return a string that contains `i`, left-padded with 0's up to PAD_LEN digits

def paddedInt(i):
    '''
    return a string that contains `i`, left-padded with 0's up to PAD_LEN digits
    '''
    i_str = str(i)
    pad = PAD_LEN - len(i_str)
    return (pad * "0") + i_str