Using typing in a target function signature breaks strategies.builds

[hoefling]

In our code, we rely a lot on typing feature that was first introduced in Python 3.5 (see PEP-484) so the breakage of typed signatures is a major issue for us.

To reproduce run the following script with latest hypothesis==3.33.0 and any of python{3.5,3.6,3.7dev}:

from typing import Tuple
from hypothesis import strategies as st

def target_untyped(num, flag):
    return (num, flag, )

def target_typed(num: int, flag: bool) -> Tuple[int, bool]:
    return (num, flag, )

StrategyUntypedPositionalArgs = st.builds(
    target_untyped,
    st.integers(),
    st.booleans(),
)

StrategyTypedPositionalArgs = st.builds(
    target_typed,
    st.integers(),
    st.booleans(),
)

# target with untyped args works
print(StrategyUntypedPositionalArgs.example())

# target with typed args raises TypeError
print(StrategyTypedPositionalArgs.example())

Bisecting shows the first failing commit is:

7f9fe4ae8aa156f54a4a66a0044763129a8d1d37 is the first bad commit
commit 7f9fe4ae8aa156f54a4a66a0044763129a8d1d37
Author: Zac-HD <zac.hatfield.dodds@gmail.com>
Date:   Sun Jul 23 20:55:59 2017 +1000

    Use from_type for missing required args to builds()

:040000 040000 89f12436a970300b9f9500ace096ff4c0bbe29bd 1369e4cef83506b9a5bf17b3b2329a1670a846d9 M	src

Workaround suggestion: do not update to 3.14.0, last working version is 3.13.0.

[Zac-HD]

Thanks for reporting this - providing a reproducing case and the bisection is really useful. I'll post a fix as soon as possible 😄

[hoefling]

Glad if this helps; if you need any additional info, don't hesitate to ask.

[Zac-HD]

I've already tracked it down - there's a one-line fix to the introspection where I called it with the tuple of received arguments but handled a tuple of argument names. Just writing a regression test now, then changelog and it'll be released as soon as someone can review it.

[hoefling]

Nice, thanks!