Python-How to add a struct.pack value to an integer that would be sent through the serial port

Question

I'm a newbie in python and recently I made a similar question about how to convert a decimal value to an integer with low byte first and then high byte is here and is solved but now I have a problem because I want python to take a decimal value from a raw_input convert it to an integer with low byte first and then high byte and insert it into a 16-bit integer to send it to the serial port to a servo controller (devantech sd84). I've contained the raw_input into a value and then the struc.pack in other value then I put the second value into the 16-bit integer but before this can occurs python returns this error:

    Traceback (most recent call last):
    File "C:/Users/Pablo/Desktop/kdfkldkfgv", line 28, in <module>
    POS_SERVO = struct.pack('<h', 'posicion')
    error: cannot convert argument to integer

Here is the complete code:

    # -*- coding: utf-8 -*-
  import serial
  import time
  import struct
  # Para cambiar de Sistema Operativo cambiar puerto 
  #en la siguiente línea: Win COM# linux /dev/ttyS# /dev/ttyUSB#
  # #=un número asignado por tu sistema.
  port='COM3'
  sync='\xAA\xA0\x55'
  SET_SERVO='\x01'
  GET_SERVO='\x02'
  SET_SPEED='\x03'
  SET_MODE='\x04'
  GET_MODE='\x05'
  SET_AD_CNT='\x06' #Controla el número de canales analógicos.
  GET_AD_CNT='\x07' #Devuelve el número de canales analógicos actuales.
  GET_INPUT='\X08' # Devuelve el estado de una entrada.
  GET_ADC='\X09' #Devuelve el valor de un canal analógico.
  GET_VERSION='\x0A' #Devuelve la versión del procesador seleccionado.
  GET_BULK='\x15' #Usado para test en fábriica.
  TEST='\X16' #Usado para test en fábrica.
  ser = serial.Serial(port, baudrate=115200, bytesize=8, parity='N', stopbits=2,timeout=1)

  if ser.isOpen():
print "Estado del puerto: Correcto."
print "Procedo a enviar modo del canal 1 y posiciones del mismo."
posicion = raw_input('Inserta un numero entre 400 y 2200:')
POS_SERVO = struct.pack('<h', 'posicion')
ser.write(sync+SET_MODE+'\x01\x01\x19')
ser.write(sync+SET_SERVO+'\x01\x02'+POS_SERVO)
time.sleep(1.6) 
ser.close
print "Todo el proceso completado correctamente."

   else:
print "El puertito está cerrado"

   print "Hasta Luego :D"

For the interest of people who have the same controler I'll explain that 16-bit integer first three bytes are sync (\xAA\xA0\x55) then the type of command SET_SERVO (position) (\x01) then the channel 1-84 for 1 is (\x01) then a byte count in this case 2 (\x02) and the servo position low byte and then the high byte (POS_SERVO) (for example for 1500 \xDC\x05)

I don't know how to solve it, because it's not an argument in fact it is a value and I don't know how to tell that to python.

Solution

You're passing a string literal; you should be passing a value instead, indicated by using the bare name. But you'll need to convert it to an integer before it can be handled.

POS_SERVO = struct.pack('<h', int(posicion))

You should consider validating the input though, to make sure that it's the corect type and within the desired range.