Hypothesis is a powerful, flexible, and easy to use library for property-based testing.
Hypothesis is a family of testing libraries which let you write tests parametrized by a source of examples. A Hypothesis implementation then generates simple and comprehensible examples that make your tests fail. This simplifies writing your tests and makes them more powerful at the same time, by letting software automate the boring bits and do them to a higher standard than a human would, freeing you to focus on the higher level test logic.
This sort of testing is often called "property-based testing", and the most widely known implementation of the concept is the Haskell library
QuickCheck_, but Hypothesis differs significantly from QuickCheck and is designed to fit idiomatically and easily into existing styles of testing that you are used to, with absolutely no familiarity with Haskell or functional programming needed.
Hypothesis for Python_ is the original implementation, and the only one that is currently fully production ready and actively maintained.
The core ideas of Hypothesis are language agnostic and in principle it is suitable for any language. We are interested in developing and supporting implementations for a wide variety of languages, but currently lack the resources to do so, so our porting efforts are mostly prototypes.
The two prototype implementations of Hypothesis for other languages are:
Hypothesis for Ruby_ is a reasonable start on a port of Hypothesis to Ruby.
Hypothesis for Java_ is a prototype written some time ago. It's far from feature complete and is not under active development, but was intended to prove the viability of the concept.
Additionally there is a port of the core engine of Hypothesis, Conjecture, to Rust. It is not feature complete but in the long run we are hoping to move much of the existing functionality to Rust and rebuild Hypothesis for Python on top of it, greatly lowering the porting effort to other languages.
Any or all of these could be turned into full fledged implementations with relatively little effort (no more than a few months of full time work), but as well as the initial work this would require someone prepared to provide or fund ongoing maintenance efforts for them in order to be viable.