from machine import Pin
import utime

gpio18 = Pin(18,Pin.OUT)
gpio19 = Pin(19,Pin.OUT)
gpio20 = Pin(20,Pin.OUT)
gpio21 = Pin(21,Pin.OUT)

a = [gpio18,gpio19,gpio20,gpio21]

while True:
    for k in a:
        k.value(1)
        ultime.sleep(0.5)
        k.value(0)