如何陷入 for 循环？

Question

您好，我有一个问题，我需要在 for 循环中等待一段时间，直到布尔变量的值发生更改。我故意想在循环中等待。示例代码

check = True  

def change_check_value():
    global check
    ###
    after a while check changes to true
    ###

change_check_vale()  #running on a different thread

for i in range(0,10):
    print(i)
    check = False
    ## wait till check becomes true and continue the for loop

我想在 for 循环中等待，直到检查再次变为真。我尝试使用 while 循环，但无法实现该功能。 time.sleep() 无法使用，因为我不确定要等待多长时间。有人可以帮我弄这个吗？

谢谢。

Answer 1

尝试使用本文中提到的方法：Is there an easy way in Python to wait until certain condition is true?

import time

check = True

def wait_until():
  while true:
    if check == True: return


def change_check_value():
    global check
    ###
    after a while check changes to true
    ###

change_check_vale()  #running on a different thread

for i in range(0,10):
    print(i)
    check = False
    ## wait till check becomes true and continue the for loop
    wait_until()

希望对你有帮助。

Answer 2

您可以使用 Event 对象，它可以在 threading 和 asyncio 包下找到。

事件对象有一个 wait() 方法，调用它时代码不会继续，直到事件设置为真。 一旦事件被设置为True，代码将立即继续。

异步示例 (source)：

async def waiter(event):
    print('waiting for it ...')
    await event.wait()
    print('... got it!')

async def main():
    # Create an Event object.
    event = asyncio.Event()

    # Spawn a Task to wait until 'event' is set.
    waiter_task = asyncio.create_task(waiter(event))

    # Sleep for 1 second and set the event.
    await asyncio.sleep(1)
    event.set()

    # Wait until the waiter task is finished.
    await waiter_task

asyncio.run(main())

线程示例 (source)：

import threading
import time
import logging

logging.basicConfig(level=logging.DEBUG,
                    format='(%(threadName)-9s) %(message)s',)

def wait_for_event(e):
    logging.debug('wait_for_event starting')
    event_is_set = e.wait()
    logging.debug('event set: %s', event_is_set)

def wait_for_event_timeout(e, t):
    while not e.isSet():
        logging.debug('wait_for_event_timeout starting')
        event_is_set = e.wait(t)
        logging.debug('event set: %s', event_is_set)
        if event_is_set:
            logging.debug('processing event')
        else:
            logging.debug('doing other things')

if __name__ == '__main__':
    e = threading.Event()
    t1 = threading.Thread(name='blocking', 
                      target=wait_for_event,
                      args=(e,))
    t1.start()

    t2 = threading.Thread(name='non-blocking', 
                      target=wait_for_event_timeout, 
                      args=(e, 2))
    t2.start()

    logging.debug('Waiting before calling Event.set()')
    time.sleep(3)
    e.set()
    logging.debug('Event is set')

Answer 3

这可以通过一种非常简单的方式完成：

from threading import Event, Thread

event = Event()


def wait_for_it(e):
    print('Waiting for something to happen.')
    e.wait()
    print('No longer waiting.')
    e.clear()
    print('It can happen again.')


def make_it_happen(e):
    print('Making it happen.')
    e.set()
    print('It happened.')


# create the threads
threads = [Thread(target=wait_for_it, args=(event,)), Thread(target=make_it_happen, args=(event,))]

# start them
for thread in threads:
    thread.start()

# make the main thread wait for the others to complete
for thread in threads:
    thread.join()