[Tutor] Event loop logic question

[Phil]

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