`*args` is a list, not a tuple

Type: bug (NilPy semantics — visibly wrong output) — Track N
Found: 2026-08-02, by a differential sweep against the CPython oracle.

Measured

def m(a, *args):
    return args

print(m(1, 2, 3))                     # CPython: (2, 3)    pxx: [2, 3]
print(type(m(1, 2, 3)).__name__)      # CPython: tuple     pxx: list

Cause, and why it is nearly fixed already

pxx backs a Python tuple with TPyList and marks the instance so it RENDERS with parentheses — pylist_mark_tuple exists for exactly this, and [[bug-nilpy-str-of-tuple-is-empty]] and [[bug-nilpy-derived-tuple-loses-tupleness]] are the two tickets that built and then repaired that mechanism.

The *args packing path simply does not call it. So this is very likely a one-line fix at the site that builds the packed argument list, not a representation question — the representation already exists.

Impact

Cosmetic in the common case, real in two:

any program that prints or logs *args shows the wrong bracket
type(args).__name__ and isinstance(args, tuple) answer wrongly, which is the kind of check argument-forwarding wrappers actually make

Gate

A .npy diffed against CPython: print(args) for zero, one and several extra arguments; type(args).__name__; args forwarded to another *args function and printed there; and a genuine list parameter alongside, to confirm the marking is not applied too widely.

Resolved 2026-08-02 — commit 4ef8f4343

The guess in "Cause" was right: one call, at the point PyPackStarArgs hands the packed list to the callee. The representation already existed and this path never used it.

test/test_nilpy_star_args_is_a_tuple.npy (+ .expected, wired into make test-nilpy) is byte-identical to CPython across zero, one and several extra arguments, type(args).__name__, len(args), *args alongside **kwargs, and — the control that matters, since over-applying the mark is the only real risk — a genuine list parameter staying a list, with print((1, 2), [1, 2]) showing both bracket styles.

gate.sh quick GREEN, self-host fixedpoint byte-identical.

Found alongside, NOT fixed

inner(*args) — star-UNPACKING at a CALL site, as opposed to star-packing in a signature — does not parse ("expected expression"). Different direction of the same syntax and a separate gap; see [[feature-nilpy-starred-and-nested-unpacking]].

Log

2026-08-02 — resolved, commit 4ef8f4343.

*args is a list, not a tuple