Python - scheduler blocks interrupt

Question

I am using a system based on a raspberry to control some stuff. At the moment I am just testing it with turning a led on and off.

My plan is: Press a button to open a valve. Press the button again to close it - but if the button is not pressed, close it after a set time. My current script is as follows: (I know that this will not turn the led off on the second press)

import RPi.GPIO as GPIO # Import Raspberry Pi GPIO library
import time,sched

s = sched.scheduler(time.time, time.sleep)

def button_callback(channel):
   print("Button was pushed!")
   print(time.time())
   GPIO.output(18,GPIO.HIGH)
   s.enter(10, 1, turnoff,argument='')
   s.run()

def turnoff():
   print "LED off"
   print(time.time())
   GPIO.output(18,GPIO.LOW)

btpin=22
ledpin=18

GPIO.setwarnings(False) 
GPIO.setmode(GPIO.BOARD)
GPIO.setup(btpin, GPIO.IN, pull_up_down=GPIO.PUD_DOWN)
GPIO.setup(ledpin,GPIO.OUT)
GPIO.add_event_detect(btpin,GPIO.RISING,callback=button_callback) 
message = input("Press enter to quit\n\n")
GPIO.cleanup()

If I press the button and then leaves things alone, the led will turn off after 10 secs. But, if I press the button again immideately, nothing happens before the scheduler has finished, then a new press is registered. I had expected that the scheduler was spun of in the background so that when I pressed the button again, the callback would have ran again so I would have gotten the "Button was pushed" message (and everything happening afterwards would not have had any effect as GPIO 18 already was high and the scheduled call to turnoff would have happened after turnoff already had run.

Is it possible to use the sched library to do what I want or do I have to use some other techniques? I know I can either do it the simple way, by looping looking for a pressed button rather than registering a callback, or probably a more complicated way by creating a new thread that will pull the GPIO down after a given time - but is there something I have not understood in sched or is there some other library that gives me what I want - a way to tell python do do something a bit in the future without interfering with what else is going on.

(I do not need a very accurate timing - and also, this is just a part of what I intend to make a more complex control system, I manage to do exactly what I want using an arduino, but that will limit the further development)

I've not used sched but I would try cancelling any queued events: docs.python.org/3/library/sched.html#sched.scheduler.cancel — 001
– 001, Commented May 1, 2019 at 15:03

MortenSickel · Accepted Answer · 2019-05-01 19:08:25Z

1

Thanks to the tip from @stovfl, I rewrote the first part of my code:

import time,threading

def button_callback(channel):
   pin=18
   print("Button was pushed!")
   print(time.time())
   GPIO.output(pin,GPIO.HIGH)
   t = threading.Timer(10.0, turnoff)
   t.start()

and it works just like I wanted

answered May 1, 2019 at 19:08

MortenSickel

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