Question about packages

[djw]

I'm sturcturing my (relatively) large application into packages and am 
having trouble understanding one aspect of module/package paths.

If I have a structure like:

in /:
appA.py
__init__.py

in X/:
modX1.py
modX2.py
__init__.py

in Y:/
modY1.py
__init__.py

Now, if I do the following imports and run appA.py, all is well:

in appA.py:
from X import modX1
import Y.modY1

in modX1:
import modX2

in modY1: (as long as run from appA.py)
import X.modX1

But, if I try to create a test harness inside modY1 (in an if statement 
checking __name__ with "__main__") and run modY1.py with the last import 
(import X.modX1), I get an ImportError.

So, the question is, how do you import into a sub-package to allow both 
a script at the root level to import the module ("library mode") and for 
the module itself to run in "test harness" mode with the same import?

Any hints appreciated.

-Don