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A library for property based testing

Project description

Hypothesis is a library for property based testing in Python. You write tests encoding some invariant that you believe should always be true for a variety of inputs and then Hypothesis tries to prove you wrong.

For example:

from hypothesis import given

def test_strings_are_palindromic(x):
    assert x == ''.join(reversed(x))

You can then run this with your favourite testing framework and get the following output:

AssertionError: assert '01' == '10'

Hypothesis will also print out the example for you:

Falsifying example: x='01'

Hypothesis not only finds you counterexamples it finds you simple counter-examples.

Hypothesis is inspired by and strongly based on libraries for testing like Quickcheck, but in comparison has a distinctly dynamic flavour and a novel approach to data generation.

Hypothesis is not itself a testing framework, and should play nicely with your existing one. Development and testing of Hypothesis itself is done with Pytest, but pytest is not a dependency and Hypothesis should work with just about anything.

If you want to learn more about how to use Hypothesis, comprehensive documentation is available at readthedocs.

There is also a separate document describing some of the internals, mostly for the benefit of people who are interested in porting Hypothesis to other languages, but it may be of general interest.

Discussion and help

If you use or are interested in using Hypothesis, we have a mailing list. We also have the IRC channel #hypothesis on freenode.

Feel free to use these to ask for help, provide feedback, or discuss anything remotely Hypothesis related at all. When you do, please abide by the Hacker School social rules.

In particular this is an inclusive environment for people from a variety of backgrounds and skill levels. Prejudice and aggression are unwelcome and everyone should be treated with respect.

I’ll do my best to pay attention to peoples’ behaviour, but if you see anyone violating these rules and I haven’t noticed, please alert me and I’ll deal with it. Usually I will simply ask people to modify their behaviour, but for particularly severe transgressions, repeat offenders or those unwilling to change their ways I’ll ban them from the community.


Hypothesis should be considered fairly stable.

It’s highly stable in the sense that it should mostly work very well. It’s extremely solidly tested and while there are almost certainly bugs lurking in it, as with any non-trivial codebase, they should be few and far between.

For the moment, until 1.0 is reached, the API may still be prone to breaking between minor releases, but the 0.7 release should be considered as very close to the final version and it’s unlikely that anything will break very much between now and 1.0.

Supported versions

2.7.x, 3.2.x, 3.3.x and 3.4.x, as well as both pypy-2.5.0 and pypy3-2.5.0 are all fully supported and tested on.

Builds are checked with Travis and Appveyor.

Versions of Python earlier than 2.7 will not work and will probably never be supported. Jython and IronPython might work but I haven’t checked and will probably only fix bugs with them if it’s really easy to do so.


External contributions to Hypothesis are currently less easy than I would like them to be. You might want to consider any of the following in preference to trying to work on the main Hypothesis code base:

  • Submit bug reports
  • Submit feature requests
  • Write about Hypothesis
  • Build libraries and tools on top of Hypothesis outside the main repo

And indeed I’ll be delighted with you if you do! If you need any help with any of these, get in touch and I’ll be extremely happy to provide it.

However if you really really want to submit code to Hypothesis, the process is as follows:

You must own the copyright to the patch you’re submitting as an individual. I’m not currently clear on how to accept patches from organisations and other legal entities.

If you have not already done so, you must sign a CLA assigning copyright to me. Send an email to with an attached copy of the current version of the CLA and the text in the body “I, (your name), have read the attached CLA and agree to its terms” (you should in fact have actually read it).

Note that it’s important to attach a copy of the CLA because I may change it from time to time as new things come up and this keeps a record of which version of it you agreed to.

Then submit a pull request on Github. This will be checked by Travis and Appveyor to see if the build passes.

Advance warning that passing the build requires:

  1. All the tests to pass, naturally.
  2. Your code to have 100% branch coverage.
  3. Your code to be flake8 clean.
  4. Your code to be a fixed point for a variety of reformatting operations (defined in

It is a fairly strict process.

Once all this has happened I’ll review your patch. I don’t promise to accept it, but I do promise to review it as promptly as I can and to tell you why if I reject it.

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