How to do transparent (opaque) windows from Python

[Don Taylor]

I want to build an application in Python that can show an opaque window 
so that you can still see and type into any window that it covers.

Sort of like a software mylar transparency sheet placed over the screen. 
  I need to be able to type 'through' the transparency into the 
underlying application, focus remains on the underlying window.

My application is assisted typing, but I can envisage other uses such as 
  on-screen rulers.

While I would like this to be multi-platform, I need it in MS-Windows.

Does anyone know of any Python packages that can do this type of thing?

Thanks,

Don.