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An analysis tool for Python that blurs the line between testing and type systems.

THE LATEST NEWS: The new CrossHair VSCode extension can work in the background and provide in-line errors when it finds a counterexample, just like a linter or type-checker. Try it out and tell me what you think!

If you have a function with type annotations and add a contract in a supported syntax, CrossHair will attempt to find counterexamples for you:

Animated GIF demonstrating the verification of a python function

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.

Try CrossHair right now, in your browser, at!

CrossHair has IDE integrations for VS Code, PyCharm, and more.

Finally, CrossHair can do more than check contracts. It can also generate unit tests and find behavioral differences between functions.

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