# CrossHair [](https://discord.gg/rUeTaYTWbb) [](https://github.com/pschanely/CrossHair/actions/workflows/test_crosshair.yml?query=branch%3Amain) [](https://pepy.tech/project/crosshair-tool) An analysis tool for Python that blurs the line between testing and type systems. > **_THE LATEST NEWS:_** Python's most popular property-based testing tool, [Hypothesis](https://hypothesis.readthedocs.io/en/latest/), now supports running CrossHair as an [optional backend](https://hypothesis.readthedocs.io/en/latest/strategies.html#alternative-backends)! If you have a function with [type annotations](https://www.python.org/dev/peps/pep-0484/) and add a contract [in a supported syntax](https://crosshair.readthedocs.io/en/latest/kinds_of_contracts.html), CrossHair will attempt to find counterexamples for you:  CrossHair works by repeatedly calling your functions with symbolic inputs. It uses an [SMT solver] (a kind of theorem prover) to explore viable execution paths and find counterexamples for you. This is not a new idea; a Python approach was first described in [this paper]. However, to my knowledge, CrossHair is the most complete implementation: it can use symbolic reasoning for the built-in types, user-defined classes, and much of the standard library. [SMT solver]: https://en.wikipedia.org/wiki/Satisfiability_modulo_theories [this paper]: https://hoheinzollern.files.wordpress.com/2008/04/seer1.pdf Try CrossHair right now, in your browser, at [crosshair-web.org]! CrossHair has [IDE integrations] for [VS Code], [PyCharm], and more. Finally, CrossHair can do more than check contracts. It can also [generate unit tests](https://crosshair.readthedocs.io/en/latest/cover.html) and [find behavioral differences](https://crosshair.readthedocs.io/en/latest/diff_behavior.html) between functions. [IDE integrations]: https://crosshair.readthedocs.io/en/latest/ide_integrations.html [VS Code]: https://marketplace.visualstudio.com/items?itemName=CrossHair.crosshair [PyCharm]: https://plugins.jetbrains.com/plugin/16266-crosshair-pycharm [crosshair-web.org]: https://crosshair-web.org Want to help? Sign up for [email](http://eepurl.com/hGTLRH) or [RSS](https://pschanely.github.io/feed.xml) updates. Star ⭐️ the repository. There are [other ways to help](https://crosshair.readthedocs.io/en/latest/how_can_i_help.html) too. ## [Documentation](https://crosshair.readthedocs.io/en/latest) * [Introduction](https://crosshair.readthedocs.io/en/latest/introduction.html) * [Get Started](https://crosshair.readthedocs.io/en/latest/get_started.html) * [The Details: Contracts](https://crosshair.readthedocs.io/en/latest/contracts.html) * [The Details: cover](https://crosshair.readthedocs.io/en/latest/cover.html) * [The Details: diffbehavior](https://crosshair.readthedocs.io/en/latest/diff_behavior.html) * [Kinds of Contracts](https://crosshair.readthedocs.io/en/latest/kinds_of_contracts.html) * [Hints for Your Classes](https://crosshair.readthedocs.io/en/latest/hints_for_your_classes.html) * [IDE Integrations](https://crosshair.readthedocs.io/en/latest/ide_integrations.html) * [Case Studies](https://crosshair.readthedocs.io/en/latest/case_studies.html) * [Limitations](https://crosshair.readthedocs.io/en/latest/limitations.html) * [Plugins](https://crosshair.readthedocs.io/en/latest/plugins.html) * [How Does It Work?](https://crosshair.readthedocs.io/en/latest/how_does_it_work.html) * [How Can I Help?](https://crosshair.readthedocs.io/en/latest/how_can_i_help.html) * [Related Work](https://crosshair.readthedocs.io/en/latest/related_work.html) * [Contributing](https://crosshair.readthedocs.io/en/latest/contributing.html) * [Changelog](https://crosshair.readthedocs.io/en/latest/changelog.html)