I want to send some analog sensor data from a Teensy 3.2 to a mac running a python script (I am using the usual pyserial module and I send data using Serial.print() in the arduino sketch). I am using Teensy but I guess the same would be true for an Arduino Uno. The teensy is correctly sending on the serial port the data, as I can see it using the serial monitor. However, only the first time that I run the python script the data are handled by the script itself, if I stop the script then no data seem to be read in the serial buffer. To make the script work I necessarily have to re-launch the Arduino IDE and then upload the sketch.

Indeed the second time I run the script (after having stopped it), I can see that the pyserial function serial_port.inWaiting() does not give any data.

if (serial_port.inWaiting() > 0):
   print("serial_port.inWaiting() > 0") # does not print anything the second time I launch the script

I have checked that the second time that I launch the script the serial connection is open (using self.serial_port.isOpen())

What is preventing the python script to get the serial data the second time? Why serial_port.inWaiting() is not seeing that there is data in the input buffer?

In the arduino sketch code I have added the line while(!Serial); in void setup()

EDIT: I post the code. This is the arduino code (I am testing on teensy so no initial handshaking is required as teensy will not reboot)

int sensor = 0;

void setup() {

void loop() {

  sensor = analogRead(0);

And this is the python code:

#!/usr/bin/env python3

import serial
import time
import signal
import sys

def keyboard_interrupt(signal, frame):

serial_port = serial.Serial("/dev/tty.usbmodem1225061", 115200)

signal.signal(signal.SIGINT, keyboard_interrupt)
signal.signal(signal.SIGTERM, keyboard_interrupt)

if serial_port.isOpen():
    print(serial_port.name + ' is open...')

while True:
    if (serial_port.inWaiting() > 0):
        sensor = serial_port.read().decode("utf-8") 
        print('sensor: "{}" '.format(sensor))

  • I discovered so far that in_waiting is returning 0 after the script is launched the second time. I can't figure out why. Any idea?
    – L_T
    May 26, 2019 at 19:38

1 Answer 1


Serial communication is always terminated with a specific character (usually \n). The receiving device will accumulate the incoming bytes in a buffer until it receives this terminating character.

To fix the problem simply change the last line of your arduino loop to Serial.println(sensor);

Also note that the arduino might be faster than your python script, meaning that you will get several sensor values per readout. If that's the case you have to add a short delay in the arduino loop.

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