How to wait until signal

Question

I want make a while loop to wait and continue only when it receives signal

For example in while.py

        while queue:
             #wait until signal
             #bla bla

And when I hit button on my flask server it should send signal to this loop to continue: in main.py

         def alarm():
             #trigger loop to continue

Is there a way to do it?

Answer 1

You need to understand the simple producer-consumer example, (see source)

from threading import Thread, Lock
import time
import random

queue = []
lock = Lock()

class ProducerThread(Thread):
    def run(self):
        nums = range(5) #Will create the list [0, 1, 2, 3, 4]
        global queue
        while True:
            num = random.choice(nums) #Selects a random number from list [0, 1, 2, 3, 4]
            lock.acquire()
            queue.append(num)
            print "Produced", num 
            lock.release()
            time.sleep(random.random())


class ConsumerThread(Thread):
    def run(self):
        global queue
        while True:
            lock.acquire()
            if not queue:
                print "Nothing in queue, but consumer will try to consume"
            num = queue.pop(0)
            print "Consumed", num 
            lock.release()
            time.sleep(random.random())


ProducerThread().start()
ConsumerThread().start()

Explanation:

We started one producer thread (hereafter referred as producer) and one consumer thread (hereafter referred as consumer). The producer keeps adding to the queue while consumer keeps on removing from the queue. Since queue is a shared variable, we keep it inside the lock to avoid a race condition.

At some point, the consumer has consumed everything and the producer is still sleeping. The consumer tries to consume more but since queue is empty, an IndexError is raised. But on every execution, before the IndexError is raised you will see the print statement for "Nothing in queue, but consumer will try to consume", which explains why you are getting the error.