with open('filename.txt', 'r') as stdin_fh:
    with open('filename.gpg', 'w') as stdout_fh:
        proc = subprocess.Popen(shlex.split(cmd), stdin=stdin_fh, stdout=
            stdout_fh)
proc.communicate()
os.close(fd)
Write to file descriptor 3 of a Python subprocess.Popen object