I have a Python script that performs some intensive processing of user's files and can take some time. I've build a user interface to it using Kivy, that allows the user to select the file, processing mode and shows them some messages as the process goes on.

My problem is that when the main Kivy loop passes calls the underlying user interface, the window freezes.

From what I've understood, the proper way of resolving this is to create a separate process to which the script would be off-loaded and from which it would send the updates to the user interface.

However, I was not able to find an example of how to do this or any specification on how to send messages from a separate thread back into application.

Could someone please give an example of how to do this properly or point me to the documentation pertaining to the subject?

Update:

For the sake of keeping the program maintainable I would like to avoid calling the elements of loops of processor from the main thread and instead call one long process that comes back to updated elements of the GUI, such as the progress bar or a text field. It looks like those elements can be modified only from the main kivy thread. How do I gain access to them from the outside?

Use publisher/consumer model as described here. Here's an example from that link modified to use separate threads:

from kivy.app import App
from kivy.clock import Clock, _default_time as time  # ok, no better way to use the same clock as kivy, hmm
from kivy.lang import Builder
from kivy.factory import Factory
from kivy.uix.button import Button
from kivy.properties import ListProperty

from threading import Thread
from time import sleep

MAX_TIME = 1/60.

kv = '''
BoxLayout:
    ScrollView:
        GridLayout:
            cols: 1
            id: target
            size_hint: 1, None
            height: self.minimum_height

    MyButton:
        text: 'run'

<MyLabel@Label>:
    size_hint_y: None
    height: self.texture_size[1]
'''

class MyButton(Button):
    def on_press(self, *args):
        Thread(target=self.worker).start()

    def worker(self):
        sleep(5) # blocking operation
        App.get_running_app().consommables.append("done")

class PubConApp(App):
    consommables = ListProperty([])

    def build(self):
        Clock.schedule_interval(self.consume, 0)
        return Builder.load_string(kv)

    def consume(self, *args):
        while self.consommables and time() < (Clock.get_time() + MAX_TIME):
            item = self.consommables.pop(0)  # i want the first one
            label = Factory.MyLabel(text=item)
            self.root.ids.target.add_widget(label)

if __name__ == '__main__':
    PubConApp().run()