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Between Python 3.4 (3.4.5 I've tested with) and Python 3.5 (3.5.2), the behavior of ipaddress's subnet() method changed.
This code:
$ cat iat.py
import ipaddress
p = ipaddress.IPv4Network("172.0.0.4/30")
subnets = p.subnets(new_prefix=31)
n = next(subnets)
print(list(n.hosts()))
has two different outcomes on 3.4 vs. 3.5:
$ python3.4 iat.py 
[IPv4Address('172.0.0.4'), IPv4Address('172.0.0.5')]
$ python3.5 iat.py 
[]
AIUI, the 3.4 behavior is the correct one (or the docs need to be fixed).

If someone wants to bisect to find out which changeset introduced the behavior change that would be helpful.

The change was introduced in issue21487.

I just bisected it and the specific commit is this one:
https://hg.python.org/cpython/rev/2711677cf874 

Since this is a regression, I'm going to claim this has higher than normal priority, for whatever good that will do :)

Here's a patch that fixes this bug, test included. I made this against the 3.5 branch, but should apply cleanly on default.

The bug appears to be in the new form of the constructor. Here's a more minimal reproduction:
In python3.5:
>>> list(ipaddress.IPv4Network(("127.0.0.4", 31)).hosts())
[]
In python3.4
>>> list(ipaddress.IPv4Network("127.0.0.4/31").hosts())
[IPv4Address('127.0.0.4'), IPv4Address('127.0.0.5')]

As Stephen notes, the underlying problem appears to be a behavioural difference between two theoretically equivalent ways of defining a network:
>>> list(ipaddress.IPv4Network(('127.0.0.4', 31)).hosts())
[]
>>> list(ipaddress.IPv4Network(('127.0.0.4/31')).hosts())
[IPv4Address('127.0.0.4'), IPv4Address('127.0.0.5')]
Now, the first case is the documented behaviour: hosts() is *supposed to* exclude the network and broadcast address, and those are the only addresses in a /31.
If you want to iterate over all the *addresses* in a network (including the network and broadcast addresses) then you need to iterate over the network object directly:
>>> list(ipaddress.IPv4Network(('127.0.0.4', 31)))
[IPv4Address('127.0.0.4'), IPv4Address('127.0.0.5')]
>>> list(ipaddress.IPv4Network(('127.0.0.4/31')))
[IPv4Address('127.0.0.4'), IPv4Address('127.0.0.5')]
However, as Emanuel found when writing his patch, there's currently an undocumented special case for /31 networks: the definition of "hosts" is *implicitly changed* for such instances to include the nominal network and broadcast address (by setting "self.hosts = self.__iter__"), presumably on the assumption that such networks represent a point-to-point link between two hosts, so the concepts of "network address" and "broadcast address" don't really apply.
That special case seems pragmatically useful, so I think the right fix would be to:
- document the special case that for /31 networks, hosts() includes the network and broadcast addresses (on the assumption the "network" is actually a point-to-point link between two hosts)
- refactor IPv4Network.__init__ to first map the supplied input to a "candidate_address" and "candidate_netmask" and then use *common* validation logic to determine the actual network address and netmask (this will also address the "# fixme" comment for the int/bytes case)
- check whether or not IPv6Network is affected by the same behavioural discrepancy

Ack, special cases! I can look into making a patch, but I'm not really acquainted with ipaddress or the relevant protocols, so it might not be optimal. I'll give it a shot next week if nobody else does.

issue27683.patch tries to fix this. It alters the doc and refactors the constructor, so there is no difference between different arguments
As for IPv6Network, it has the same problem.

ping :)

Ping. ;-)

#28577 requests a similar special case for /32

New changeset 10b134a07c898c2fbc5fd3582503680a54ed80a2 by Xiang Zhang in branch 'master':
bpo-27683: Fix a regression for host() of ipaddress network objects (GH-6016)
https://github.com/python/cpython/commit/10b134a07c898c2fbc5fd3582503680a54ed80a2

New changeset 3326c9267f9df15fa77094b8a740be4eaa4b9374 by Miss Islington (bot) in branch '3.7':
bpo-27683: Fix a regression for host() of ipaddress network objects (GH-6016)
https://github.com/python/cpython/commit/3326c9267f9df15fa77094b8a740be4eaa4b9374

New changeset ae2feb32e7eff328199ce7d527593b3c2aa1fcab by Miss Islington (bot) in branch '3.6':
bpo-27683: Fix a regression for host() of ipaddress network objects (GH-6016)
https://github.com/python/cpython/commit/ae2feb32e7eff328199ce7d527593b3c2aa1fcab