Add Hypothesis[django]

[jamescooke]

Use Hypothesis's django extras package and call for an example.

[jamescooke]

The reason that Hypothesis[django] is RED 🔴 is because model(Item).example() does not reliably generate valid data. There could be two solutions, if either were provided by the library then it would be bumped up to GREEN 💚 ...

• Call example so that it returns only valid data? Something like example(valid=True).

  This would mean that the do_draw function here https://github.com/HypothesisWorks/hypothesis-python/blob/master/src/hypothesis/extra/django/models.py#L174-L175 would be called multiple times if the data was invalid.

• Add some kind of inline assume. This would look something like model(Item).example().assume('full_clean') which would mean that the generated instance's full_clean passing would be a condition of returning the instance.

Maybe either or both of these exist already and I've missed them - that would be optimal 😄

[DRMacIver]

You could do model(Item).filter(lambda item: item.full_clean()).

I actually thought this was already happening, but I think maybe that was in a pull request that got abandoned.

[DRMacIver]

Hmm. Actually that won't work perfectly. There will still be invalid items in the database, but it won't return them from the example.

[jamescooke]

@DRMacIver Thanks for checking this out. I feel sad that this doesn't work with Hypothesis out of the box (yet). Hopefully I'll be able to look some more at Hypothesis[django] and see if there's any quick win that could mean that example(valid=True) could become a reality.

If you find that abandoned PR please stick it on here or tweet it to me - I'd be excited to look it over.

[jamescooke]

Note to self - abandoned PR in Hypothesis is here: HypothesisWorks/hypothesis#335