PySerial not talking to Arduino

Can not verify this but it could be that you try and read before there is any data there, thus you get no reply back.

To test this you could try and poll until there is data

value = None
while not value:
   value = sp.readline()
print value

Edit

The Arduino will reset when you open a serial connection, any data written during bootup will likely go to bit heaven. You could use a sleep for 2 seconds (could not find the exact time it takes, will likely vary anyway) before you do any reads/writes.

Alternatively you could write to it until you get a response back, after you get a return you start doing "real work".

For the time being I am using a workaround. I have set the timeout to 1.5 seconds and put a readline call in before the first write.

So now the Python code looks like:

import serial

def main():
    sp = serial.Serial()
    sp.port = 'COM4'
    sp.baudrate = 19200
    sp.parity = serial.PARITY_NONE
    sp.bytesize = serial.EIGHTBITS
    sp.stopbits = serial.STOPBITS_ONE
    sp.timeout = 1.5 #1.5 to give the hardware handshake time to happen
    sp.xonxoff = False
    sp.rtscts = False
    sp.dsrdtr = False

    sp.open()

    sp.readline() #to give the hardware handshake time to happen

    sp.write("GV\r\n".encode('ascii'))
    value = sp.readline()
    print value
    sp.write("GI\r\n".encode('ascii'))
    value = sp.readline()
    print value

    sp.close()

if __name__ == "__main__":
    main()

I've also encountered this problem recently and here's my solution:

import serial

ser = serial.Serial(4, timeout=2)
ser.setRTS(True)
ser.setRTS(False)
while 1:
    line = ser.readline()
    print(line)
ser.close

Turns out this will successfully reset the Arduino board.