PySerial API认为在write（）期间com端口仍处于打开状态，为什么？

import serial
com = serial.Serial()
com.port = 19
com.timeout = 0 #also tried a timeout value greater than 0.
try:
    com.open()
    # at this point I turn off the phone.
    com.write(\"AT\\r\\n\")
    print com.readlines()
except SerialException, e:
    print e