Hypothesis strategies for generating Python programs, something like CSmith

python, testing, fuzzing, property-based-testing, hypothesis
pip install hypothesmith==0.1.2



Hypothesis strategies for generating Python programs, something like CSmith.

This is definitely pre-alpha, but if you want to play with it feel free! You can even keep the shiny pieces when - not if - it breaks.

Get it today with pip install hypothesmith, or by cloning the GitHub repo.

You can run the tests, such as they are, with tox on Python 3.6 or later. Use tox -va to see what environments are available.


This package provides two Hypothesis strategies for generating Python source code.

The generated code will always be syntatically valid, and is useful for testing parsers, linters, auto-formatters, and other tools that operate on source code.


It could do literally anything that running Python code is able to do, including changing, deleting, or uploading important data. Arbitrary code can be useful, but "arbitrary code execution" can be very, very bad.

hypothesmith.from_grammar(start="file_input", *, auto_target=True)

Generates syntactically-valid Python source code based on the grammar.

Valid values for start are "single_input", "file_input", or "eval_input"; respectively a single interactive statement, a module or sequence of commands read from a file, and input for the eval() function.

If auto_target is True, this strategy uses hypothesis.target() internally to drive towards larger and more complex examples. We recommend leaving this enabled, as the grammar is quite complex and only simple examples tend to be generated otherwise.

hypothesmith.from_node(node=libcst.Module, *, auto_target=True)

Generates syntactically-valid Python source code based on the node types defined by the LibCST project.

You can pass any subtype of libcst.CSTNode. Alternatively, you can use Hypothesis' built-in from_type(node_type).map(lambda n: libcst.Module([n]).code, after Hypothesmith has registered the required strategies. However, this does not include automatic targeting and limitations of LibCST may lead to invalid code being generated.

Notable bugs found with Hypothesmith


0.1.2 - 2020-05-17

  • Emit more debug info to diagnose a compile() issue in CPython nightly

0.1.1 - 2020-05-17

  • Emit some debug info to help diagnose a possible upstream bug in CPython nightly

0.1.0 - 2020-04-24

  • Added auto_target=True argument to the from_node() strategy.
  • Improved from_node() generation of comments and trailing whitespace.

0.0.8 - 2020-04-23

  • Added a from_node() strategy which uses LibCST to generate source code. This is a proof-of-concept rather than a robust tool, but IMO it's a pretty cool concept.

0.0.7 - 2020-04-19

  • The from_grammar() strategy now takes an auto_target=True argument, to drive generated examples towards (relatively) larger and more complex programs.

0.0.6 - 2020-04-08

  • support for non-ASCII identifiers

0.0.5 - 2019-11-27

  • Updated project metadata and started testing on Python 3.8

0.0.4 - 2019-09-10

  • Depends on more recent Hypothesis version, with upstreamed grammar generation.
  • Improved filtering rejects fewer valid examples, finding another bug in Black.

0.0.3 - 2019-08-08

Checks validity at statement level, which makes filtering much more efficient. Improved testing, input validation, and code comments.

0.0.2 - 2019-08-07

Improved filtering and fixing of source code generated from the grammar. This version found a novel bug: "pass #\\r#\\n" is accepted by the built-in compile() and exec() functions, but not by black or lib2to3.

0.0.1 - 2019-08-06

Initial release. This is a minimal proof of concept, generating from the grammar and rejecting it if we get errors from black or tokenize. Cool, but while promising not very useful at this stage.