How to send a value from Arduino to Python and then use that value

Question

I am in the process of building a robot that is remote controlled using Python to send control messages via the Internet through a simple GUI.

I have gotten part of my code working pretty well, the GUI and control systems, but I am stuck. I am trying to use a parallax ping sensor to get distance to objects information from an Arduino Mega, and send that value to my Python control script to be displayed on the remote GUI.

The main problem that I am having is how to integrate Python code that will use the already established COM port with the Arduino and send a message to tell the Arduino to poll the ping sensor and then send to a Python program which will receive the value, and then let me insert that value into my GUI.

I already have this code to control the Arduino, and it works, with my simple GUI.

import serial
ser = serial.Serial('/dev/ttyUSB0', 9600)

from PythonCard import model

class MainWindow(model.Background):
    def on_SpdBtn_mouseClick(self, event):
       spd = self.components.SpdSpin.value
    def on_FBtn_mouseClick(self, event):
       spd = self.components.SpdSpin.value
       ser.write('@')
       ser.write('F')
       ser.write(chr(spd))
    def on_BBtn_mouseClick(self, event):
       spd = self.components.SpdSpin.value
       ser.write('@')
       ser.write('B')
       ser.write(chr(spd))
    def on_LBtn_mouseClick(self, event):
       spd = self.components.SpdSpin.value
       ser.write('@')
       ser.write('L')
       ser.write(chr(spd))
    def on_RBtn_mouseClick(self, event):
       spd = self.components.SpdSpin.value
       ser.write('@')
       ser.write('R')
       ser.write(chr(spd))
    def on_SBtn_mouseClick(self, event):
       spd = self.components.SpdSpin.value
       ser.write('@')
       ser.write('S')
       ser.write('0')
    def on_PngDisBtn_mouseClick(self, event):
       ser.write('~')
       ser.write('P1')
       ser.write('p2')

app = model.Application(MainWindow)
app.MainLoop()

What I would really like to do is improve the above code and add a button to click to tell Python to send a message to the Arduino to check the ping sensor and return the value. I am very literate with the Arduino code, but I just started playing with Python in the last two weeks.

Answer 1

Basically, you'd just send a suitable command to the Arduino, much like you're already doing, but then wait for the Arduino to send something back; the python end of it might look something like this

ser.write('foo')
retval = ser.readline() # read a complete line (\r\n or \n terminated), 
    #or you could use read(n) where n is the number of bytes you want (default=1)
ping_data = retval.strip() # strip out the newline, if you read an entire line

of course, that'll get you a string, you'll probably want to convert it to an int or float in order to use it in calculations later (use int(ping_data) or float(ping_data) for strings, or struct.unpack in case its a byte sequence that needs unpacking to something sane first, but it all depends on how you represent the sensor data).

Answer 2

Maybe check out the Pyduino project:

pyduino is a library which allows you to communicate with Arduino boards loaded with the Firmata protocol from within Python. It currently supports version 2 of the Firmata protocol.