Respect PEP 3102 keyword-only arguments in nopython mode

[astrojuanlu]

Reporting a bug

• I am using the latest released version of Numba (most recent is visible in
  the change log (https://github.com/numba/numba/blob/master/CHANGE_LOG).
• I have included below a minimal working reproducer (if you are unsure how
  to write one see http://matthewrocklin.com/blog/work/2018/02/28/minimal-bug-reports).

Keyword-only arguments work fine in nopython mode:

In [1]: from numba import njit                                                                                                                                                                                                                

In [2]: def f(a, *, b): 
   ...:     return a * b 
   ...:                                                                                                                                                                                                                                       

In [3]: f(1, b=2)                                                                                                                                                                                                                             
Out[3]: 2

In [4]: f(1, 2)                                                                                                                                                                                                                               
---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-4-c9c271413adf> in <module>
----> 1 f(1, 2)

TypeError: f() takes 1 positional argument but 2 were given

In [5]: nf = njit(f)                                                                                                                                                                                                                          

In [6]: nf(1, b=2)                                                                                                                                                                                                                            
Out[6]: 2

However, I expected nf(1, 2) to fail in the same way as f(1, 2), but it doesn't:

In [7]: nf(1, 2)                                                                                                                                                                                                                              
Out[7]: 2

(perhaps related to #2994?)

[esc]

@astrojuanlu I can reproduce with the following snippet:

from numba import njit


@njit
def f(a, *, b):
    return a * b


print(f(1, b=2))
print(f(1, 2))

I am not sure if this should be classified as a bug or as a feature request.

[stuartarchibald]

I think feature request, Numba's only just got out of having to support both Python 2 and 3. IIRC this is a Python 3 only feature and, as signature handling is very hard, it wasn't of a particularly high priority to handle version specific features when supporting both major versions of Python.

[flying-sheep]

This is a bug. Numba corrupts the function signature.