I'm just toying around with some Python program which uses twisted and twisted plug-ins. I hot errors since twisted wants to write a cache file into /usr/lib/python2.6/site-packages/, which a normal user is not allowed to do. Solution is simple, though:
If twisted is not installed properly by you distribution, you will get errors like
exceptions.IOError: [Errno 13] Permission denied: '/usr/lib/python2.6/site-packages/twisted/plugins/dropin.cache.new'
In this case you need to (re-) build the twisted plugin cache by running as root:
python -c 'import twisted.plugin as P; list(P.getPlugins(P.IPlugin))'