property-based testing for serialization pairs, parsers, normalizers, validators, and pure functions with complex input domains. Avoid it for simple CRUD operations, UI logic, integration tests with external dependencies, or throwaway prototype code.

Failures can stem from wrong properties, strategies generating inputs outside the documented domain, or ambiguous specifications. Verify the property against docstrings and type annotations, check strategy realism, and confirm the input violates a documented guarantee before reporting a bug.

Use Echidna or Medusa, which are property-based fuzzers for EVM contracts. Write invariant functions prefixed with echidna_ that return a boolean, such as checking that contract balance stays non-negative, and the fuzzer generates transaction sequences to violate them.

Critical issues include tautological assertions that compare an expression to itself and vacuous tests where contradictory assume() calls filter out all inputs. High-severity issues include tests with no assertions or assertions that reimplement the function's own logic.