def swap_bytes(bits, bytesize=8):
    """
    Bits is a b'' object containing a binary representation. Assuming each
    bytesize bits constitute a bytes, perform a endianness byte swap. Example:

        >>> swap_bytes(b'00011011', 2)
        b'11100100'
    """
    i = 0
    l = len(bits)
    output = [b''] * (l // bytesize + 1)
    j = len(output) - 1
    while i < l:
        output[j] = bits[i:i + bytesize]
        i += bytesize
        j -= 1
    return b''.join(output)
