did you by chance install a different version of Python that is in the fedora 18 base or update repo
using a mismatched python can cause errors
Also did you by chance turn on the "rawhide" repo ?
rawhide is the NEXT version of fedora ( right now the ALPHA testing fedora 19 )
if this outputs the listing ( from the root
text only login )
then it is not a python error
and please post that list
it should have the fedora base ( std) repo
the update repo
and maybe rpmfusion and fusion update
( the adobe repo is not needed )
It is possible that the fedora 18 repos are a bit out of sync
try the update again
if you are using the propitiatory ATI/AMD or NVIDIA drivers ( from the ati or nvidia websites )
then you need to reinstall them after EVERY kernel update .