An augmented subscript evaluates its index twice

• Type: bug (semantic divergence) — Track N
• Found: 2026-08-06, while fixing [[bug-nilpy-augmented-assign-through-a-variant-subscript-is-dropped]]. Pre-existing on both subscript paths, not introduced by that fix.

Measured

calls = []
def key():
    calls.append(1)
    return "n"

e = {"n": 0}
e[key()] += 1
print(e, len(calls))       # CPython {'n': 1} 1     pxx {'n': 1} 2

Same on a variant base (d["a"][key()] += 1). The stored value is correct in every case — only the extra evaluation is observable.

Why it is prio 30 and not higher

It is a deliberate, documented trade, not an oversight. [[feature-nilpy-augmented-subscript-assign]] chose it when it made d[k] += 1 work at all: "the index expression is evaluated twice, which is the same trade the del d[k] rewrite makes and is invisible for the pure index expressions the corpus uses." That is still true — an index with side effects is rare, and a NilPy program that hits this is unusual.

It is filed because "rare" is not "never", and because the reasoning above lives in a resolved ticket where nobody will find it. A future reader measuring len(calls) deserves to find this rather than re-derive it.

Shape of a fix

Bind the base and the index to hidden temps once, then read and write through them — the same thing PyMakeVariantSetItem already does for the value (__py_setval, added for chained assignment). Both paths want it: the default-indexed-property desugar in parser.inc and the variant arm beside it.

Do both together or neither, so the two spellings cannot drift apart — the last two bugs in this family were exactly "one path was fixed and its sibling was not".

Gate

Per-fix loop. A .npy test counting calls to a side-effecting index function under += on a static base, a variant base, and del, diffed against CPython with tools/pydiff.py.