Problem with imp.load_source

[Kevin Smith]

I am trying to implement a new source loader using the imp.load_source() 
function, but I get some weird side-effects.  If I use a '.' in the 
first argument of the load_source() call, classes imported in both the 
main program and the imported file don't appear to be the same class 
anymore.  See the example below.

#------- testimp -------
#!/usr/bin/env python

import sys, imp, os
from UserList import UserList

mod = imp.load_source('my.testing',
                      '%(HOME)s/filters/testing.py' % os.environ)

OtherUserList = vars(mod)['UserList']

# Returns 0
print issubclass(OtherUserList, UserList)
#-------- end testimp -------

#--------- $HOME/filters/testing.py
from UserList import UserList
#--------- end testing.py -------