Technically, this is not the same because Signature.parameters contains everything.

I think

len([p for p in inspect.signature(func).parameters.values()
    if p.kind == inspect.Parameter.POSITIONAL_OR_KEYWORD])

is equivalent, but haven't checked for cases with variable positional args or kwonly.

Some example code which may be useful to play around with:

>>> from pprint import pprint
>>> import inspect
>>> def f(a, b, c=0, **kwargs):
...     pass
... 
>>> inspect.getfullargspec(f)
FullArgSpec(args=['a', 'b', 'c'], varargs=None, varkw='kwargs', defaults=(0,), kwonlyargs=[], kwonlydefaults=None, annotations={})
>>> pprint(inspect.signature(f).parameters)
mappingproxy(OrderedDict([('a', <Parameter "a">),
                          ('b', <Parameter "b">),
                          ('c', <Parameter "c=0">),
                          ('kwargs', <Parameter "**kwargs">)]))

However, the only thing we use nargs for is in the next line to detect if there if it less than 2 to detect if this is something that is named like a setter, but does not take enough arguments to actually be a setter.

For the following

len(inspect.getfullargspec(func).args) < 2 is true but len(inspect.signature(func).parameters) < 2 is false:

Line2D.set_data(self, *args)
Axes.set_prop_cycle(self, *args, **kwargs)
Figure.set_constrained_layout_pads(self, **kwargs)

I did not check the full code path, but I assume they will be considered setters after the change - not sure if that's actually desired or not, but it's a change to be aware of.

fair enough, will make the changes!

Thanks for pushing on this.

Actually that's just str(p) (which will print out as name=default in presence of a default, isn't the stdlib wonderful? :p)

and the whole thing can be simplified down to

        argstr = (str(inspect.signature(cls))[1:-1]  # Strip parentheses.
                  or "None")

because style classes don't take other arguments, and inspect.signature(cls) helpfully drops self from the signature of __init__...

"if something seems like it is too hard, it probably is"