Arduino, Python, Serial and Unpacking Values: "ValueError: need more than 1 value to unpack" -

this first question , relatively new python, kindly bear me. guidance problem appreciated!

i have pulse oximeter used particular arduino shield produce 2 simple signals: pulse rate , oxygen saturation content. see arduino code:

#include <pinchangeint.h> #include <ehealth.h>  int cont = 0;  void setup() {   serial.begin(115200);     ehealth.initpulsioximeter();    //attach inttruptions using pulsioximeter.      pcintport::attachinterrupt(6, readpulsioximeter, rising); }  void loop() {    //bpm   serial.print(ehealth.getbpm());   serial.print(",");    //sp02   serial.print(ehealth.getoxygensaturation());      //new line   serial.print('\n');     delay(500); }   //========================================================================= void readpulsioximeter(){      cont ++;    if (cont == 50) { //get of 1 50 measures reduce latency     ehealth.readpulsioximeter();       cont = 0;   } } 

so output serial monitor, in other words when pyserial readline() called, comma-delimited lines looks like:

65,95 67,96 67,95 70,94 

simple, right?

well, use python program in combination pyserial read these values, assign them vector, print them on graph in real time , save .csv file.

for reference, here program:

import serial import time  import matplotlib.pyplot plt import numpy np import sys import os import csv  #time load timestr = time.strftime("%y_%m_%d")  #spacer spacer = "_"  #user inputs name name = raw_input('enter full name:  ') file_name = name + spacer + timestr file_name_str = file_name+'.csv'  #change directory path = '/home/pi/desktop/pulseox/data' os.chdir(path) text_file = open(file_name, "w") full_path = path + "/" + file_name_str  #check print full_path  #establish serial connection acm0 ser = serial.serial('/dev/ttyacm0', 115200)  #establish vectors thymel = [ ] bpml = [ ] sp02l = [ ] array_data = thymel, bpml, sp02l   #declare time thyme = 1  #establish plot values #plt.axis([0,50,0,120]) plt.ion() plt.title("pulse [bpm] & spo2 [%] v. time [s]", fontsize = "16") plt.xlabel("time [s]", fontsize = "14") plt.ylabel("pulse (red) [bpm] & spo2 (blue) [%]", fontsize = "14")  #print data terminal, define time while true:              data_in = ser.readline()     bpm, sp02 = data_in.split(",")     thyme = float(thyme)     bpm = float(bpm)     sp02 = float(sp02)     print "time [s]: %s" % (thyme)     print "hr [bpm]: %s" % (bpm)     print "spo2 [%%]: %s" % (sp02)       print       thymel.append(thyme)     bpml.append(bpm)     sp02l.append(sp02)      plt.scatter(thyme,bpm,color="red")     plt.scatter(thyme,sp02,color="blue")     plt.pause(0.1)     time.sleep(0.05)      thyme = thyme + 1      open(full_path, 'w') f:         writer = csv.writer(f)         t, b, s in zip(array_data[0], array_data[1], array_data[2]):             writer.writerow([t, b, s]) 

25% of time works fine. other 75% of time, randomly, program return following error:

traceback (most recent call last)l     file "pulseox.py", line 74, in <module>         bpm, sp02 = data_in.split(",") valueerror: need more 1 value unpack 

so ignore rest of program except following lines:

data_in = ser.readline() spm,sp02 = data_in.split(",") 

if, in fact, serial monitor outputting 2 comma-separated values such as:

67, 95 71, 95 

then why there unpack error when program instructed unpack 2 variables , assign them arrays? problem occurring in program of mine , bottom of it! appreciated!

the problem python code might read buffer when arduino has printed partially. using 4 single prints arduino , while true: data_in = ser.readline() in python code. following may happen:

void loop() {    //bpm   serial.print(ehealth.getbpm());   serial.print(",");    // *** @ point python code may read has been written far, i.e. receives invalid data set , valueerror. ***    //sp02   serial.print(ehealth.getoxygensaturation());      //new line   serial.print('\n');     delay(500); } 

you should first construct string of form x,y\n , print string using single print. python part either reads data tuple or doesn't won't read incomplete tuples leading unpacking failure.

so following do:

void loop() {    char buffer[32];  // make sure buffer large enough;   sprintf(buffer, "%d,%d\n", ehealth.getbpm(), ehealth.getoxygensaturation());   serial.print(buffer);   delay(500); }