Is this added check catching a case not already caught by the in addrs check which immediately follows?

Can you please give an example of a value which you ran into? Are we talking about leading / trailing spaces or spaces in the middle of the IP address?

LGTM then.

Can you please give an example of a value which you ran into?

This was my problem this time:

> export ROS_IP=$(ip address show eth0 | sed -n 's:inet \([^ /]*\).*:\1:; T; p')
> echo $ROS_IP
1.2.3.4
> roswtf
...
WARNING ROS_IP may be incorrect: ROS_IP [    1.2.3.4] does not appear to be a local IP address [u'127.0.0.1', ...]

There is probably a better way to extract the intended IP address, but a bug in the sed script accidentally added 4 spaces before the IP. In theory this is shown in the warning, but in practice it could also be random padding in the output.

Now this will say

WARNING ROS_IP may be incorrect: ROS_IP [    1.2.3.4] contains whitespace. This is not a valid IP.

Additionally I disambiguate the previous warning when the IP does not match a local adapter.
It will now print the more comprehensive warning:

WARNING ROS_IP may be incorrect: ROS_IP [1.2.3.4] does not appear to be an IP address of a local network interface (one of [u'127.0.0.1', ...]).

Thank you for the patch and the explanation.