Python 3 garbage data into the struct.unpack

I am trying to send and receive messages via socket using Python 3.

BUFFER_SIZE = 1024
# Create message
MLI = struct.pack("!I", len(MESSAGE))
MLI_MESSAGE = MLI + str.encode(MESSAGE)

When the message receive:

s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
s.connect((TCP_IP, TCP_PORT))
s.send(MLI_MESSAGE)
print ("Sent data: ‘", MESSAGE, "’")
# Receive MLI from response (you might want to add some timeout handling as well
resp = s.recv(BUFFER_SIZE)
resp = struct.unpack("!I", resp)[0]
print(resp)

resp:

b'\x00\t\xeb\x07\xdf\x01\x00\xdf\x02\x010'

I am getting that error:

struct.error: unpack requires a buffer of 4 bytes

I think it is related with \t char into the resp but I am not sure. How can I remove that \t char and how to solve that issue?

You are basically trying to do the following (sockets removed):

1 import struct
2 
3 msg = "foobar"
4 mli = struct.pack("!I", len(msg))
5 mli_msg = mli + str.encode(msg)
6 
7 len = struct.unpack("!I", mli_msg)[0]
8 print(len)

The extraction of the length in line 7 will fail since you put the whole mli_msg as argument to unpack, not only the expected 4 bytes for the len. Instead you should do:

7 len = struct.unpack("!I", mli_msg[:4])[0] 

Apart from that it is wrong to first take the length of the message and then convert the message to bytes. The first takes the number of characters while the latter takes the number of bytes, which will differ when non-ASCII characters are involved - check len("ü") vs. len(str.encode("ü")). You need to first convert the message to bytes thus and then take the length to provide the correct byte length for what you send.

4 encoded_msg = str.encode(msg)
5 mli_msg = struct.pack("!I", len(encoded_msg))  + encoded_msg