"%(k)s" % {...} — the mapping form of %-formatting

print("%(k)s" % {"k": "v"})

Unhandled exception: ValueError: unsupported format character "("

Fails at RUN time with a named ValueError quoting the character it could not handle — visible, not a wrong string.

Positional %-formatting is complete: a sweep against CPython matched %s %d %.2f, %x/%o, %e/%g, sign flags, thousands separators and * width. The mapping form is the remaining spelling, and it is the one logging and templating code uses because it survives reordering.

The parenthesised NAME is the whole feature: %(name)s looks up name in the right-hand mapping instead of consuming the next positional argument, and the rest of the conversion (flags, width, precision, type) is unchanged — so the existing spec handling is reused and only the argument SOURCE differs.

Gate

make test-nilpy + self-host byte-identical, CPython-diffed over a mapping with several keys, a key used twice, a missing key (KeyError, now that a KeyError names its key), mapping keys combined with flags/width/precision, and a literal %% alongside.

2026-08-09 — IMPLEMENTED

The key is parsed BEFORE the flags (%(name)-10s) and the rest of the conversion is unchanged, so the existing spec grammar is reused exactly as the ticket expected — only the argument SOURCE differs.

The subtlety worth keeping: a mapping placeholder consumes no positional argument, so the positional counter must not advance. "%(k)s %(k)s" is what pins it — one key read twice, which a counter-advancing implementation gets wrong on the second placeholder only.

A missing key raises KeyError NAMING the key, which works because the dict fetch already raises a proper KeyError as of the same day's bug-nilpy-exception-str-and-repr-diverge-from-cpython fix — so that case also guards this fix from the other direction.

Verified against CPython: repeated keys, out-of-order keys, every flag/width/precision combination through a mapping key, a literal %% beside one, the missing-key KeyError, and the positional forms as controls. gate.sh quick GREEN.