{"id":6296,"date":"2023-01-04T00:48:03","date_gmt":"2023-01-04T00:48:03","guid":{"rendered":"https:\/\/www.pythontutorial.net\/?page_id=6296"},"modified":"2023-06-03T12:34:21","modified_gmt":"2023-06-03T12:34:21","slug":"python-threading-event","status":"publish","type":"page","link":"https:\/\/www.pythontutorial.net\/python-concurrency\/python-threading-event\/","title":{"rendered":"Python Threading Event"},"content":{"rendered":"\n

Summary<\/strong>: in this tutorial, you’ll learn how to use the Python threading Event object to communicate between threads.<\/p>\n\n\n\n

Introduction to the Python threading Event object #<\/a><\/h2>\n\n\n\n

Sometimes, you need to communicate between the threads<\/a>. To do that, you can use a lock<\/a> (mutex) and a boolean variable.<\/p>\n\n\n\n

However, Python provides you with a better way to communicate between threads using the Event<\/code> class from the threading<\/code> module.<\/p>\n\n\n\n

The Event<\/code> class offers a simple but effective way to coordinate between threads: one thread signals an event while other threads wait for it.<\/p>\n\n\n\n

The Event<\/code> object wraps a boolean flag that can be set (True<\/code>) or cleared (False<\/code>). Multiple threads can wait for an Event<\/code> to be set before proceeding or can reset the Event<\/code> back to the cleared state.<\/p>\n\n\n\n

The following illustrates the steps for using the Event<\/code> object:<\/p>\n\n\n\n

First, import the Event<\/code> from the threading<\/code> module:<\/p>\n\n\n

from<\/span> threading import<\/span> Event<\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
Next, create a new Event<\/code> object:<\/p>\n\n\n
event = Event()<\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
By default, the event is not set (cleared). The is_set()<\/code> method of the event object will return False<\/code>:<\/p>\n\n\n
if<\/span> event.is_set():\n   # ...<\/span><\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
Then, set an event using the set()<\/code> method: <\/p>\n\n\n
event.set()<\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
Once an event is set, all the threads that wait on the event will be notified automatically.<\/p>\n\n\n\n
After that, unset an event via the clear()<\/code> method:<\/p>\n\n\n
event.clear()<\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
Finally, threads can wait for the event to be set via the wait()<\/code> method:<\/p>\n\n\n
event.wait()<\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
The wait()<\/code> method blocks the execution of a thread until the event is set. In other words, the wait()<\/code> method will block the current thread until another thread call the set()<\/code> method to set the event.<\/p>\n\n\n\n
If an event is set, the wait()<\/code> function returns immediately.<\/p>\n\n\n\n
To specify how long the thread is going to wait, you can use the timeout argument. For example:<\/p>\n\n\n
event.wait(timeout=5<\/span>) # wait for 5 seconds<\/span><\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
Python threading event example #<\/a><\/h2>\n\n\n\n
The following example shows a simple example of using the Event<\/code> object to communicate between threads:<\/p>\n\n\n
from<\/span> threading import<\/span> Thread, Event\nfrom<\/span> time import<\/span> sleep\n\ndef<\/span> task<\/span>(event: Event, id: int)<\/span> -> None<\/span>:<\/span>\n    print(f'Thread {id}<\/span> started. Waiting for the signal....'<\/span>)\n    event.wait()\n    print(f'Received signal. The thread {id}<\/span> was completed.'<\/span>)\n\ndef<\/span> main<\/span>()<\/span> -> None<\/span>:<\/span>\n    event = Event()\n\n    t1 = Thread(target=task, args=(event,1<\/span>))\n    t2 = Thread(target=task, args=(event,2<\/span>))\n\n    t1.start()\n    t2.start()\n\n    print('Blocking the main thread for 3 seconds...'<\/span>)\n    sleep(3<\/span>) \n    event.set()\n\n\n\nif<\/span> __name__ == '__main__'<\/span>:\n    main()<\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
Output:<\/p>\n\n\n
Thread 1<\/span> started. Waiting for<\/span> the signal....\nThread 2<\/span> started. Waiting for<\/span> the signal....\nBlocking the main thread for<\/span> 3<\/span> seconds...\nReceived signal. The thread 1<\/span> was completed.\nReceived signal. The thread 2<\/span> was completed.<\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
How it works.<\/p>\n\n\n\n
First, define the task()<\/code> function that accepts an Event<\/code> object and an integer:<\/p>\n\n\n
def<\/span> task<\/span>(event: Event, id: int)<\/span> -> None<\/span>:<\/span>\n    print(f'Thread {id}<\/span> started. Wait for the signal....'<\/span>)\n    event.wait()\n    print(f'Receive signal. The thread {id}<\/span> was completed.'<\/span>)<\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
Inside, the task()<\/code> function, we call the wait()<\/code> method of the event object to wait for the event to be set by the main thread.<\/p>\n\n\n\n
Second, create an Event<\/code> object inside the main()<\/code> function:<\/p>\n\n\n
event = Event()<\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
Third, create two child threads that execute the task()<\/code> function with the same event object and different id 1 and 2:<\/p>\n\n\n
t1 = Thread(target=task, args=(event,1<\/span>))\nt2 = Thread(target=task, args=(event,2<\/span>))<\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
Fourth, start both threads by calling the start()<\/code> method:<\/p>\n\n\n
t1.start()\nt2.start()<\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
Fifth, call the sleep()<\/code> method to block the main thread for three seconds:<\/p>\n\n\n
sleep(3<\/span>)<\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
Since the task()<\/code> function call the wait()<\/code> method of the event object, both t1<\/code> and t2<\/code> threads will wait for the event to be set before continuing.<\/p>\n\n\n\n
Finally, set the event by calling the set()<\/code> method from the main thread:<\/p>\n\n\n
event.set()<\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
Both t1<\/code> and t2<\/code> threads will be notified and continue executing until the end.<\/p>\n\n\n\n
Note that you’ll learn how to use the Event object to stop a child thread<\/a> from the main thread in the next tutorial.<\/p>\n\n\n\n
Practical example of using Threading event #<\/a><\/h2>\n\n\n\n
The following example illustrates hwo to use the threading event to synchronize between two threads:<\/p>\n\n\n\n