...

...

...

import leo Traceback (most recent call last): File "<pyshell#0>", line 1, in ? import leo ImportError: No module named leo

If the module is opened and executed via f5 from IDLE, the path of its directory is added to the syspath. So (to take from an unquoted part of the message), if I run my helloworld.py the first time using this method, then later I'll be able to import (using >>> import foo) anything else that I wrote and kept in this directory. Yeah, doesn't help with leo. 'Course, Numeric, pygame, and others, have nice installers that add themselves to the os.path (at least, they run, and I didn't touch my os.path).