InvalidArgument when using @given with a function with positional-only args

[mx781]

Hi and thanks for the hard work on hypothesis, it's been super useful already.

I can't seem to use @given with a function that has pos-only args:

@given(st.integers())
def test(a: int, /, b: str, *, q: int = 3) -> None:
    pass

test()

raises InvalidArgument: positional arguments to @given are not supported with varargs, varkeywords, positional-only, or keyword-only arguments

while

@given(st.integers())
def test(a: int, /, b: str, *, q: int = 3) -> None:
    pass

test()

throws InvalidArgument: test() got an unexpected keyword argument 'a', from a=integers() in @given.

Is there a way to do this? I see some tests relating to pos-only args, so I suspect this has been though about, but I also haven't found examples of this (somewhat hard to search for).

[Zac-HD]

It looks like you've pasted the first example test twice, so I'm not sure what's different in the second? If it's meant to show @given(st.integers()), then you'll have two problems: no argument given for b, and passing a by keyword when it's a positional-only argument.

Unfortunately we had to lock in the details for how Hypothesis translates the arguments to @given() into arguments to your test function back when we supported Python 2, before positional-only or even keyword-only arguments (or inspect.Signature.bind()) existed. As a result, I've been unable to support generating positional-only arguments using Hypothesis - although we can now generate the other parameters for a test function-or-method which has some posonly params - and we've also had to exclude mixing positional and keyword arguments to the @given() decorator.

While this is definitely not ideal, restricting the signatures of supported test functions is a relatively small cost compared to breaking many existing tests which would change the signature binding semantics. For what it's worth, I'm not convinced that offering more power is a good tradeoff against simplicity either 😕

[mx781]

Ooof, my bad. The example that raises "InvalidArgument: test() got an unexpected keyword argument 'a', from a=integers() in @given" should've indeed been this:

@given(a=st.integers(), b=st.text(), q=st.integers())
def test(a: int, /, b: str, *, q: int = 3) -> None:
    pass

test()

Posonly-args are indeed rare so what you said makes sense - thanks for the speedy response and for clearing this up! 🙂