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?

• Juraj: the new USB connection resets the Mega and it spends some time in bootloader waiting for upload.
• Dave X: Maybe add some delay on the Arduino after initializing the serialData in startup() before it starts sending data in the loop().

Answer 1

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.

Answer 2

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