Tkinter with Multiprocessing

Tkinter is mainly based on single-threaded event model. The mainloop(), callbacks, event handlers and raising tkinter exceptions are all handled in single thread. That is why, it happens quite often that tkinter GUI becomes unresponsive ( for the user or is considers to be unresponsive by the user : which is a bad user experience) when event handlers try to do long blocking operations. In order to avoid it and improve user experience, it is generally advised that long running background tasks should be shifted to other threads.

But, we have two choices to incorporate parallelism in application using Python:

  • Multithreading
  • Multiprocessing

But, Python’s multithreading module is notorious due to GIL in Python. ( Google about GIL, you will find about it). It prevents Python multithreaded applications from taking full advantage of multiple processors. I used multithreading module with my application, with no significant improvement in performance. However, if we have used basic multithreading like :

new_thread = multithreading.Thread(target=some_function)
new_thread.start()

It was very easy to convert it into multiprocessing module by replacing ‘multithreading’ with ‘multiprocessing’. And also we have used the queues to send messages from processes running in secondary threads to tkinter mainloop() (primary thread), then those queues should be replaced multiprocessing safe queues : multiprocessing.Queue(). But, it is important to remember that only objects which can be pickled can be allowed to pass from process X to process Y. The general thumb rule I came to know about it was that generally all Python native objects like lists, tuples, strings, integers, dictionaries are picklable. But, complex classes or objects can’t be pickled. I found about it here.

We need to ask us questions to decided which objects can be pickled :

  • Can you capture the state of the object by reference (i.e. a function defined in __main__ versus an imported function)? [Then, yes]
  • Does a generic
    __get_state__
    __set_state__

    rule exists for the given object type? [Then, yes]

  • Does it depend on a Frame object (i.e. rely on the GIL and global execution stack)? Iterators are now an exception to this, by “replaying” the iterator on unpickling. [Then, no]
  • Does the object instance point to the wrong class path (i.e. due to being defined in a closure, in C-bindings, or other __init__ path manipulations)? [Then, no]

A better option is check the docs here ( a more definite answer ) :

    These objects can be pickled :

  • None, True, and False
  • integers, long integers, floating point numbers, complex numbers
  • normal and Unicode strings
  • tuples, lists, sets, and dictionaries containing only picklable objects
  • functions defined at the top level of a module
  • built-in functions defined at the top level of a module
  • classes that are defined at the top level of a module
  • instances of such classes whose __dict__ or the result of calling
    __getstate__() is picklable (see section The pickle protocol for details).

My next post will be just a code sample on how I used multiprocessing with Tkinter.

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Author: scorpiocoder

I am a 3rd year, IIIT-Chittoor student enthusiastic and confused about various software technologies : Python, Machine Learning, Networks, Natural Language Processing and Android Development

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