ipaddress.IPv6Interface.with_prefixlen drops IPv6 scope IDs

[augusto-rehfeldt]

Bug report

Bug description:

Documented behaviour: The IPv6Interface documentation inherits the IPv4Interface attribute with_prefixlen: "A string representation of the interface with the mask in prefix notation." The IPv6Address documentation permits a scope zone ID appended with '%'.

Expected: with_prefixlen returns 'fe80::1%eth/0', preserving scope 'eth' and prefix length 0 on reconstruction.

Actual: with_prefixlen returns 'fe80::1/0'; reconstruction retains prefix length 0 but loses the scope ID.

import ipaddress as ip

s = 'fe80::1%eth/0'
address, prefix = s.split('/')
base, zone = address.split('%')
p = int(prefix)
try:
    if not (zone.isascii() and zone.isalpha() and 0 <= p <= 128):
        raise ValueError('input outside documented domain')
    i = ip.IPv6Interface(s)
    expected = (int(ip.IPv6Address(base)), zone, p)
    if (int(i), i.scope_id, i.network.prefixlen) != expected:
        raise ValueError('constructor did not retain the valid input')
except ValueError as e:
    print('REFUTATION REJECTED:', str(e))
else:
    text = i.with_prefixlen
    try:
        j = ip.IPv6Interface(text)
        actual = (int(j), j.scope_id, j.network.prefixlen)
    except ValueError as e:
        actual = str(e)
    if actual != expected:
        print('REFUTATION CONFIRMED:', s, 'actual:', (text, actual),
              'expected:', expected)
    else:
        print('REFUTATION REJECTED:', 'scope and prefix survived roundtrip')

Output on Python 3.14.6 (Windows-11-10.0.26220-SP0), standard library ipaddress:

REFUTATION CONFIRMED: fe80::1%eth/0 actual: ('fe80::1/0', (338288524927261089654018896841347694593, None, 0)) expected: (338288524927261089654018896841347694593, 'eth', 0)

This report was found and written by an automated property-testing tool I run (bugforge). The reproducer above was executed and its output is pasted unedited; no person reviewed the report before it was filed. The search script is in https://github.com/augusto-rehfeldt/bugforge-results/tree/main/ipaddress-20261003-155745-c2

CPython versions tested on:

3.14

Operating systems tested on:

Windows

[ZeroIntensity]

See #158748 (comment).