program in interactive mode

[B.G.R.]

Hi all,

I'm working on an interpreter for a university subject, which is programmed
in python under linux.
I got most of the work done and I'm just trying to settle some problems I've
found on my way.
Right now, the most important is reading the user input. Everything goes
through the standard input, and now I got something like this:

numline=0
for line in sys.stdin:
    numline+=1
    workwithline(line)

A little bit more complex, but that's the idea. That will work if the user
does something like "./myprog.py < code" or "cat code | ./myprog.py", and
that's ok, but if the user only does "./myprog.py" then I got to get into
interactive mode and show a prompt in every line expecting the user input
for that line. Problem is I don't know how to tell if I've been "piped" or
not.
I used to think that the piped program doesn't know anything and it's just
OS dependant to close and open the right descriptors, but I'm not sure
anymore. Any help or pointer in the right direction would be greatly
appreciated.

Happy christmas everyone,

RGB