Nick Coghlan added the comment:
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
----------
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<http://bugs.python.org/issue27683>
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