I am transmitting a JSON payload from an Arduino microcontroller and attempting to receive it using a Python script:
import bluetooth #pybluez
import json
sensor_address = "18:D9:31:YY:C7:4A"
socket = bluetooth.BluetoothSocket(bluetooth.RFCOMM)
socket.connect((sensor_address, 1))
buffer = ""
print("Listening...")
while True:
data = socket.recv(1024)
buffer += str(data, encoding='ascii')
print(buffer) # used to check json payload
try:
data = json.loads(buffer)
print("Received:", data)
buffer = ""
except json.JSONDecodeError as e:
print(e)
continue
Examining the value stored in buffer before entering the try statement, I see what appears to be perfectly valid JSON:
{"a_x":957.5195,"a_y":-0.488281,"a_z":315.918,"g_x":-0.625954,"g_y":-1.305344}{"a_x":961.914,"a_y":-1.953125,"a_z":297.8516,"g_x":-2.816794,"g_y":2.572519}{"a_x":964.8437,"a_y":3.417969,"a_z":303.2227,"g_x":-1,"g_y":0.374046}
However the result of my script is only Expecting value: line 1 column 1 (char 0) repeatedly.
Why doesn't the code inside the try statement execute once a complete payload has been received?
My hunch is that at no time does a valid JSON payload appear in buffer, but instead valid payloads appear along with incomplete payloads.
Is it possible to use a regular expression to extract a valid payload from a string?
The data in buffer is not valid JSON so that is why you are seeing the error.
The buffer seems to have the information in the format of a Python dictionary so you could use Python re module to extract the dictionary and then use ast.literal_eval to turn the string in to a Python dictionary.
In the example below I've mocked the reading of the socket as I don't have your device.
import re
pattern = re.compile(r'{.*?}')
class socket:
"""Mock reading data from socket"""
pointer = 3
chunk_size = 8
feed = (b'{"a_x":957.5195,"a_y":-0.488281,"a_z":315.918,"g_x":-0.625954,"g_y":-1.305344}'
b'{"a_x":961.914,"a_y":-1.953125,"a_z":297.8516,"g_x":-2.816794,"g_y":2.572519}'
b'{"a_x":964.8437,"a_y":3.417969,"a_z":303.2227,"g_x":-1,"g_y":0.374046}')
@classmethod
def recv(cls):
data = cls.feed[cls.pointer:cls.pointer + cls.chunk_size]
cls.pointer += cls.chunk_size
return data
def process_reading(buffer):
match = re.search(pattern, buffer)
start_idx, end_idx = match.span()
reading = literal_eval(buffer[start_idx:end_idx])
buffer = buffer[end_idx:]
return buffer, reading
def main():
buffer = ''
data = True
while data:
data = socket.recv()
# print("Data:", data)
buffer += str(data, encoding='ascii')
print("Buffer contents", buffer)
if re.search(pattern, buffer):
buffer, measurement = process_reading(buffer)
print("\tMeasurement:", measurement.get('g_x'))
if __name__ == '__main__':
main()
This gave the following output:
Buffer contents _x":957.
Buffer contents _x":957.5195,"a_
Buffer contents _x":957.5195,"a_y":-0.48
Buffer contents _x":957.5195,"a_y":-0.488281,"a_
Buffer contents _x":957.5195,"a_y":-0.488281,"a_z":315.9
Buffer contents _x":957.5195,"a_y":-0.488281,"a_z":315.918,"g_x"
Buffer contents _x":957.5195,"a_y":-0.488281,"a_z":315.918,"g_x":-0.6259
Buffer contents _x":957.5195,"a_y":-0.488281,"a_z":315.918,"g_x":-0.625954,"g_y"
Buffer contents _x":957.5195,"a_y":-0.488281,"a_z":315.918,"g_x":-0.625954,"g_y":-1.3053
Buffer contents _x":957.5195,"a_y":-0.488281,"a_z":315.918,"g_x":-0.625954,"g_y":-1.305344}{"a_x
Buffer contents _x":957.5195,"a_y":-0.488281,"a_z":315.918,"g_x":-0.625954,"g_y":-1.305344}{"a_x":961.91
Buffer contents _x":957.5195,"a_y":-0.488281,"a_z":315.918,"g_x":-0.625954,"g_y":-1.305344}{"a_x":961.914,"a_y":
Buffer contents _x":957.5195,"a_y":-0.488281,"a_z":315.918,"g_x":-0.625954,"g_y":-1.305344}{"a_x":961.914,"a_y":-1.95312
Buffer contents _x":957.5195,"a_y":-0.488281,"a_z":315.918,"g_x":-0.625954,"g_y":-1.305344}{"a_x":961.914,"a_y":-1.953125,"a_z":
Buffer contents _x":957.5195,"a_y":-0.488281,"a_z":315.918,"g_x":-0.625954,"g_y":-1.305344}{"a_x":961.914,"a_y":-1.953125,"a_z":297.8516
Buffer contents _x":957.5195,"a_y":-0.488281,"a_z":315.918,"g_x":-0.625954,"g_y":-1.305344}{"a_x":961.914,"a_y":-1.953125,"a_z":297.8516,"g_x":-
Buffer contents _x":957.5195,"a_y":-0.488281,"a_z":315.918,"g_x":-0.625954,"g_y":-1.305344}{"a_x":961.914,"a_y":-1.953125,"a_z":297.8516,"g_x":-2.816794
Buffer contents _x":957.5195,"a_y":-0.488281,"a_z":315.918,"g_x":-0.625954,"g_y":-1.305344}{"a_x":961.914,"a_y":-1.953125,"a_z":297.8516,"g_x":-2.816794,"g_y":2
Buffer contents _x":957.5195,"a_y":-0.488281,"a_z":315.918,"g_x":-0.625954,"g_y":-1.305344}{"a_x":961.914,"a_y":-1.953125,"a_z":297.8516,"g_x":-2.816794,"g_y":2.572519}
Measurement: -2.816794
Buffer contents {"a_x":9
Buffer contents {"a_x":964.8437,
Buffer contents {"a_x":964.8437,"a_y":3.
Buffer contents {"a_x":964.8437,"a_y":3.417969,"
Buffer contents {"a_x":964.8437,"a_y":3.417969,"a_z":303
Buffer contents {"a_x":964.8437,"a_y":3.417969,"a_z":303.2227,"g
Buffer contents {"a_x":964.8437,"a_y":3.417969,"a_z":303.2227,"g_x":-1,"
Buffer contents {"a_x":964.8437,"a_y":3.417969,"a_z":303.2227,"g_x":-1,"g_y":0.3
Buffer contents {"a_x":964.8437,"a_y":3.417969,"a_z":303.2227,"g_x":-1,"g_y":0.374046}
Measurement: -1
Buffer contents