{"id":307,"date":"2024-01-28T10:38:40","date_gmt":"2024-01-28T10:38:40","guid":{"rendered":"https:\/\/learnpython.elegantwallp.com\/?p=307"},"modified":"2024-01-28T10:38:41","modified_gmt":"2024-01-28T10:38:41","slug":"python-threading-event","status":"publish","type":"post","link":"https:\/\/learnpython.elegantwallp.com\/2024\/01\/28\/python-threading-event\/","title":{"rendered":"Python Threading Event"},"content":{"rendered":"\n

Summary<\/strong>: in this tutorial, you\u2019ll 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<\/h2>\n\n\n\n

Sometimes, you need to communicate between the\u00a0threads. To do that, you can use a\u00a0lock\u00a0(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\u00a0Event<\/code>\u00a0from the\u00a0threading<\/code>\u00a0module:<\/p>\n\n\n\n

from threading import Event<\/code>Code language: Python (python)<\/small><\/code><\/pre>\n\n\n\n
Next, create a new\u00a0Event<\/code>\u00a0object:<\/p>\n\n\n\n
event = Event()<\/code>Code language: Python (python)<\/small><\/code><\/pre>\n\n\n\n
By default, the event is not set (cleared). The\u00a0is_set()<\/code>\u00a0method of the event object will return\u00a0False<\/code>:<\/p>\n\n\n\n
if event.is_set(): # ...<\/em><\/code>Code language: Python (python)<\/small><\/code><\/pre>\n\n\n\n
Then, set an event using the\u00a0set()<\/code>\u00a0method:<\/p>\n\n\n\n
event.set()<\/code>Code language: Python (python)<\/small><\/code><\/pre>\n\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\u00a0clear()<\/code>\u00a0method:<\/p>\n\n\n\n
event.clear()<\/code>Code language: Python (python)<\/small><\/code><\/pre>\n\n\n\n
Finally, threads can wait for the event to be set via the\u00a0wait()<\/code>\u00a0method:<\/p>\n\n\n\n
event.wait()<\/code>Code language: Python (python)<\/small><\/code><\/pre>\n\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\n
event.wait(timeout=5) # wait for 5 seconds<\/em><\/code>Code language: Python (python)<\/small><\/code><\/pre>\n\n\n\n
Python threading event example<\/h2>\n\n\n\n
The following example shows a simple example of using the\u00a0Event<\/code>\u00a0object to communicate between threads:<\/p>\n\n\n\n
from threading import Thread, Event from time import sleep def task(event: Event, id: int) -> None: print(f'Thread {id} started. Waiting for the signal....') event.wait() print(f'Received signal. The thread {id} was completed.') def main() -> None: event = Event() t1 = Thread(target=task, args=(event,1)) t2 = Thread(target=task, args=(event,2)) t1.start() t2.start() print('Blocking the main thread for 3 seconds...') sleep(3) event.set() if __name__ == '__main__': main()<\/code>Code language: Python (python)<\/small><\/code><\/pre>\n\n\n\n
Output:<\/p>\n\n\n\n
Thread 1 started. Waiting for the signal.... Thread 2 started. Waiting for the signal.... Blocking the main thread for 3 seconds... Received signal. The thread 1 was completed. Received signal. The thread 2 was completed.<\/code>Code language: Python (python)<\/small><\/code><\/pre>\n\n\n\n
How it works.<\/p>\n\n\n\n
First, define the\u00a0task()<\/code>\u00a0function that accepts an\u00a0Event<\/code>\u00a0object and an integer:<\/p>\n\n\n\n
def task(event: Event, id: int) -> None: print(f'Thread {id} started. Wait for the signal....') event.wait() print(f'Receive signal. The thread {id} was completed.')<\/code>Code language: Python (python)<\/small><\/code><\/pre>\n\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\u00a0Event<\/code>\u00a0object inside the\u00a0main()<\/code>\u00a0function:<\/p>\n\n\n\n
event = Event()<\/code>Code language: Python (python)<\/small><\/code><\/pre>\n\n\n\n
Third, create two child threads that execute the\u00a0task()<\/code>\u00a0function with the same event object and different id 1 and 2:<\/p>\n\n\n\n
t1 = Thread(target=task, args=(event,1)) t2 = Thread(target=task, args=(event,2))<\/code>Code language: Python (python)<\/small><\/code><\/pre>\n\n\n\n
Fourth, start both threads by calling the\u00a0start()<\/code>\u00a0method:<\/p>\n\n\n\n
t1.start() t2.start()<\/code>Code language: Python (python)<\/small><\/code><\/pre>\n\n\n\n
Fifth, call the\u00a0sleep()<\/code>\u00a0method to block the main thread for three seconds:<\/p>\n\n\n\n
sleep(3)<\/code>Code language: Python (python)<\/small><\/code><\/pre>\n\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\u00a0set()<\/code>\u00a0method from the main thread:<\/p>\n\n\n\n
event.set()<\/code>Code language: Python (python)<\/small><\/code><\/pre>\n\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\u2019ll learn how to use the Event object to\u00a0stop a child thread\u00a0from the main thread in the next tutorial.<\/p>\n\n\n\n
Practical example of using Threading event<\/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
    \n
  • Thread #1 downloads a text file from URL https:\/\/www.ietf.org\/rfc\/rfc793.txt<\/code>, once completed, it notifies the second thread to count the words from the downloaded text file.<\/li>\n\n\n\n
  • Thread #2 starts and wait for the completed signal from the thread #1. Once, receiving the signal, it starts counting the words from the downloaded file.<\/li>\n<\/ul>\n\n\n\n
    Here\u2019s the complete program:<\/p>\n\n\n\n
    from threading import Thread, Event from urllib import request def download_file(url, event): # Download the file form URL<\/em> print(f\"Downloading file from {url}...\") filename, _ = request.urlretrieve(url, \"rfc793.txt\") # File download completed, set the event<\/em> event.set() def process_file(event): print(\"Waiting for the file to be downloaded...\") event.wait() # Wait for the event to be set<\/em> # File has been downloaded, start processing it<\/em> print(\"File download completed. Starting file processing...\") # Count the number of words in the file<\/em> word_count = 0 with open(\"rfc793.txt\", \"r\") as file: for line in file: words = line.split() word_count += len(words) # Print the word count<\/em> print(f\"Number of words in the file: {word_count}\") def main(): # Create an Event object<\/em> event = Event() # Create and start the file download thread<\/em> download_thread = Thread(target=download_file, args=(\"https:\/\/www.ietf.org\/rfc\/rfc793.txt\", event)) download_thread.start() # Create and start the file processing thread<\/em> process_thread = Thread(target=process_file, args=(event,)) process_thread.start() # Wait for both threads to complete<\/em> download_thread.join() process_thread.join() print(\"Main thread finished.\") if __name__ == '__main__' : main()<\/code><\/code><\/pre>\n","protected":false},"excerpt":{"rendered":"
    Summary: in this tutorial, you\u2019ll learn how to use the Python threading Event object to communicate between threads. Introduction to the Python threading Event object Sometimes, you need to communicate between the\u00a0threads. To do that, you can use a\u00a0lock\u00a0(mutex) and a boolean variable. However, Python provides you with a better way to communicate between threads […]<\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[41],"tags":[],"class_list":["post-307","post","type-post","status-publish","format-standard","hentry","category-1-python-concurrency"],"_links":{"self":[{"href":"https:\/\/learnpython.elegantwallp.com\/wp-json\/wp\/v2\/posts\/307","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/learnpython.elegantwallp.com\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/learnpython.elegantwallp.com\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/learnpython.elegantwallp.com\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/learnpython.elegantwallp.com\/wp-json\/wp\/v2\/comments?post=307"}],"version-history":[{"count":1,"href":"https:\/\/learnpython.elegantwallp.com\/wp-json\/wp\/v2\/posts\/307\/revisions"}],"predecessor-version":[{"id":308,"href":"https:\/\/learnpython.elegantwallp.com\/wp-json\/wp\/v2\/posts\/307\/revisions\/308"}],"wp:attachment":[{"href":"https:\/\/learnpython.elegantwallp.com\/wp-json\/wp\/v2\/media?parent=307"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/learnpython.elegantwallp.com\/wp-json\/wp\/v2\/categories?post=307"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/learnpython.elegantwallp.com\/wp-json\/wp\/v2\/tags?post=307"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}