Getting the Output of (ERROR: sequence item 0: expected str instance, bytes found) as a result of my function

binascii.unhexlify returns a bytes object in Python 3, so the .join should also use a bytes string:

from binascii import unhexlify

def swap_endian_words(hex_words):
  '''Swaps the endianness of a hexidecimal string of words and converts to binary string.'''
  message = unhexlify(hex_words)
  if len(message) % 4 != 0: raise ValueError('Must be 4-byte word aligned')
  return b''.join(([ message[4 * i: 4 * i + 4][::-1] for i in range(0, len(message) // 4) ]))
  #      ^ here

print(swap_endian_words('aabbccdd'))

Output:

b'\xdd\xcc\xbb\xaa'

Here's a faster version using array.array:

import array

def swap_endian_words(hex_words):
  '''Swaps the endianness of a hexadecimal string of words and converts to binary string.'''
  # 'L' is unsigned 32-bit integer item type
  arr = array.array('L', bytes.fromhex(hex_words))
  arr.byteswap()  # may want the final 32-bit integers
  return arr.tobytes() # or dump back as bytes

print(swap_endian_words('00112233445566778899aabbccddeeff').hex(' '))

Output:

33 22 11 00 77 66 55 44 bb aa 99 88 ff ee dd cc