Error reading Struct in python on Raspberry pi 3b+ sent from Arduino using NRF24L01+

Question

I've been struggling to send sensor values from arduino to raspberry pi 3b+. I'm using NRF24L01+ module to communicate. I'm trying to send accelerometer values (double type) from arduino to raspberry pi. A part of Arduino code that's sending values:

typedef struct {
  double rollx;
  double pitchy;
}myStruct;

myStruct duomenys;

  duomenys.rollx = kalAngleX;
  duomenys.pitchy = kalAngleY;
  radio.write(&duomenys,sizeof(duomenys));

  Serial.print("Roll: ");
  Serial.println(duomenys.rollx);
  Serial.print("Pitch: ");
  Serial.println(duomenys.pitchy);
  Serial.print("\t");

Here's the arduino serial monitor output:

Pitch: -12.98
    Roll: 89.85
Pitch: -12.97
    Roll: 89.85
Pitch: -12.96
    Roll: 89.86

However on the raspberry side, I'm not able to Unpack the received structure (Note: I'm pretty new to python). What I've tried is:

while True:

    while not radio.available(0):
      ##  print("Nepareina")
        time.sleep(1)

    duomenys = []

    radio.read(duomenys, radio.getDynamicPayloadSize())
    data = struct.unpack('ff',duomenys)
    rollx = data [0]
    pitchy = data[1]
    print(rollx)
    print("                              ")
    print(pitchy)

When compiling this code, I'm getting an error:

Traceback (most recent call last):
  File "/home/pi/Desktop/NRF24L01/receiveArduino.py", line 41, in <module>
    data = struct.unpack('ff',duomenys)
TypeError: a bytes-like object is required, not 'list'

If I change the line

data = struct.unpack('ff',duomenys)

to

data = struct.unpack('ff',bytes(duomenys))

I get an error:

Traceback (most recent call last):
  File "/home/pi/Desktop/NRF24L01/receiveArduino.py", line 41, in <module>
    data = struct.unpack('ff',bytes(duomenys))
struct.error: unpack requires a bytes object of length 8

If anyone has any suggestions on how to read the received struct in python, feel free to share.

EDIT: Updated the arduino serial monitor output. Previously had posted the wrong output.

EDIT: This is the NRF24L01library I'm using. https://github.com/matmany/lib_nrf24

Answer 1

Hey I do not know if you solved it or not. I was having the same issue and I managed to solve it like this:

Instead of declaring duomenys = [] ... try duomenys = bytearray(nbBytesData), where nbBytesData should be the number of bytes you are expecting (I'm assuming 8 bytes)

Then the data = struct.unpack('ff',duomenys) should work.