Python Serial reading a USB Arduino, all I get is 0s

Question

I am trying to read an ads1115 monitoring a current shunt so it's reading in millivolts. All I need is to be able to read the serial output from the arduino serial. When I bring the arduino up with the IDE in macos sierra I get changing one or two digit values. I'm confused, am I reading the serial wrong in the Python? I know I have the correct port and only one arduino is plugged in.

Arduino code:

Adafruit_ADS1115 ads;  /* Use this for the 16-bit version */
void setup(void)
{
  Serial.begin(9600);
   ads.setGain(GAIN_SIXTEEN);    // 16x gain  +/- 0.256V  1 bit = 0.125mV  0.0078125mV

  ads.begin();
}

void loop(void)
{
  int16_t results;
  results = ads.readADC_Differential_0_1();  
  Serial.println(results);
  delay(1000);
}

Python code on RPi:

import io
import sys
import serial
import time

logfilename = 'temp' + time.strftime("%Y-%m-%d")
file = open('/home/pi/' + logfilename + '.log', 'w+')

ser = serial.Serial('/dev/ttyACM0', 9600)
ser.flushInput()
ser.flushOutput()
while True :
    try:
        bytesToRead = ser.inWaiting()
        state=ser.read(bytesToRead)
        file.write( "%s\n" % (state) )
        time.sleep(1)
    except ValueError:
        print "error"

file.close()

What is contained in the file

pi@pi:~$ more ~/temp2017-04-01.log

0


0


0


0


0


0

What I see from the IDE serial monitor:

11
12
0
18
2
0
3
0
14
6

Edit: I didn't figure it out, but I did get an ethernet shield and webserver to work. I'm still curious why this didn't work.

Answer 1

The Arduino outputs each result with a line terminator.

I suggest you use a blocking read line at the Pi.

while True:
   line = ser.readline()
   file.write("%s\n" % (line))

Answer 2

First, are you sure you connected the cable right? Also, I think the Raspi comes with the serial configured for terminal emulation, you have to do something to make it available for your programs. See this: How do I make serial work on the Raspberry Pi3