Newbie question: Sub-interpreters for CAD program

[sonicSpammersGoToHellSmooth]

Hi all,

I'm a newbie to Python, so I have a question about writing an
application that also has a scripting ability.  I'm thinking of Eric3
as an example.  It's written in Python, but it also has an interpreter
window.  The user doesn't have access (I don't think...) to all the
internal stuff that makes the IDE work.

In my case I'd like to write a CAD program which allows the user to
write Python scripts, and to provide an API to do CAD stuff, manipulate
parameters, circuits, layouts, simulations, etc.  The user should not
have access to the internals of the CAD program itself.  The CAD
program is written primarily in Python, with possibly C++ extensions
for speed critical stuff.

There is another posting currently asking about how many interpreters
are needed with how many thread states each.  Since this is new to me,
can someone please explain how this sort of thing is "supposed" to
work, from a high level?

I have a strong EE and hardware background (hence my need to write a
CAD program that doesn't piss me off), but not a CS background.

Thanks
Michael