ipaddress: lru_cache on instance methods causes memory leak (use cached_property instead)

[bastitva0-blip]

Bug report

Bug description:

Lib/ipaddress.py uses @functools.lru_cache() on four instance method properties:

• IPv4Address.is_private (line 1321)
• IPv4Address.is_global (line 1343)
• IPv4Network.is_private (line 1552)
• IPv6Address.is_private (line 2089)

Using lru_cache on instance methods causes a memory leak: the global cache holds
a reference to self, preventing garbage collection even after all other references
to the instance are dropped.

This can be demonstrated with:

import ipaddress, gc, weakref

a = ipaddress.IPv4Address('192.168.1.1')
ref = weakref.ref(a)
a.is_private  # trigger the cache
del a
gc.collect()
print(ref() is not None)  # True — instance is NOT collected (leak!)

The same file already uses @functools.cached_property for other properties
(e.g. with_prefixlen, compressed), which correctly ties cache lifetime to
the instance. Those do not leak:

n = ipaddress.IPv4Network('192.168.1.0/24')
ref = weakref.ref(n)
_ = n.with_prefixlen
del n
gc.collect()
print(ref() is not None)  # False — correctly collected

Fix: Replace @functools.lru_cache() with @functools.cached_property on
the four affected properties.
I would like to work on a fix for this issue.

CPython versions tested on:

CPython main branch

Operating systems tested on:

Linux