A Basic Example of Threads Synchronization in Python, python中的线程同步示例

http://zulko.github.io/blog/2013/09/19/a-basic-example-of-threads-synchronization-in-python/

We will see how to use threading Events to have functions in different Python threads start at the same time.

I recently coded a method to view movies in Python : it plays the video, and in the same time, in a parralel thread, it renders the audio. The difficult part is that the audio and video should be exactly synchronized. The pseudo-code looks like this:

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def view(movie): new_thread( play_audio( movie ) ) play_video( movie )

In this code,

play_audio()

and

play_video()

will start at approximately the same time and will run parallely, but these functions need some preparation before actually starting playing stuff. Their code looks like that:

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def play_audio(movie): audio = prepare_audio( movie ) audio.start_playing() def play_video(movie): video = prepare_video( movie ) video.start_playing()

To have a well-synchronized movie we need the internal functions

audio.start_playing()

and

video.start_playing()

, which are run in two separate threads, to start at exactly the same time. How do we do that ?

The solution seems to be using

threading.Event

objects. An

Event

is an object that can be accessed from all the threads and allows very basic communication between them : each thread can set or unset an Event, or check whether this event has already been been set (by another thread).

For our problem we will use two events

video_ready

and

audio_ready

which will enable our two threads to scream at each other “I am ready ! Are you ?”. Here is the Python fot that:

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import threading def play_audio(movie, audio_ready, video_ready): audio = prepare_audio( movie ) audio_ready.set() # Say "I'm ready" to play_video() video_ready.wait() # Wait for play_video() to say "I'm ready" audio.start_playing() def play_video(movie, audio_ready, video_ready): video = prepare_video( movie ) video_ready.set() # Say "I'm ready" to play_audio() audio_ready.wait() # Wait for play_audio() to say "I'm ready" video.start_playing()

and finally the code for

view(movie)

:

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def view(movie): audio_ready = threading.Event() video_ready = threading.Event() # launch the parrallel audio thread audiothread = threading.Thread(target=play_audio, args = (movie, audio_ready, video_ready)) audiothread.start() play_video(movie, audio_ready, video_ready)

A few tips tips to go further:

Here I am using the module

threading

, and the two threads will be played in parrallel on the same processor. If you have a computer with several processors you can also use the

multiprocessing

module to have your threads played on two different processors (which can be MUCH faster). Nicely enough the two modules have the same syntax: simply replace

threading

by

multiprocessing

and

Thread

by

Process

in the example above and it should work.

In my original program, I also use an Event to terminate

play_video

and

play_audio

at the same time: when the video playing is exited,

play_video

unsets that Event. In

play_audio

, this event is regularly checked, and when it is seen to be unset,

play_audio

exits too.

Instead of using

wait

to wait for an Event to be set, you can use a loop to you decide at which frequency you want to check the Event. Only do that if don’t mind a lag of a few milliseconds between your processes :

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import time while not audio_ready.is_set(): time.sleep(0.002) # sleep 2 milliseconds

Posted by Zulko Sep 19th, 2013 MoviePy, Python,, threads,