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[英]Can threading.Event be used to protect variable access in Python?
[英]Python threading: can I sleep on two threading.Event()s simultaneously?
如果我有兩個threading.Event()
對象,並希望睡眠,直到其中任何一個設置為止,是否有一種有效的方法在python中執行此操作? 顯然,我可以使用輪詢/超時做一些事情,但我想讓線程一直處於休眠狀態,直到設置為一個,類似於如何將select
用於文件描述符。
所以在下面的實現中, wait_for_either
的高效非輪詢實現會是什么樣的?
a = threading.Event()
b = threading.Event()
wait_for_either(a, b)
這是一個非輪詢的非過度線程解決方案:修改現有的Event
以在它們發生更改時觸發回調,並處理在該回調中設置新事件:
import threading
def or_set(self):
self._set()
self.changed()
def or_clear(self):
self._clear()
self.changed()
def orify(e, changed_callback):
e._set = e.set
e._clear = e.clear
e.changed = changed_callback
e.set = lambda: or_set(e)
e.clear = lambda: or_clear(e)
def OrEvent(*events):
or_event = threading.Event()
def changed():
bools = [e.is_set() for e in events]
if any(bools):
or_event.set()
else:
or_event.clear()
for e in events:
orify(e, changed)
changed()
return or_event
樣品用法:
def wait_on(name, e):
print "Waiting on %s..." % (name,)
e.wait()
print "%s fired!" % (name,)
def test():
import time
e1 = threading.Event()
e2 = threading.Event()
or_e = OrEvent(e1, e2)
threading.Thread(target=wait_on, args=('e1', e1)).start()
time.sleep(0.05)
threading.Thread(target=wait_on, args=('e2', e2)).start()
time.sleep(0.05)
threading.Thread(target=wait_on, args=('or_e', or_e)).start()
time.sleep(0.05)
print "Firing e1 in 2 seconds..."
time.sleep(2)
e1.set()
time.sleep(0.05)
print "Firing e2 in 2 seconds..."
time.sleep(2)
e2.set()
time.sleep(0.05)
結果是:
Waiting on e1...
Waiting on e2...
Waiting on or_e...
Firing e1 in 2 seconds...
e1 fired!or_e fired!
Firing e2 in 2 seconds...
e2 fired!
這應該是線程安全的。 歡迎任何評論。
編輯:哦,這是你的wait_for_either
函數,雖然我編寫代碼的方式,最好制作和傳遞or_event
。 請注意,不應手動設置或清除or_event
。
def wait_for_either(e1, e2):
OrEvent(e1, e2).wait()
一種解決方案( 使用輪詢 )將在循環中對每個Event
執行順序等待
def wait_for_either(a, b):
while True:
if a.wait(tunable_timeout):
break
if b.wait(tunable_timeout):
break
我認為如果你足夠好地調整超時,結果就可以了。
我能想到的最好的非輪詢是在一個不同的線程中等待每一個,並設置一個共享的Event
,你將在主線程中等待。
def repeat_trigger(waiter, trigger):
waiter.wait()
trigger.set()
def wait_for_either(a, b):
trigger = threading.Event()
ta = threading.Thread(target=repeat_trigger, args=(a, trigger))
tb = threading.Thread(target=repeat_trigger, args=(b, trigger))
ta.start()
tb.start()
# Now do the union waiting
trigger.wait()
非常有趣,所以我寫了以前解決方案的OOP版本:
class EventUnion(object):
"""Register Event objects and wait for release when any of them is set"""
def __init__(self, ev_list=None):
self._trigger = Event()
if ev_list:
# Make a list of threads, one for each Event
self._t_list = [
Thread(target=self._triggerer, args=(ev, ))
for ev in ev_list
]
else:
self._t_list = []
def register(self, ev):
"""Register a new Event"""
self._t_list.append(Thread(target=self._triggerer, args=(ev, )))
def wait(self, timeout=None):
"""Start waiting until any one of the registred Event is set"""
# Start all the threads
map(lambda t: t.start(), self._t_list)
# Now do the union waiting
return self._trigger.wait(timeout)
def _triggerer(self, ev):
ev.wait()
self._trigger.set()
我認為標准庫為這個問題提供了一個非常規范的解決方案,我在這個問題中沒有看到: 條件變量 。 您的主線程在條件變量上等待,並在每次通知時輪詢事件集。 只有在其中一個事件更新時才會通知它,因此沒有浪費的輪詢。 這是一個Python 3示例:
from threading import Thread, Event, Condition
from time import sleep
from random import random
event1 = Event()
event2 = Event()
cond = Condition()
def thread_func(event, i):
delay = random()
print("Thread {} sleeping for {}s".format(i, delay))
sleep(delay)
event.set()
with cond:
cond.notify()
print("Thread {} done".format(i))
with cond:
Thread(target=thread_func, args=(event1, 1)).start()
Thread(target=thread_func, args=(event2, 2)).start()
print("Threads started")
while not (event1.is_set() or event2.is_set()):
print("Entering cond.wait")
cond.wait()
print("Exited cond.wait ({}, {})".format(event1.is_set(), event2.is_set()))
print("Main thread done")
示例輸出:
Thread 1 sleeping for 0.31569427100177794s
Thread 2 sleeping for 0.486548134317051s
Threads started
Entering cond.wait
Thread 1 done
Exited cond.wait (True, False)
Main thread done
Thread 2 done
請注意,沒有額外的線程或不必要的輪詢,您可以等待任意謂詞變為真(例如,對於要設置的事件的任何特定子集)。 還有一個wait_for
包裝器用於while (pred): cond.wait()
模式,它可以使您的代碼更容易閱讀。
啟動額外的線程似乎是一個明確的解決方案,但不是很有效。 函數wait_events將阻止util設置任何一個事件。
def wait_events(*events):
event_share = Event()
def set_event_share(event):
event.wait()
event.clear()
event_share.set()
for event in events:
Thread(target=set_event_share(event)).start()
event_share.wait()
wait_events(event1, event2, event3)
擴展Claudiu的答案,您可以等待:
from threading import Thread, Event, _Event
class ConditionalEvent(_Event):
def __init__(self, events_list, condition):
_Event.__init__(self)
self.event_list = events_list
self.condition = condition
for e in events_list:
self._setup(e, self._state_changed)
self._state_changed()
def _state_changed(self):
bools = [e.is_set() for e in self.event_list]
if self.condition == 'or':
if any(bools):
self.set()
else:
self.clear()
elif self.condition == 'and':
if all(bools):
self.set()
else:
self.clear()
def _custom_set(self,e):
e._set()
e._state_changed()
def _custom_clear(self,e):
e._clear()
e._state_changed()
def _setup(self, e, changed_callback):
e._set = e.set
e._clear = e.clear
e._state_changed = changed_callback
e.set = lambda: self._custom_set(e)
e.clear = lambda: self._custom_clear(e)
示例用法與以前非常相似
import time
e1 = Event()
e2 = Event()
# Example to wait for triggering of event 1 OR event 2
or_e = ConditionalEvent([e1, e2], 'or')
# Example to wait for triggering of event 1 AND event 2
and_e = ConditionalEvent([e1, e2], 'and')
這是一個老問題,但我希望這可以幫助來自Google的人。
接受的答案相當陳舊,將導致兩次“orified”事件的無限循環。
這是使用concurrent.futures
的實現
import concurrent.futures
from concurrent.futures import ThreadPoolExecutor
def wait_for_either(events, timeout=None, t_pool=None):
'''blocks untils one of the events gets set
PARAMETERS
events (list): list of threading.Event objects
timeout (float): timeout for events (used for polling)
t_pool (concurrent.futures.ThreadPoolExecutor): optional
'''
if any(event.is_set() for event in events):
# sanity check
pass
else:
t_pool = t_pool or ThreadPoolExecutor(max_workers=len(events))
tasks = []
for event in events:
tasks.append(t_pool.submit(event.wait))
concurrent.futures.wait(tasks, timeout=timeout, return_when='FIRST_COMPLETED')
# cleanup
for task in tasks:
try:
task.result(timeout=0)
except concurrent.futures.TimeoutError:
pass
測試功能
import threading
import time
from datetime import datetime, timedelta
def bomb(myevent, sleep_s):
'''set event after sleep_s seconds'''
with lock:
print('explodes in ', datetime.now() + timedelta(seconds=sleep_s))
time.sleep(sleep_s)
myevent.set()
with lock:
print('BOOM!')
lock = threading.RLock() # so prints don't get jumbled
a = threading.Event()
b = threading.Event()
t_pool = ThreadPoolExecutor(max_workers=2)
threading.Thread(target=bomb, args=(event1, 5), daemon=True).start()
threading.Thread(target=bomb, args=(event2, 120), daemon=True).start()
with lock:
print('1 second timeout, no ThreadPool', datetime.now())
wait_for_either([a, b], timeout=1)
with lock:
print('wait_event_or done', datetime.now())
print('=' * 15)
with lock:
print('wait for event1', datetime.now())
wait_for_either([a, b], t_pool=t_pool)
with lock:
print('wait_event_or done', datetime.now())
不漂亮,但你可以使用兩個額外的線程來復用事件......
def wait_for_either(a, b):
flag = False #some condition variable, event, or similar
class Event_Waiter(threading.Thread):
def __init__(self, event):
self.e = event
def run(self):
self.e.wait()
flag.set()
a_thread = Event_Waiter(a)
b_thread = Event_Waiter(b)
a.start()
b.start()
flag.wait()
請注意,如果事件太快到達,您可能不得不擔心意外得到兩個事件。 輔助線程(a_thread和b_thread)應該在嘗試設置標志時鎖定同步,然后應該殺死另一個線程(如果消耗了該線程的事件,可能會重置該事件)。
def wait_for_event_timeout(*events):
while not all([e.isSet() for e in events]):
#Check to see if the event is set. Timeout 1 sec.
ev_wait_bool=[e.wait(1) for e in events]
# Process if all events are set. Change all to any to process if any event set
if all(ev_wait_bool):
logging.debug('processing event')
else:
logging.debug('doing other work')
e1 = threading.Event()
e2 = threading.Event()
t3 = threading.Thread(name='non-block-multi',
target=wait_for_event_timeout,
args=(e1,e2))
t3.start()
logging.debug('Waiting before calling Event.set()')
time.sleep(5)
e1.set()
time.sleep(10)
e2.set()
logging.debug('Event is set')
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