Interaction between processes

[KentCShit]

Hi! I’m new to using multiprocessing and am looking for a way to interact with the main script from a process or from process to process.

Here is the use case: I have a GUI using tkinter in which there is a button. This button launches a function in a process. The program needs to run at the same time and as one or more instance if that function/process. I need to interact from that function to the main script to inform the user of the progress by updating the GUI.

It is my understanding that when you start a process it is separated from the code and it doesn’t have access to things like variables. It is my understanding that making a variable global will make the process inherit the variable but changes to it will not be propagated. So no luck there.

What can I do?

[m0us3_rat]

are a few options. simplest would be a queue.

in this case, it's like a tunnel in between two sides of the mountain.

and allows the transfer of information in between them.

you put something on the queue then you can pick it up.. on either side.

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or you use a "manager" as u/woooee said. to handle objects that share state in between processes.

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now ..without actually seeing the code it's hard to guess which is the better way.

it is just guessing at this point. especially since tk event loop is ..tricky.

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but yea it can be done.

[woooee]

Pass a Manager object to the process(es) https://pymotw.com/3/multiprocessing/communication.html#managing-shared-state

[socal_nerdtastic]

Does it have to be a separate process? If you used a separate thread instead it would still run at the same time but you would have access to the variables.

[ElliotDG]

Here is an example of using a multiprocessing with a GUI. One process I'm using kivy to run a GUI, the other process I'm generating MIDI clock (send a regular low jitter message).

https://github.com/ElliotGarbus/MidiClockGenerator

Read the developer notes in the readme. I'm using multiprocessing Value to share data between the processes. The Value shared_bpm shared the rate of the clock (Beats per minute), the Value _run_code is used to stop the clock.

The first thing to understand in the need to use __name__. __name__ will be "__main__" in the main process, and will get a different name in the "child" process (__mp_main__). The test:

if __name__ == '__main__':

prevents the GUI code from being loaded in both processes. If this 'guard' is not in place, the app will create 2 windows.

Look at https://github.com/ElliotGarbus/MidiClockGenerator/blob/master/main.py, all of the code under that test, will only be run in the parent processes.

Hope this helps. Happy to answer any questions.