问为什么我在python中不能使用mysql Insert命令？

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    def callback(channel):
    print("\nflame detected\n")

GPIO.add_event_detect(channel, GPIO.BOTH, bouncetime=300)#pin if HIGH or LOW
GPIO.add_event_callback(channel, callback)       


while True:

    db = MySQLdb.connect(host="localhost", user="root", passwd="password", db="results")
    #create a cursor for the select
    cur = db.cursor()
    result = instance.read()
    if result.is_valid():
        print("Last valid input: " + str(datetime.datetime.now()))
        print("Temperature: %d C" % result.temperature)
        print("Temperature: %d F" % ((result.temperature*9/5)+32))
        print("Humidity: %d %%" % result.humidity)

        cel = str(result.temperature)
        far = str((result.temperature*9/5)+32)
        hum = str(result.humidity)
        fla = "No Flame"




    #execute an sql query
    sql = "INSERT INTO res(celsius,fahrenheit,humidity,flame) VALUES('"+cel+"','"+far+"','"+hum+"','"+fla+"')"
    cur.execute(sql)

        # close the cursor
    cur.close()

    # close the connection
    db.close ()
    time.sleep(1)