wraps does not correctly handle keyword-only arguments

[FMeinicke]

I have a function that takes some positional arguments and a keyword-only argument that I'd like to wrap with @ureg.wraps as follows:

@ureg.wraps(None, (None, ureg.kilogram, ureg.meter, ureg.meter_per_second, None))
def func(arg1: str, arg2: float, arg3: np.float, arg4: float, *, kwarg1: str = ""):
    ...

However, this results in the following error:

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/florian/.local/lib/python3.10/site-packages/pint/registry_helpers.py", line 289, in wrapper
    result = func(*new_values, **kw)
TypeError: func() takes 4 positional arguments but 5 were given

I've traced the problem down to this function:

pint/pint/registry_helpers.py

Lines 173 to 186 in 5314065

	def _apply_defaults(func, args, kwargs):
	"""Apply default keyword arguments.
	Named keywords may have been left blank. This function applies the default
	values so that every argument is defined.
	"""
	sig = signature(func)
	bound_arguments = sig.bind(*args, **kwargs)
	for param in sig.parameters.values():
	if param.name not in bound_arguments.arguments:
	bound_arguments.arguments[param.name] = param.default
	args = [bound_arguments.arguments[key] for key in sig.parameters.keys()]
	return args, {}

Here, kwargs are "converted" to positional arguments which in my case causes the error because kwarg1 cannot be used as a positional argument but only a keyword argument.

A possible fix would be to check the kind of the parameter and construct a kwargs dictionary with all keyword-only arguments. This, for example, works in my case:

def _apply_defaults(func, args, kwargs):
    """Apply default keyword arguments.

    Named keywords may have been left blank. This function applies the default
    values so that every argument is defined.
    """

    sig = signature(func)
    bound_arguments = sig.bind(*args, **kwargs)
    for param in sig.parameters.values():
        if param.name not in bound_arguments.arguments:
            bound_arguments.arguments[param.name] = param.default
    args = []
    kwargs = {}
    for key, param in sig.parameters.items():
        if param.kind == Parameter.KEYWORD_ONLY:
            kwargs[key] = bound_arguments.arguments[key]
        else:
            args.append(bound_arguments.arguments[key])
    return args, kwargs

[hgrecco]

Yes. We need to improve wraps and check. But we should make a wish list in a separate issue.

[hgrecco]

See #1753