socket.recv() response varies in length

I have a normal TCP client connection which I use to retrieve data from a climate sensor (SCD30). In theory, it does what it is supposed to do, but I always get different numbers of bytes.

with socket.socket(socket.AF_INET, socket.SOCK_STREAM) as tcp:
 try:
  tcp.settimeout(self.timeout)
  tcp.connect((self.ip, self.port))
  tcp.settimeout(None)
  tcp.sendall(b"\x61\x03\x00\x20\x00\x01\x8c\x60")
  data = tcp.recv(self.bytes)
  tcp.close()

The correct response is:

b'a\x03\x02\x03B\xb8\x8d'
Bytes: 7

what else I received:

b'a'
Bytes: 1

b'a\x03'
Bytes: 2

b'\x02\x03B\xb8\x8da\x03\x02\x03B\xb8\x8d'
Bytes: 12

b'\x03\x02\x03B\xb8\x8d'
Bytes: 6

Solution

TCP is a byte stream, it has no concept of message boundaries. As such, recv() can return fewer bytes than requested. By default, it returns an arbitrary number of available bytes up to the buffer size you specify.

So, you will need to call recv() in a loop until you have received all of the bytes you are expecting, in this case 7 bytes total, eg:

with socket.socket(socket.AF_INET, socket.SOCK_STREAM) as tcp:
try:
  tcp.settimeout(self.timeout)
  tcp.connect((self.ip, self.port))
  tcp.settimeout(None)
  tcp.sendall(b"\x61\x03\x00\x20\x00\x01\x8c\x60")
  data = []
  while len(data) < 7:
    bytesRead = tcp.recv(7-len(data))
    if len(bytesRead) = 0:
      break
    data += bytesRead
  tcp.close()

Alternatively, if you are running your code on a 'Nix-based system, you can use the MSG_WAITALL flag when calling recv() instead of using a loop, eg:

with socket.socket(socket.AF_INET, socket.SOCK_STREAM) as tcp:
try:
  tcp.settimeout(self.timeout)
  tcp.connect((self.ip, self.port))
  tcp.settimeout(None)
  tcp.sendall(b"\x61\x03\x00\x20\x00\x01\x8c\x60")
  data = tcp.recv(7, socket.MSG_WAITALL)
  tcp.close()