CODEWITHSUNDEEP
pythonpyserial
John Sly
John Sly

Reputation: 769

Python's serial.readline is not receiving my entire line

I'm having a problem with a block of Python code reading in a string from an Arduino connected over USB. I understand that serial doesn't know what a string is or care. I'm using serial.readline, which from the documentation sounds like the perfect match, but my string isn't always complete. The weird problem is, the string doesn't always have the front of the string, but it always has the end of the string. I'm really lost on this and I'm sure it's just my lack of understanding about the nuances of reading serial data or how Python handles it.

In the code below, I loop through the serial interfaces until I find the one I'm looking for. I flush the input and give it a sleep for a couple seconds to make sure it has time to get a new read.

arduinoTemp = serial.Serial(iface, 9600, timeout=1)
arduinoTemp.flushInput()
arduinoTemp.flushOutput()
arduinoTemp.write("status\r\n".encode())
time.sleep(2)
read = arduinoTemp.readline().strip()
if read != "":
    #check the string to make sure it's what I'm expecting.

I'm sending the string in JSON.

I'm expecting something in line with this:

 {"id": "env monitor","distance": {"forward": {"num":"0","unit": "inches"}},"humidity": {"num":"0.00","unit": "%"},"temp": {"num":"0.00","unit": "fahrenheit"},"heatIndex": {"num":"0.00","unit": "fahrenheit"}}

I might get something back like this:

 ": t": "%"},"temp": {"num":"69.80","unit": "fahrenheit"},"heatIndex": {"num":"68.13","unit": "fahrenheit"}}

or this:

atIndex": {"num":"0.00","unit": "fahrenheit"}}

At first I thought it was the length of the string that might be causing some issues, but the cut off isn't always consistent, and since it has the end of the string, it stands to reason that it should have gotten everything before that.

I've verified that my Arduino is broadcasting correctly by interfacing with it directly and the Arduino IDE and serial monitor. This is definitely an issue with my Python code.

Upvotes: 1

Views: 3225

Answers (1)

Roland Smith
Roland Smith

Reputation: 43533

In (serial) communications you should always expect to receive partial answers.

A usual solution in this case is to add whatever you read from the serial to a string/buffer until you can parse it successfully with json.loads.

import serial
import json
import time

ser = serial.Serial('/dev/ttyACM0', 9600)

buffer = ''
while True:
    buffer += ser.read()
    try:
        data = json.loads(buffer)
        print(data)
        buffer = ''
    except json.JSONDecodeError:
        time.sleep(1)

(From this answer).

Note that if you flush, you will lose data!

Also note that this is a somewhat simplified solution. Ideally the buffer should be reset to whatever remains after the successful parse. But as far as I know, the json module doesn't offer that functionality.

Upvotes: 2

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