I just spent some time struggling to upgrade to QGIS 2.18.
I originally had these lines in my /etc/apt/sources.list:
deb http://qgis.org/ubuntugis xenial main deb-src http://qgis.org/ubuntugis xenial main deb http://ppa.launchpad.net/ubuntugis/ubuntugis-unstable/ubuntu xenial main
but when I tried to install I was getting error messages like:
Some packages could not be installed. This may mean that you have requested an impossible situation or if you are using the unstable distribution that some required packages have not yet been created or been moved out of Incoming. The following information may help to resolve the situation:
The following packages have unmet dependencies: python-qgis : Depends: libqgis-analysis2.18.14 but it is not going to be installed Depends: libqgis-server2.18.14 but it is not going to be installed
When I changed the sources.list entries to the following the problem was fixed:
deb http://qgis.org/debian xenial main deb-src http://qgis.org/debian xenial main
My question is, why?