Had a bit of trouble with Smart on my laptop this week. After I had to cancel out of an update the whole thing kept hanging when updating the cache at the start, the GUI would sit there doing nothing. I tried running it direct from the shell and got this error:

# smart update
Loading cache...
Traceback (most recent call last):#####################################  ( 98%)
  File "/usr/bin/smart", line 194, in ?
    main(sys.argv[1:])

I found the answer in the link above:

# rm /var/lib/smart/channels/*
# rm /var/lib/smart/cache
# smart update

And everything is working fine again