typing.Hashable and typing.Sized error on inference

[leaprovenzano]

typing.Hashable and typing.Sized error on inference

Version: 4.53.2
Python : 3.7.4

heres an minimum example to reproduce the error:

from typing import Hashable
from hypothesis import given, infer


@given(key=infer)
def test_setitem_with_hashable(key: Hashable):
    d = dict()
    d[key] = 1
    assert d[key] == 1

expected behaviour: infer infers a strategy of hashable objects from my type hint OR errors gracefully and tells me I can't use typing.Hashable.

actual behaviour:

inferring a strategy based on typing.Hashable type fails when we try and to index an empty __args__ tuple in resolve_FrozenSet as you can see from the tailend of this traceback:

env/lib/python3.7/site-packages/hypothesis/strategies/_internal/core.py:1452: in from_type
    return types.from_typing_type(thing)
env/lib/python3.7/site-packages/hypothesis/strategies/_internal/types.py:182: in from_typing_type
    for k, v in mapping.items()
env/lib/python3.7/site-packages/hypothesis/strategies/_internal/types.py:183: in <listcomp>
    if sum(try_issubclass(k, T) for T in mapping) == 1
env/lib/python3.7/site-packages/hypothesis/strategies/_internal/types.py:340: in really_inner
    return func(thing)
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _

thing = typing.Hashable

    @register(typing.FrozenSet, st.builds(frozenset))
    def resolve_FrozenSet(thing):
>       return st.frozensets(st.from_type(thing.__args__[0]))
E       IndexError: tuple index out of range

env/lib/python3.7/site-packages/hypothesis/strategies/_internal/types.py:377: IndexError

typing.Hashable does not accept type args via __class_getitem__ (it would make no sense) and typing.Hashable.__args__ exists but is an empty tuple ( which is also a bit silly but whatever).
to be clear this works just fine:

from collections.abc import Hashable
from hypothesis import given, infer


@given(key=infer)
def test_setitem_with_hashable(key: Hashable):
    d = dict()
    d[key] = 1
    assert d[key] == 1

which is good because typing.Hashable.__origin__ is collections.abc.Hashable.

I had a look for other things in the typing module with these properties and i can see that typing.Sized has precisely the same issue (and also works fine with from_type(typing.Sized.__origin__)).

---

how can I help?

I'm happy to put up a PR to patch these in to strategies._internal.types.py

Let me know if you're happy for me to go ahead i will get to work on a PR.

[Zac-HD]

Go for it - I think we'll want to take the first element of __args__ if it's non-empty, falling back to __origin__ otherwise. A helper function might make this easier!

[leaprovenzano]

Go for it - I think we'll want to take the first element of __args__ if it's non-empty, falling back to __origin__ otherwise. A helper function might make this easier!

cool. thanks for tip!

you can assign me to this issue if you like.