gettext.c2py mistranslates unary `!` before a binary operator (wrong operator precedence)

[winklemad]

Bug report

gettext.c2py() compiles a C plural-form expression into an equivalent Python function. In C the unary ! operator binds tighter than every binary operator, so !n + 1 means (!n) + 1. But c2py emits a bare not prefix (Lib/gettext.py, in _parse) and then appends the binary operators onto the whole string. Because Python's not binds looser than arithmetic/comparison operators, !n + 1 is compiled to not n + 1, i.e. not (n + 1) — a different expression.

Reproduction

import gettext

gettext.c2py('!n + 1')(0)   # -> 0   ; C says (!0)+1 == 2
gettext.c2py('!n + 1')(1)   # -> 0   ; C says (!1)+1 == 1
gettext.c2py('!n < 3')(0)   # -> 0   ; C: (!0) < 3 == 1  (!n is 0/1, always < 3)
gettext.c2py('!n*2')(0)     # -> 1   ; C: (!0)*2 == 2

The generated source for '!n + 1' is int(not n + 1) (i.e. int(not (n + 1))) instead of int((not n) + 1).

Only two forms happen to work today: a bare !x with nothing after it (which is what test_negation covers, e.g. !!!n), and !(...) with no trailing binary operator.

Impact

c2py is public API and is also used by GNUTranslations._parse to compile the plural= rule from a catalog's Plural-Forms header, so a valid .mo whose plural rule applies ! before a binary operator selects the wrong plural form from ngettext().

Why it's not caught

test_gettext.test_negation only exercises !!!n (no following binary operator), which is exactly the case that works, so the precedence bug is not pinned by any test. The neighbouring cases (chained comparisons, /→//) are handled deliberately; unary ! was overlooked.

Fix

Negate the operand as a self-contained parenthesised unit ((not <operand>)) before the binary-operator loop, so !n + 1 compiles to (not n) + 1. Double negation still normalises to 0/1 (!!n → (not (not n))). I have a small patch + tests and will open a PR referencing this issue.

Reproduced on main (3.16.0a0). Found with AI assistance; I've verified and understand the behaviour and the fix.

Linked PRs

• gh-157451: Fix gettext.c2py precedence of unary ! before a binary operator #157453