NotImplementedError: This should never happen in a test chaining st.datetimes()

[honno]

For the following test

# bad_test.py
from datetime import datetime
from hypothesis import given, strategies as st, assume


@given(start_date=st.datetimes(), data=st.data())
def test_foo(start_date, data):
    end_date = data.draw(st.datetimes(min_value=start_date))
    assert False

you will sometimes give you a scary NotImplementedError: This should never happen error raised in hypothesis.internal.conjecture.shrinker.

Full traceback

Traceback (most recent call last):
  File "bad_test.py", line 31, in <module>
    test_foo()
  File "bad_test.py", line 26, in test_foo
    def test_foo(start_date, data):
  File ".../hypothesis/hypothesis-python/src/hypothesis/core.py", line 1471, in wrapped_test
    raise the_error_hypothesis_found
  File ".../hypothesis/hypothesis-python/src/hypothesis/core.py", line 1438, in wrapped_test
    state.run_engine()
  File ".../hypothesis/hypothesis-python/src/hypothesis/core.py", line 1006, in run_engine
    runner.run()
  File ".../hypothesis/hypothesis-python/src/hypothesis/internal/conjecture/engine.py", line 461, in run
    self._run()
  File ".../hypothesis/hypothesis-python/src/hypothesis/internal/conjecture/engine.py", line 874, in _run
    self.shrink_interesting_examples()
  File ".../hypothesis/hypothesis-python/src/hypothesis/internal/conjecture/engine.py", line 939, in shrink_interesting_examples
    self.shrink(example, predicate)
  File ".../hypothesis/hypothesis-python/src/hypothesis/internal/conjecture/engine.py", line 971, in shrink
    s.shrink()
  File ".../hypothesis/hypothesis-python/src/hypothesis/internal/conjecture/shrinker.py", line 442, in shrink
    self.explain()
  File ".../hypothesis/hypothesis-python/src/hypothesis/internal/conjecture/shrinker.py", line 578, in explain
    raise NotImplementedError("This should never happen")
NotImplementedError: This should never happen

I never see this issue once .hypothesis is generated, but if it's not (e.g. if you keep on deleting it before running the tests again) then I see it more often than not.

This looks to be an issue from #3749, as I get this issue on the latest release/master, but not on say the 6.84.2 release just before the aforementioned explain-mode patch.

This was found in a PyCon UK "sprint" I ran with someone who's never used the library before—not the best timing 😅 Huge kudos to @End-of-Eternity for giving me a minimal reproducer!

[End-of-Eternity]

Glad I could help :)

[Zac-HD]

Thanks from me as well! The fix should be released in half an hour or so 😁