Basic interaction with another program

[ETP]

I have a dos program (run in a window) that I would like to control
with a script.  It needs only text input.  For example, I only need to
tell it:

L
u
100 <cr>
u

It will then wait for a file to be created, rename the file, then
loop.  Simple.

I'd like to run this on Lucid Puppy Linux as it will be implemented on
a very old laptop, but can probably deal with windows if needed.

I only need direction on getting Python to interact with another
program; how do I get it to direct text onto another window?