Nicely formats hex strings

def _pretty_hex(hex_str):
    '''
    Nicely formats hex strings
    '''
    if len(hex_str) % 2 != 0:
        hex_str = '0' + hex_str
    return ':'.join(
        [hex_str[i:i + 2] for i in range(0, len(hex_str), 2)]).upper()