inhibit buttons whilst running script?

Question

Hi there I'm trying to use my Raspberry Pi 1B and PiFace to run a little interactive museum exhibition.

Process is Pi is in standby, Standby.py runs from rc.local Relay 1 is closed as this is the main light in the room,

Button Press (SW1) initiates the show

GoShow.py runs, this closes a number of replays which change the lights in the space and trigger a media player for the video the Pi plays a audio file.

I want to inhibit the button when GoShow is active, I've tried to 'de-listen' under the GoShow.py but no luck.

https://github.com/Brewj/Event-Trigger

Anyone any ideas?

Cheers

Brew

jack.chaney56 · Answer

Seems the thing you are looking for is what is called a semaphore. Which is an OS feature. Simply put, a semaphore is a switch that is tested for state, prior to an activity. If the state is available, the task runs, if the state is not available, the task sleeps until the state becomes available. The OS usually manages semaphore operation by putting a task in a "sleep" state when a semaphore is not available, and waking the task back up when the state is changed. For your case, it could be something as simple as a flag that is examined in the interrupt (SW1 input). If the switch was off, it turns it on, and starts the presentation, if the switch is on, the event is ignored. The presentation routine sets the switch off upon completion. A "counter" can also be implemented for queued SW1 inputs, so the presentation will repeat if more than one SW1 input is received, so the presentation can loop instead of shutting off. Hope this helps, Jack

shabaz · Answer

As mentioned, you could just check the process list. This worked, could be adapted to do what you want.. import psutil
p=psutil.pids()
found=0
for i, elem in enumerate(p):
 process=psutil.Process(elem)
 pname=process.name()
 print "process name %s " % pname
 if (pname=="vi"):
 found = 1
 break
print "found = %d " % found Replace the text in line 8 with the name of your program (e.g. goshow). I tested it with 'vi'. I did this on a PC, not a Pi, so I've no idea if it will work. I needed to do 'pip install psutil' first. If that works, then in your toggle_led0 function, use a routine (perhaps call it goshow_found() similar to that. Pseudocode (because I'm not familiar enough with Python to do it properly): if goshow_found()==1:
// do nothing
else
subprocess.call("/home/pi/goshow.sh", shell=True)

rew · Answer

Premature optimization is the source of all evil!

The idea of a "pi" is that you can easily do a lot of things "the easy way". So the file based semaphore is a great way to implement the locking that James wants.

So, James, it is worth knowing that the computer-nerds call what you are asking a "semaphore". Feel free to forget that word if you want. This can be programmed efficiently with the system calls that are available. But another way, which is probably MUCH easier for you to implement is to simply us a file. Remember to remove that file when the system boots. Otherwise, at one point in time someone may shut down the system with the presentation running and then the whole system would become unresponsive....

shabaz · Answer

I couldn't follow the entire thread, but generally one trick is to just change the problem if there really is no other easy way out. Not looking into the details of it since I'm worse than useless at Python, and now it looks like the high-level API you're using is spitting out lower-level library related messages, another option would have been to revert to the method mentioned earlier of just checking the process list before calling your player. But your code has changed significantly now. Alternatively, if you now want to keep close to the current code you have instead of doing a deactivate, you could change the problem by electrically deactivating the switch. For instance, instead of connecting the switch to 0V, you could connect it to another GPIO pin, which is set to logic low when you want the switch activated, and set to logic high when you want the switch deactivated. It is an ugly hack, but it is an option.

clem57 · Answer

Reading and Writing Files in Python all you need. I will return shortly. Clem

mconners · Answer

Technically, what you are suggesting is a file based semaphore. Although historically these were not recommended due to overhead, this is probably not as concerning these days, but performance should always be a consideration.

Both python 2 and python 3 have support for semaphores in the threading and or asyncio libraries respectively.

Mike

brew · Answer

Hi, so got it all in had to adjust some of the indents but this is where I'm at #!/usr/bin/python 
import pifacedigitalio 
import subprocess 
from time import sleep 
 
pfd = pifacedigitalio.PiFaceDigital()
pifacedigital = pifacedigitalio.PiFaceDigital() 
listener = pifacedigitalio.InputEventListener(chip=pifacedigital)
listener.register(0, pifacedigitalio.IODIR_FALLING_EDGE, toggle_led0)
listener.activate() 
 
pfd.leds[5].turn_on() #Room light ON 
 
def toggle_led0(event): 
 listener.deactivate() #Inhibit button
 pfd.leds[1].turn_on() #On Air and Cabinet ON 
 sleep (2) #Delay for lights 
 pfd.leds[0].turn_off() #Room light OFF 
 sleep (1) 
 os.system("aplay /home/pi/withy_edit.wav") #Audio GO 
 sleep (3) #End Delay 
 pfd.leds[0].turn_on() #Room light ON 
 sleep (1) 
 pfd.leds[1].turn_off() 
 listener.activate() #re arm button 
 
if __name__ == "__main__": 
 
 while True: 
 pass however I get the following error now pi@raspberrypi : ~ $ python goshow2.py Traceback (most recent call last): File "goshow2.py", line 9, in <module> listener.register(0, pifacedigitalio.IODIR_FALLING_EDGE, toggle_led0) NameError: name 'toggle_led0' is not defined