FutureWarning from a nested set operand is not attributed to the caller

[fedonman]

Bug description:

A FutureWarning raised while parsing a nested set inside a set-operation operand is attributed to a frame inside the re package rather than to the caller, and lands further away the deeper the nesting. _parse_operand() in Lib/re/_parser.py adds two frames but advances nested by one, so the stacklevel is one short.

import re, warnings

for pattern in [r'[[b]]', r'[a--[[b]]]']:
    with warnings.catch_warnings(record=True) as w:
        warnings.simplefilter('always')
        re.compile(pattern)
    print(pattern, '->', w[0].filename == __file__)

[[b]] -> True
[a--[[b]]] -> False

Expected: True for both. test_set_operations in Lib/test/test_re.py already asserts w.filename == __file__ for the top-level spelling. Because warning filters and the default duplicate suppression key on the reported location, -W 'error::FutureWarning:__main__' raises on the first pattern and misses the second.

CPython versions tested on:

CPython main branch

Operating systems tested on:

Linux

Linked PRs

• gh-156187: Fix the warning stacklevel inside a nested set operand #156188