rising edge of detection

[peter14]

Hello, i do an other code who works, i just have a problem whith the lcd. When he displays a big number an after i have an other smaller number, the lcd keep figures that exess. So i want that the first linechange all the number. Can you help me?

# coding: utf-8
import RPi.GPIO as GPIO
import lcddriver
import time
 
GPIO.setmode(GPIO.BCM)
GPIO.setup(23, GPIO.IN)
 
lcd = lcddriver.lcd()
start_time = time.time()
 
def effacer_ligne():
    lcd.lcd_display_string("                ",1)
 
GPIO.add_event_detect(23, GPIO.RISING)
def delta_angle():
    counter = 0
    base_temps=time.time()
    delta_temps=0
    while delta_temps<1 :
        if GPIO.event_detected(23):
            counter +=1
        delta_temps=(time.time())-(base_temps)
    return(counter)
while 1:
    start_time=time.time()#definition base des temps
    vitesse = delta_angle()/((time.time())-(start_time))#delta angle / delta temps
    vitesse2 = ((vitesse/128)*258)
    vitesse3 = "%.0f"% vitesse2
    lcd.lcd_display_string("     "+(vitesse3),1)
    lcd.lcd_display_string("     cm/min",2)
    #time.sleep(1)
    #effacer_ligne()

thank you