uPy stepper driver

from machine import Pin

a = Pin(4, Pin.OUT)
b = Pin(5, Pin.OUT)
c = Pin(12, Pin.OUT)
d = Pin(13, Pin.OUT)

steps = ((a, 1), (d, 0), (b, 1), (a,0), (c, 1), (b, 0), (d, 1), (c, 0))
s = 0
l = len(steps)

def step():
    global s
    st = steps[s]
    st[0].value(st[1])
    s += 1
    if s >= l:
        s = 0
    print(s)