A keyword argument through a callable field is refused

Repro

class Holder:
    def go(self, x, mode=1):
        return x

def call(o):
    return o.zap(1, mode=2)

class Box:
    def __init__(self):
        self.v = 0

b = Box()
b.zap = lambda n, mode=0: n + mode
print(call(b))          # CPython: 3.  pxx: TypeError at run time.

Holder.go is there only to keep zap off every other path: no class declares zap, so the call is dispatched on the receiver at run time.

test/nilpy_open_world_kwarg_fail.npy is this program and asserts the refusal, so closing this ticket means changing that fixture from a refusal row to a value row.

Why it refuses

PyDynMethN looks the name up on the receiver's RTTI. A METHOD hit goes to PyHostCall, which binds a kwNames list against the parameter names the RTTI records — that path is correct and is what test_nilpy_open_world_keyword_dispatch.npy asserts. A miss falls through to pydynattr_get and then pyvar_callv0..4, which take positional slots only. Binding mode=2 to a slot there is the mis-binding bug-nilpy-pyeval-fallback-still-binds-host-kwargs-by-position was filed about, so it raises instead.

The measurement to do first

Whether the callable's own value already carries its parameter names. A def and a lambda are boxed by pybound_new_sig / PyMakeFuncValueFor and feature-n-a-callable-value-carries-its-signature-type says a signature travels with the value — if the NAMES are in it, this is a binder call and not a design question. If only arity and defaults are there, it is a bigger job and should be ranked on how much real code writes a keyword through a callable attribute, which is not obviously much.