Logged In: YES 
user_id=92689

If you install a hook on sys.path_hooks and want it to
affect sys.path items that may already have been used you'd
need to clear sys.path_importer_cache. Say if you install a
directory caching hook after site.py has been imported.
Tricky, but possibly not uncommon. But at the very least the
test script needs it <wink>.

I think it's on a similar level as sys.modules: you normally
don't need it, except when you're mucking with custom import
hooks... (I know, that's not an entirely fair comparison,
but still.)