[Bug?] .filter ignored on just strategy

[mristin]

Hi,
I hope this has not been reported before. I checked the issues and read the changelog but could not find anything related and this seems to be related to issue #2036.

I noticed an unexpected difference in the behavior when .filter is applied on just strategy between Hypothesis 5.43.3 and Hypothesis 6.12.0 (the former ran on my machine, while the remote CI machine had 6.12.0 installed).

With Hypothesis 5.43.3 (expected behavior):

import hypothesis.strategies
import hypothesis.strategies._internal.strategies

strategy = hypothesis.strategies.just(1).filter(lambda x: x > 10)
assert str(strategy) == "just(1).filter(lambda x: x > 10)"
assert type(strategy) == hypothesis.strategies._internal.strategies.FilteredStrategy

With Hypothesis 6.12.0 (unexpected behavior):

import hypothesis.strategies
import hypothesis.strategies._internal.misc

strategy = hypothesis.strategies.just(1).filter(lambda x: x > 10)
assert str(strategy) == "just(1)"
assert type(strategy) == hypothesis.strategies._internal.misc.JustStrategy

This bug (?) is relevant for icontract-hypothesis when we test instance methods automatically where pre-conditions need to be applied on self. I'd expect the health check to be raised rather than the pre-conditions to be silently ignored.

[Zac-HD]

This changed in #2688, version 5.43.7. The preconditions aren't silently ignored - so our bug is limited to an incorrect __repr__ for just() strategies, which I'll fix momentarily. The return type is not part of our public/stable API though - note the _internal! - and so to keep your check working you'll need to add a special case for _transformations on a SampledFromStrategy.

[mristin]

Thanks a lot, @Zac-HD!

Just to confirm: I tried this in my Python console with Hypothesis 6.12.0 and indeed, the filter is applied:

>>> import hypothesis.strategies as st
>>> st.just(1).filter(lambda x: x > 10)
just(1)
>>> st.just(1).filter(lambda x: x > 10).example()
Traceback (most recent call last):
  ...
hypothesis.errors.Unsatisfiable: Unable to satisfy assumptions of hypothesis example_generating_inner_function.