How can I find the big endian key in a message?

Question

I am trying to read a binary message from an ESP32 using a broker; i wrote a phyton script where I subscribe the topic. the message that i actually receive is:

b'\x00\x00\x00?'

this is a float binary little endian message but I don't the key to decode it. Is there a way to find the decode key based on this data? This is my python code:

    import paho.mqtt.client as mqtt


def on_connect1(client1,  userdata1, flags1, rc1):
    
    client1.subscribe("ESP32DevKit123/mytopic")

def on_message1(client1, userdata1, msg1):
    print(msg1.topic+" "+ "TESTENZA: "+str(msg1.payload))

client1 = mqtt.Client()

client1.username_pw_set(username="myuser",password="mypassword")

client1.on_connect = on_connect1

client1.on_message = on_message1

client1.connect("linkclient", portnumber, 60)


def twosComplement_hex(hexval):
    bits = 16 # Number of bits in a hexadecimal number format
    on_message1 = int(hexval, bits)
    if on_message1 & (1 << (bits-1)):
    on_message1 -= 1 << bits
    return on_message1


client1.loop_forever()

It also gives me an error in the line on_message1 -= 1 << bits; the error says: Expected intended block pylance. Any solutions?

Answer 1

The data you provided is b'\x00\x00\x00?' - I'm going to assume that this is 0000003f (please output hex with msg1.payload.hex()).

I'll also assume that by "float binary little endian" you mean a big endian floating point (IEE754) - note that this does not match up with the algorithm you are using (twos compliment). Plugging this input into an online tool indicates that the expected result ("Float - Big Endian (ABCD)") is 8.82818e-44 (it's worth checking with this tool; sometimes the encoding may not be what you think it is!).

Lets unpack this using python (see the struct docs for more information):

>>> from struct import unpack
>>> unpack('>f', b'\x00\x00\x00\x3f')[0]
8.828180325246348e-44

Notes:

• The [0] is there because unpack returns an array (you can unpack more than one item from the input)
• >f - the > means big-endian and the f float (standard size = 4 bytes)

The reason your original code gives the error "Expected intended block" is due to the lack of indentation in the line on_message1 -= 1 << bits (as it follows an if it needs to be indented). The algorithm does not appear relevant to your task (but there may be details I'm missing).