ipaddress: Wrong behavior with ::ffff:1.2.3.4 style IPs

[ThiefMaster]

BPO	27269
Nosy	@socketpair, @ThiefMaster, @arp11
PRs	bpo-27269: Treat ::FFFF: addresses as IPv4 for ordering and contains. #1741

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

Show more details

GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2016-06-08.14:55:23.856>
labels = ['type-bug', 'library', 'docs']
title = 'ipaddress: Wrong behavior with ::ffff:1.2.3.4 style IPs'
updated_at = <Date 2017-05-23.00:08:58.753>
user = 'https://github.com/ThiefMaster'

bugs.python.org fields:

activity = <Date 2017-05-23.00:08:58.753>
actor = 'arp11'
assignee = 'docs@python'
closed = False
closed_date = None
closer = None
components = ['Documentation', 'Library (Lib)']
creation = <Date 2016-06-08.14:55:23.856>
creator = 'ThiefMaster'
dependencies = []
files = []
hgrepos = []
issue_num = 27269
keywords = []
message_count = 4.0
messages = ['267861', '269984', '269986', '294198']
nosy_count = 5.0
nosy_names = ['pmoody', 'docs@python', 'socketpair', 'ThiefMaster', 'arp11']
pr_nums = ['1741']
priority = 'normal'
resolution = None
stage = None
status = 'open'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue27269'
versions = ['Python 3.5', 'Python 3.6']

[ThiefMaster]

I'd expect the IPv4 address to be considered part of that network (or actually parsed as an IPv4Address and not IPv6Address) even if it's written in IPv6 notation. It's an IPv4 after all.

Python 3.5.1 (default, Jun  7 2016, 09:20:44)
[GCC 4.9.3] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import ipaddress
>>> ipaddress.ip_address('::ffff:127.0.0.1')
IPv6Address('::ffff:7f00:1')
>>> ipaddress.ip_address('::ffff:127.0.0.1') in ipaddress.ip_network('127.0.0.0/8')
False
>>> ipaddress.ip_address('127.0.0.1') in ipaddress.ip_network('127.0.0.0/8')
True

[socketpair]

see also bpo-20446

[socketpair]

Also, it should be marked explicitly, if cross-type lookup is safe. I mean code like that:

    some_net = ipaddress.ip_network(some_input1)
    some_addr = ipaddress.ip_address(some_input2)
    return some_addr in some_net

Is it safe to write such code? Docs say:

21.28.2.3. Operators
Address objects support some operators. Unless stated otherwise,
operators can only be applied between compatible objects
(i.e. IPv4 with IPv4, IPv6 with IPv6).

1. It is not said if exception will be raised if incompatible objects are used, and not said if result is defined.
2. "Unless stated otherwise" - Seem it is stated nowhere.

[arp11]

5cc2742

... seems to implement the principle in this bug, but I'm not convinced this is a good idea.