How Do You Get Redirected Input?

[not your business]

I have a shell tool that accepts arguments on the command line. I would like
to check if the input is being piped in.  That is,
 
        $ mytool.py < cmdlst.txt

In this case sys.argv is empty. So I added

        pipein = os.read(sys.stdin.fileno(),256)
        if (pipein):
            input_args = pipein.split()
        else:
            input_args = sys.argv[1:]
 
Problem is that if nothing is redirected in, the script waits for a Enter
pressed on the the keyboard. Anyone know a solution to this?  Thanx in
advance for any help.