Pause main thread in Python

Question

I'm writing some code in Python so that it shows me info on LCD display when I press a button. This is part of my code:

GPIO.setmode(GPIO.BCM)
GPIO.setup(18, GPIO.IN, pull_up_down=GPIO.PUD_UP)

def action1(self):
   temp = "15"
   lcd.clear()
   lcd.message("  Temp. Est.: \n" + "    " + temp + " " + chr(223) + "C")
   time.sleep(5.0) 
   lcd.clear

GPIO.add_event_detect(18, GPIO.RISING,callback=action1, bouncetime=800) 

while True:
date = datetime.now().strftime('%m-%d-%Y %H:%M')
lcd.message(date)
time.sleep(5.0)
lcd.clear()

This code is working but when I press the button it shows me the temperature, then the time and the temperature again (it depends on when I press the button). I've read that "GPIO.add_event_detect" runs a second thread for callback functions and it does not pause the main thread. I would like that after the button is pressed it stays on the LCD whenever i push the button and then it starts the code from bottom, in my case with the time. How may I achieve it? Thank you!

Answer 1

Multithreading is full of counter-intuitive surprises. To avoid them, I'd rather avoid updating the display from two different threads.

Instead, I'd rather send a message from the handler thread, or at least updated a shared flag from it.

The main loop would run faster and react on the updates (a rough sketch):

desired_state = None  # the shared flag, updated from key press handler.
current_state = 'show_date'  # what we used to display
TICK = 0.1  # we check the state every 100 ms.
MAX_TICKS = 50  # we display a value for up to 5 seconds.
ticks_elapsed = 0
while True:
  if desired_state != current_state:
    # State changed, start displaying a new thing.
    current_state = desired_state
    ticks_elapsed = 0
    if desired_state == 'show_date':
      put_date_on_lcd()
    elif desired_state == 'show_temp':
      put_temp_on_lcd()
    elif desired_state == 'blank':
      clear_lcd()
    else:  # unknown state, we have an error in the program.
       signal_error('What? %s' % desired_state)
  # Advance the clock.
  time.sleep(TICK)
  ticks_elapsed += 1
  if ticks_elapsed >= MAX_TICKS:
    # Switch display to either date or blank.
    desired_state = 'show_date' if current_state == 'blank' else 'blank' 
    ticks_elapsed = 0

Disclaimer: I did not test this code.