[python] Python: Writing to and Reading from serial port

I've read the documentation, but can't seem to find a straight answer on this. I have a list of all COM Ports in use by Modems connected to the computer. From this list, I try to open it, send it a command, and if it says anything back, add it to another list. I'm not entirely sure I'm using pyserial's read and write functions properly.

i=0
for modem in PortList:
    for port in modem:
        try:
            ser = serial.Serial(port, 9600, timeout=1)
            ser.close()
            ser.open()
            ser.write("ati")
            time.sleep(3)
            print ser.read(64)
            if ser.read(64) is not '':
                print port
        except serial.SerialException:
            continue
        i+=1

I'm not getting anything out of ser.read(). I'm always getting blank strings.

a piece of code who work with python to read rs232 just in case somedoby else need it

ser = serial.Serial('/dev/tty.usbserial', 9600, timeout=0.5)
ser.write('*99C\r\n')
time.sleep(0.1)
ser.close()