ipaddress.ip_interface __lt__ check seems to be broken

[s-sanjay]

BPO	29931
Nosy	@ncoghlan, @berkerpeksag, @serhiy-storchaka, @zhangyangyu, @Mariatta, @s-sanjay
PRs	bpo-29931 fix __lt__ check in ipaddress.ip_interface for both v4 and v6. #879 [3.6] bpo-29931 fix __lt__ check in ipaddress.ip_interface for both v4 and v6. (GH-879) #2217 [3.5] bpo-29931 fix __lt__ check in ipaddress.ip_interface for both v4 and v6. (GH-879) #2218

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

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GitHub fields:

assignee = None
closed_at = <Date 2017-06-15.14:17:26.165>
created_at = <Date 2017-03-28.10:37:20.702>
labels = ['3.7', 'type-bug', 'library']
title = 'ipaddress.ip_interface __lt__ check seems to be broken'
updated_at = <Date 2017-06-15.14:17:26.164>
user = 'https://github.com/s-sanjay'

bugs.python.org fields:

activity = <Date 2017-06-15.14:17:26.164>
actor = 'serhiy.storchaka'
assignee = 'none'
closed = True
closed_date = <Date 2017-06-15.14:17:26.165>
closer = 'serhiy.storchaka'
components = ['Library (Lib)']
creation = <Date 2017-03-28.10:37:20.702>
creator = 'Sanjay'
dependencies = []
files = []
hgrepos = []
issue_num = 29931
keywords = []
message_count = 7.0
messages = ['290695', '290700', '290964', '291112', '291684', '296098', '296099']
nosy_count = 7.0
nosy_names = ['ncoghlan', 'pmoody', 'berker.peksag', 'serhiy.storchaka', 'xiang.zhang', 'Mariatta', 'Sanjay']
pr_nums = ['879', '2217', '2218']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue29931'
versions = ['Python 3.5', 'Python 3.6', 'Python 3.7']

[s-sanjay]

The less than check for ip_interface behavior seems weird. I am not sure if this is by design. We are just comparing the network address but when network address is equal we should compare the ip address.
The expectation is if a < b is False then b <= a must be True
>>> import ipaddress
>>> a = ipaddress.ip_interface("1.1.1.1/24")
>>> b = ipaddress.ip_interface("1.1.1.2/24")
>>> a < b
False
>>> b <= a
False
>>> a == b
False
>>> 
This happens with both v4 and v6
The tests were passing because in ComparisonTests we were testing with prefix length of 32 which means the whole ip address became the network address.
I have made a fix here:
https://github.com/s-sanjay/cpython/commit/14975f58539308b7af5a1519705fb8cd95ad7951
I can add more tests and send PR but before that I wanted to confirm the behavior.

[serhiy-storchaka]

Indeed, this looks as a bug.

>>> a < b
False
>>> b > a
True

[serhiy-storchaka]

New changeset 7bd8d3e by Serhiy Storchaka (s-sanjay) in branch 'master':
bpo-29931 fix __lt__ check in ipaddress.ip_interface for both v4 and v6. (#879)
7bd8d3e

[Mariatta]

Serhiy, just checking whether this needs backport? The PR has the backport to 3.5 and 3.6 labels, but it's not indicated in this ticket.
If it doesn't need backport, then perhaps we can close this issue.
Thanks.

[berkerpeksag]

I think we should backport this to at least 3.6.

[serhiy-storchaka]

New changeset cf58dfb by Serhiy Storchaka in branch '3.6':
[3.6] bpo-29931 fix __lt__ check in ipaddress.ip_interface for both v4 and v6. (GH-879) (bpo-2217)
cf58dfb

[serhiy-storchaka]

New changeset c5a6fb6 by Serhiy Storchaka in branch '3.5':
[3.5] bpo-29931 fix __lt__ check in ipaddress.ip_interface for both v4 and v6. (GH-879) (bpo-2218)
c5a6fb6