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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In this code,
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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To have a well-synchronized movie we need the internal functions
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
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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and finally the code for
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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
multiprocessingmodule 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
Processin the example above and it should work.
- In my original program, I also use an Event to terminate
play_audioat the same time: when the video playing is exited,
play_videounsets that Event. In
play_audio, this event is regularly checked, and when it is seen to be unset,
- Instead of using
waitto 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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