%-*s — dynamic width from an argument — was unsupported

• Type: bug (NilPy, loud) — Track N
• Found: 2026-08-09, writing a table-formatting program and diffing it against CPython.
• Status: FIXED the same session.

W = 8
print("%-*s|" % (W, "ab"))    # CPython "ab      |"
                              # pxx     ValueError: unsupported format character "*"

%*s and %.*f failed the same way.

Why it matters more than "one format flag"

* is what a report uses when the column width is COMPUTED rather than literal (W = max(len(r) for r in rows)), so the entire idiom was unavailable — and the failure is at run time, inside the formatting call, not at the point the width was decided.

Fix

pypercent_format reads the width (and, after ., the precision) from the argument list when it sees *, consuming it BEFORE the value. That ordering is the part an implementation gets wrong: "%s=%*d" % ("k", 4, 7) must read "k", then 4 as the width, then 7 as the value — asserted.

Two CPython rules that are not obvious, both pinned:

• a negative starred width means left-align with the magnitude as the width (%*s with −8 equals %-8s);
• a negative starred precision is treated as absent.

Verified

test/test_nilpy_percent_star_width.{npy,expected} (.expected from CPython): starred width left/right/negative/zero, starred precision, both at once (%*.*f), the mixed ordering case, and the plain forms as controls — * shares the width slot with the digit parse, so a fix that captured too much would break every ordinary %-8s. gate.sh quick GREEN.