Raise if register_random is passed unreferenced object

[rsokl]

A pattern like

from hypothesis import register_random

class TorchRandomWrapper:
    # stuff

register_random(TorchRandomWrapper())

will lead to unexpected behavior: Hypothesis will instantly "forget" that it was ever passed an instance of TorchRandomWrapper. This is due to the fact that register_random uses a weakref-valued dict, and here we passed it an unreferenced object, which immediately gets garbage collected after being passed to register_random.

This PR will modify register_random to raise under these circumstances and to emit a warning when it looks like the input was referenced only within a temporary scope. It also fixes an overly-narrow type annotation for register_random so that it now permits structural subtypes of random.Random.

This is the new behavior:

from hypothesis import register_random

class TorchRandomWrapper:
    # stuff

register_random(TorchRandomWrapper())  # raises  ReferenceError

r = TorchRandomWrapper()
register_random(r)  # OK

def f():
    register_random(TorchRandomWrapper())

f()  # raises  ReferenceError


def g():
    r = TorchRandomWrapper()
    register_random(r)

g()  # throws UserWarning

[Zac-HD]

Very nice! Thanks for taking this one 😁

[rsokl]

@Zac-HD this looks like it is good to go. https://github.com/HypothesisWorks/hypothesis/actions/runs/3500864463/jobs/5863954158#step:6:539 is an unrelated flaky fail case.

[Zac-HD]

🤩