Fade?

#!/usr/bin/python3

from PWM import Pwm
from time import sleep

# Use for PWM on /dev/bone/pwm/
# see https://pastebin.com/R70P1wAn

led = Pwm('/dev/bone/pwm/1/b', frequency=500, value=0)

def led_fade( to_value, seconds, steps=None ):
    if steps is None:
        steps = round( seconds * 20 )   # default to 20 steps per second

    from_value = led.value
    for i in range( 0, steps ):
        sleep( seconds / steps )
        led.value = from_value + (1+i) * (to_value-from_value) / steps

try:
    while True:
        led_fade( 1.0, 2 )
        led_fade( 0.0, 2 )
except KeyboardInterrupt:
    led.disable()