Python and Arduino Serial, decoding issue

Question

I have a simple python script that is listening to my serial port on which my arduino send strings.

import serial

ser = serial.Serial("/dev/ttyACM0", 9600, timeout=0.5)

while True:    
    print (str(ser.readline())

The connection is etablished, but I can't compare the readed line with a string, as the line comes with unwanted caracters : [value]/r/n

I want to get rid of these caracters. I've tried the following :

ser.readline().decode().strip('\r\n')

It's working fine.. until python read an unknown caracter that it is not able to decode :

0
0
0
Traceback (most recent call last):
  File "/home/testserial.py", line 6, in <module>
    value = ser.readline().decode().strip('\r\n')
UnicodeDecodeError: 'utf-8' codec can't decode byte 0x8d in position 1: invalid start byte

I don't understand why this caracter is there. My arduino program only send bolean 0 or 1

Do you guys have any ideas ?

how often does this error happen? can you post code from the arduino side? it could be that the data is getting corrupted between the arduino and the pc, it could be that your arduino code is the problem. you could choose to just ignore that error and carry on? — James Kent
– James Kent, Commented Oct 31, 2017 at 10:52
@JamesKent I would like to ignore it indeed. But i have no clue on how to do so. The unwanted caracter seems to appear very rarely, and I can't find a link between it and my arduino program. — B.T
– B.T, Commented Oct 31, 2017 at 10:56
Why are you trying to decode it as UTF-8, if you are just sending 0 and 1? — gre_gor
– gre_gor, Commented Oct 31, 2017 at 20:17

James Kent · Accepted Answer · 2017-10-31 10:59:10Z

3

to ignore the error:

import serial

ser = serial.Serial("/dev/ttyACM0", 9600, timeout=0.5)

while True:
    try:
        print (str(ser.readline())
    except UnicodeDecodeError: # catch error and ignore it
        print('uh oh')

note that its usually better to try and find the source of the error and fix it, but if an occasional missed value is acceptable then this will do the job.

answered Oct 31, 2017 at 10:59

James Kent

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1 Comment

B.T Over a year ago

Thank you. I'm going to look for the arduino programm and will post it there later on.

Max · Accepted Answer · 2023-06-16 06:34:32Z

I think there is more to this than @JamesKent Arduino code, I had the same problem with tried and tested C code running on an XMC talking to a Pi where the Pi would send a request as an ASCII string and the XMC would respond with a fixed number of \n terminated strings of data. Rarely, (between 30 seconds and many hours) I would get the dreaded decode error. I ended up fixing it by doing this for the case that data ends up being an empty string which is essentially the same thing:

try:
    data = ser.readline().decode().strip()
except UnicodeDecodeError:  # catch serial data errors
    print('Serial Error\n')

if data:
    ..Do other stuff with data

Strangely enough the error hasn't reoccurred (yet)

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