[Python n00b alert]

I'm trying this little script, and I see an exception like:

Exception exceptions.AttributeError: "'NoneType' object
has no attribute 'refcount'" in <bound method
Shape.__del__ of Line> ignored

If I change the variable name 's1' to something like
's4', I don't see this exception. Also, if I manually
delete the object before the script ends, the exception
does not occur.

Seems that class Shape is destroyed before all it's
objects are destroyed. Is this acceptable?