STM32 uart communication

#!/usr/bin/env python3

import serial
import time
import itertools
import operator

from binascii import hexlify

NACK = b'\x1f'
ACK = b'\x79'

def hex_spaced(bin_str):
    return " ".join(hex(b) for b in bin_str)

def select_UART(ser):
    ser.write(b"\x7f") # Select UART
    ack = ser.read()          # read one byte
    print("(N)ACK: ",ack)

    if ack == ACK:
        print("UART selection ACKed")
    else:
        raise RuntimeError("UART select failed")

def send_command(ser, cmd_bin, expected=None):
    #line = ser.readline()   # read a '\n' terminated line
    xor_byte = bytes(itertools.accumulate(cmd_bin, operator.xor))[-1:]
    print("XOR checksum:", xor_byte)
    ser.write(cmd_bin + xor_byte) # Get
    time.sleep(0.1)
    ack = ser.read(1)[0]
    if (ack == ord(ACK)):
        print("Command Acked")
    elif (ack == ord(NACK)):
        print("Command Nacked")
        raise RuntimeError("Command failed")
    else:
        print("dafuq!", ack)
        raise RuntimeError("Command failed")

    if expected is None:
        s = ser.read(ser.in_waiting)        # read available bytes
    else:
        s = ser.read(expected-1)

    return s

def send_command_print(ser, cmd_bin, expected=None):
    s = send_command(ser, cmd_bin, expected)
    print("Result: %d bytes, %s" % (len(s), hex_spaced(s)))

with serial.Serial('/dev/ttyACM0', 38400, timeout=None) as ser:
        select_UART(ser)

        try:
            s = send_command_print(ser, b"\x00\xff", 15)
            s = send_command_print(ser, b"\x01\xfe")
        except KeyboardInterrupt:
            print("interrupted")
            pass