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f(1, b=7) rejected when the def also has *rest / **kw

Measured

def f1(a, b=2):                  print(f1(1, b=7))   # (1, 7)   -- always worked
def f2(a, b=2, *rest):           print(f2(1, b=7))   # REJECTED
def f3(a, b=2, **kw):            print(f3(1, b=7))   # REJECTED
def f4(a, b=2, *rest, **kw):     print(f4(1, b=7))   # REJECTED



error: Nil Python: f2 got multiple values for parameter 'b'

The star parameter is the whole difference, which is what made the message misleading — it names b, and b is written exactly once.

Why it rates 50

def f(a, b=None, **kwargs) called as f(x, b=1) is an ordinary Python shape — it is how nearly every wrapper, adapter and options-taking helper is written. And this is the direction that is not negotiable: a program CPython accepts and runs is refused. Loud, but loud in a place a real file reaches immediately.

Cause

To place the packed *args container at its parameter index, PyPackStarArgs fills the intervening DEFAULTED slots positionally (PyFillDefaultsUpTo). It filled b's default too, and the user's own b=7 then landed on the already-occupied slot, so PyBindKwArgs reported a collision.

Fix, in two halves — the second only surfaced after the first

  1. PyFillDefaultsUpTo skips a slot that a MATCHED KEYWORD in the same call already binds, advancing past it (the slot is accounted for, by the keyword rather than by a default).
  2. Once a slot can be skipped, position no longer implies index. The filled defaults and the packed *args/**kwargs containers now carry an EXPLICIT slot (ASTIVal = slot + 1) instead of relying on their place in the chain. f(a=1) is what proved this: it skips slot 0, and a positional filler for slot 1 was then counted as slot 0 and collided all over again.

Verification

test/test_nilpy_kwarg_with_star_params.{npy,expected} (.expected from CPython): all four star/kwargs combinations, keywords binding a middle parameter, keywords binding the FIRST parameter, a def with two defaulted slots before the star, and the no-star control.

Deliberately not asserted: i(1, 2, 3, 4, c=9) where a positional and a keyword genuinely hit the same slot. CPython raises TypeError and so does pxx — but at COMPILE time rather than run time. A difference in WHEN, not in whether; left alone rather than pinned.

Adjacent, filed separately

[[bug-nilpy-constructor-with-kwargs-rejects-an-unmatched-keyword]] — the same sweep found K(1, b=5, z=6) refused with "K has no field or constructor parameter named 'z'" although __init__ declares **kw.

Log