get data from arduino using python

Question

I am trying to read potential meter data from Arduino using python, with the program on Arduino as follows :

#include <cvzone.h> SerialData serialData; int sendVals[2]; void setup() { serialData.begin(9600); } void loop() { int potVal = analogRead(A0); sendVals[0]= potVal; serialData.Send(sendVals); }

the program on the arduino side is running fine

and program in python as follows

from cvzone.SerialModule import SerialObject arduino = SerialObject("COM7") while True: data = arduino.getData() print(data[0])

but I get this error:

Traceback (most recent call last): data = arduino.getData() File "C:...\site-packages\cvzone\SerialModule.py", line 68, in getData data = data.decode("utf-8") UnicodeDecodeError: 'utf-8' codec can't decode byte 0xf7 in position 0: invalid start byte

How to solve it?

the new USB connection resets the Mega and it spends some time in bootloader waiting for upload. — Juraj
– Juraj ♦, Commented Dec 6, 2021 at 13:32
Maybe add some delay on the Arduino after initializing the serialData in startup() before it starts sending data in the loop(). — Dave X
– Dave X, Commented Dec 6, 2021 at 14:45

Sim Son · Accepted Answer · 2021-12-06 13:35:30Z

I would embed the call to .getData() into a try block and handle this specific exception with except UnicodeDecodeError. There can always be a transmission error, especially when the script is started.

However, I would do it differently:

void loop() { Serial.println(analogRead(A0)); }

...and on the python side use serial.readline() from pyserial instead. But I don't know cvzone and you might have a reason to use this instead of the commonly used pyserial.

Greenonline · Accepted Answer · 2022-09-09 07:47:32Z

There seems that there may be an issue with the implementation of getData() in SerialModule.py.

From line 62

 def getData(self): """ :param numOfVals: number of vals to retrieve :return: list of data received """ data = self.ser.readline() data = data.decode("utf-8") data = data.split('#') dataList = [] [dataList.append(d) for d in data] return dataList[:-1]

The block of code containing the readLine() should probably be with in a try...except block, like so:

 def getData(self): """ :param numOfVals: number of vals to retrieve :return: list of data received """ try: data = self.ser.readline() data = data.decode("utf-8") data = data.split('#') dataList = [] [dataList.append(d) for d in data] return dataList[:-1] except Exception as e: print(e) self.ser.close

An issue has been raised on Github: SerialModule.py - readLine() should be within try block. #49

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