IPv6Network.address_exclude raises AssertionError for contained scoped IPv6 networks

[augusto-rehfeldt]

Bug report

Bug description:

Documented behaviour: "Computes the network definitions resulting from removing the given network from this one. Returns an iterator of network objects. Raises ValueError if other is not completely contained in self." — ipaddress documentation, IPv6Network.address_exclude.

Expected: Complete successfully with networks covering exactly fe80::1 through fe80::3.

Actual: AssertionError: Error performing exclusion: s1: fe80::/127 s2: fe80::2/127 other: fe80::%a/128

import ipaddress

inputs = ('fe80::%a/126', 'fe80::%a/128')
try:
    n, other = map(ipaddress.IPv6Network, inputs)
    if not other.subnet_of(n):
        raise ValueError("other is not completely contained in n")
    expected = list(range(int(n.network_address) + 1,
                          int(n.network_address) + n.num_addresses))
except Exception as e:
    print("REFUTATION REJECTED:", inputs, "invalid input:", repr(e))
else:
    try:
        result = list(n.address_exclude(other))
        actual = {
            "networks": list(map(str, result)),
            "addresses": sorted(
                a for net in result
                for a in range(int(net.network_address),
                               int(net.network_address) + net.num_addresses)
            ),
        }
        broken = actual["addresses"] != expected
    except Exception as e:
        actual = {"exception": type(e).__name__, "message": str(e)}
        broken = True
    if broken:
        print("REFUTATION CONFIRMED:", inputs, "actual =", actual,
              "expected =", expected)
    else:
        print("REFUTATION REJECTED:", inputs,
              "exclusion completed with exactly the expected coverage")

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

REFUTATION CONFIRMED: ('fe80::%a/126', 'fe80::%a/128') actual = {'exception': 'AssertionError', 'message': 'Error performing exclusion: s1: fe80::/127 s2: fe80::2/127 other: fe80::%a/128'} expected = [338288524927261089654018896841347694593, 338288524927261089654018896841347694594, 338288524927261089654018896841347694595]

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-20261003-235502-c3

CPython versions tested on:

3.14

Operating systems tested on:

Windows

[ZeroIntensity]

See #158748 (comment).