[Tutor] Event loop logic question

[avi.e.gross at gmail.com]

Phil,

I am not familiar with the package you are using and have to assume you have some code somewhere not shown that binds your functions to the task as event handlers. 

But can multiple functions be called and active at the same time. I do not see any kind of lock in place such as a semaphore that stops two or more changes to the global variables as might happen if an arrow key is held down or whatever.

And your on_key_up() function does not make stop_flag global and so any changes it makes are to a local variable that immediately disappears. 

It is not my place to mention that this may be an advanced question and someone may want to direct you to resources showing the care that must be taken when a global variable is being handled by multiple threads. Ditto for forward_flag in on_key_down()

-----Original Message-----
From: Tutor <tutor-bounces+avi.e.gross=gmail.com at python.org> On Behalf Of Phil
Sent: Saturday, June 18, 2022 1:05 AM
To: tutor at python.org
Subject: [Tutor] Event loop logic question

Thank you for reading this.

What I'm trying to achieve here is have the word 'stop' printed once whenever no keys are pressed and 'forward' printed once even when the up arrow key is held down.

I don't think this question needs any knowledge of Pygame Zero to solve and it's quite likely that I've  misunderstood the logic required. The following code is my most recent version which seems to work the first time through the loop but fails after subsequent 'up' key presses. 
Debugging the event loop is difficult.

Any hints or alternative methods will be, as always, greatly appreciated.

By the way, this project sends commands to a controller which is easily overwhelmed if too many commands are received per second.

import pgzrun


WIDTH = 20
HEIGHT = 20

direction = 'stop'
stop_flag = True
forward_flag = True

def on_key_down(key):
     global direction

     if key == keys.UP:
         print('up pressed')
         direction = 'forward'
         forward_flag = True
         print('forward_flag ', forward_flag)

     elif key == keys.SPACE:
         direction = 'quit'

def on_key_up(key):
     global direction

     #print('stop')
     direction = 'stop'
     stop_flag = True

def update():
     global direction, stop_flag, forward_flag

     if direction == 'stop' and stop_flag:
         print('stop')
         stop_flag = False

     elif direction == 'forward' and forward_flag:
         print('forward')
         forward_flag = False
         stop_flag = True

     elif direction == 'quit':
         quit()

pgzrun.go()

--
Regards,
Phil

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