How to get error to output?

tach_7 = data[7]
IndexError : string index out of range

import serial
import time
from binascii import hexlify
chuoi="\xAA\x00\x03\x25\x26\x00\x00\xBB"
serial=serial.Serial("/dev/ttyAMA0",
                     baudrate=9600,
                     stopbits=serial.STOPBITS_ONE,
                     parity=serial.PARITY_NONE,
                     bytesize=serial.EIGHTBITS,
                     timeout=0.1)
ID_rong = 128187
def RFID(str): 
        serial.write(chuoi)
        data=serial.readline()
        tach_5 = data[5]
        tach_6 = data[6]
        hex_5=hexlify(tach_5)
        hex_6=hexlify(tach_6)
        num_5 = int(hex_5,16)
        num_6 = int(hex_6,16)
        num_a = num_5 * 1000 + num_6
        if(num_a != ID_rong):
                tach_7 = data[7]
                tach_8 = data[8]
                hex_7=hexlify(tach_7)
                hex_8=hexlify(tach_8)
                num_7 = int(hex_7,16)
                num_8 = int(hex_8,16)
                num= num_7 * 1000 + num_8 + num_6*1000000 + num_5 * 1000000000
        else:
                num = num_5 * 1000 + num_6
        return num
def check_read():
    if( 'string index out of range' ?????):
        return 1
    else:
        return 0
while 1:
    num =RFID(chuoi)
    print num
    x = check_read()
    if (x==1):
        #restart read RFID