‘Ghostwriting’ property-based tests


Property-based testing is both more effective and often cheaper than traditional approaches to unit-testing, but has seen only limited adoption: even the popular Hypothesis library is used by only around 4% of Python programmers. Prospective users report that the primary barriers to adoption are uncertainty about how to identify appropriate test oracles, and whether learning and adopting property-based testing would be a good investment. In this paper I attempt to address their concerns via automatic test generation.

The Hypothesis ‘Ghostwriter’ produces idiomatic property-based tests, heuristically identifying common properties 1 in user code based on function names and infering input domains from type annotations and natural-language documentation. Unlike previous tools, the Ghostwriter does not use code coverage, execute the code to be tested, or mine usage examples - enabling “test-generation driven development” starting from function signatures. OSS-Fuzz recommends using Hypothesis and ghostwritten tests for complex inputs [Con21].

Ghostwritten tests can often be run as-is, but degrade to clearly commented stubs for user completion when insufficient information is available. Of NNN ghostwriten tests for NN open source libraries, NNN were complete, NNN required type annotations or minor edits, and only NN required nontrivial patches.


i.e. oracles such as x == load(save(x)) or f(a, b) == f(b, a); the term property-based testing derives from the use of such algebraic properties as test oracles.


Property-based testing originated with QuickCheck [CH00], a popular tool for Haskell which has been widely imitated and used in functional programming. Although property-based testing is both more effective and often cheaper than traditional manual approaches to unit-testing, it remains rare in mainstream languages. In the annual Python Software Foundation survey, 78% of respondents at least sometimes write tests for their code [PythonSoftwareFoundation20], but only 4% use Hypothesis [MHDC19,MD20].

A common theme when discussing Hypothesis at industry conferences or local user groups is that, having independently discovered it, many users are reluctant to invest time in learning or adopting property-based testing because they are uncertain how to identify testable properties of their code - i.e. partial specifications which are suitable as test oracles.

In this paper I attempt to address their concerns via automatic test generation using what [BHM+15] calls “derived oracles” based on function names or the “implicit oracle” that top-level functions should not raise exceptions. The generated tests are designed to “earn the trust of testers” [FSM+13], prioritising readability and correctness over completeness or even executability. To support experimental use, the generation process takes less than a second and is safe even if the code to be tested does IO.

Previous test-generation tools

Previous work on test generating has taken one of three basic approaches.

Note that all of these involve executing the code under test and measuring coverage. I argue that

  1. test generation should avoid assuming that the present behaviour is the correct behaviour, as it limits use for unfinished software and ignores the possible presence of bugs, and

  2. that coverage makes a poor metric - while all effective test suites will have high coverage, there is a great deal of variation at each level of coverage and attempting to reduce a test suite subject to coverage may weaken it substantially.

Random OOP test-suite generation

Randoop [PLEB07] and EvoSuite [FA11] , and the Python implementation Pynguin [LKF20]. Random generation of tests, by constructing and aggregating fragments based on type. No coincidence that it’s Java; but the generated tests are pretty weak. Randoop works “forwards” (think stateful testing); EvoSuite works “backwards” (like from_type), but IMO these are basically the same technique.

[FSM+13], from the authors of Evosuite, finds that using Evosuite

leads to clear improvements in commonly applied quality metrics such as code coverage [but] no measurable improvement in the number of bugs actually found by developers. … automatically generating a set of test cases, even high coverage test cases, does not necessarily improve our ability to test software.

Testing speculative program invariants

Daikon [EPG+07] detects program invariants, by tracking whether each of a universe of possible invariants is violated at runtime. QuickSpec [SJCA17] and Speculate [BR17] use a related technique, speculating on equational laws and using QuickCheck [CH00] to test for violating examples.

Slice programs into fuzz drivers

FUDGE [BBC+19] and FuzzGen [IAMP20] mine existing code to find chains of method calls which exercise the API of a library. They use coverage information to prioritise candidates for human inspection. WINNIE [JTH+21] does a similar trick, but using execution traces from a binary instead of source code analysis to identify targets and calling conventions.

The Hypothesis Ghostwriter

The goal of the Hypothesis Ghostwriter is to help users get started: to teach and to drive adoption of property-based testing. For novices, this means showing how to use Hypothesis’ library functions to write tests, and suggest properties which might be useful for their domain or application.

For experts, providing credible code is simply a time-saver compared to writing everything by hand.

rather than to improve productivity for experienced users. Treating the tool as a substitute for documentation drives several other design decisions:

  • Zero-config setup reduces the odds that prospective users will abandon property-based testing before they see it in action - or get stuck if they don’t read the documentation.

  • Idiomatic output means users can start immediately, by imitating ghostwritten code, and invest time studying the API and design principles only after they see value in it.

  • Incomplete is better than incorrect - it provides clear and concrete next steps for the user, which can be directed by # TODO: code comments.


At the implementation level the Ghostwriter consists of Hypothesis’ preexisting techniques to infer input-generation “strategies” from types, a set of known properties and the heuristics to recognise each, and a morass of glue code to translate between objects and source code strings in each direction.

Inferring input domains from_type()

Hypothesis’ from_type logic, recursive building (ala evosuite), registration of custom types, entry-point plugins.

Predefined properties

Predefined properties. Heuristics used to select properties. tests are named for the SUT and property; simpler than [DRF17] but same motivation.

Writing the test code

How we import objects for introspection. Skipping only as little as possible for invalid parts. Designed as an input to human tuning, not complete tests.


Ghostwritten tests achieve `XX-YY%` branch coverage on `NN` Python libraries and found `NN` previously unknown bugs. Of the `NNN` tests, `NNN` were complete, `NNN` required minor edits to compensate for missing type annotations, and only `NN` required nontrivial patches.

I evaluate the Ghostwriter by applying it to NN Python libraries, and reporting the resulting branch coverage for each, the number of bugs discovered, and the work required to complete ghostwritten tests (none, minor, or substantial).



  • Simple libraries: IDNA, colorsys, bidict, etc.

  • BugsInPy [WSL+20] dataset

  • Quixbugs [LKCSL17] dataset

  • Pynguin’s [LKF20] evaluation dataset

  • Unimplemented stubs for e.g. sorting,

  • others?

Results (plus distribution over 30+ trials each):

  • branch coverage

  • checked coverage?

  • number of test failures

  • number of bugs detected (failures can be duplicate or non-bugs)

  • report size of patch required to make each test useful (none, adjust strategies, adjust body, major revisions, discard entirely)

A different tack on evaluation: looking at the quality of generated tests, not just their coverage - supporting the idea that “guessed oracles” are helpful. [SZ11] and [SZ13] very useful here, especially if I can borrow the backwards-dynamic-slicing code from https://www.debuggingbook.org/html/Slicer.html

Future directions

The Hypothesis Ghostwriter demonstrates that fast heuristic test generation is a viable approach, and has been both adopted and imitated in the wild.

Future research could infer program semantics and input types from method names [HostOstvold09] or natural-langugage comments or documentation [PVSS+07], define test templates for additional properties such as metamorphic relations, or use ghostwritten tests as a starting point for coverage- or mutation-driven test suite modification.



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