[vtkusers] AddObserver in python?

[Tim Newsham]

Why does the vtkObject method AddObserver only take two arguments
in python (the optional priority cannot be provided)?  Is this an
oversight?  How does one add an observer to the be called before
any other callers?

On a related note -- I'm trying to observe some events and prevent
others from acting on them.  I have a RenderWindowInteractor, and I
want to occasionally catch mouse clicks and act on them and override
the default behavior or the renderer.  At other times I would like 
the click to flow through to the interactor so that navigation can
be performed.  Is this possible from python?

Tim N.