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 [WxPython] How to deal with code that blocks the mainloop, freezing the gui
#1
(The tkinter version of this can be found here)

If you have ever tried to use time.sleep or any code that takes some time to run within your gui code, you will find it becomes unresponsive like in the following example.
You will notice that the call to change the label text to running doesn't seem work,the listbox does not update until after the sleep has finished, and the button locks in the down position.

Example of the problem

import time
import wx
 
class MainFrame(wx.Frame):
 
    def __init__(self, *args, **kwargs):
        super().__init__(*args, **kwargs)
        self.panel = wx.Panel(self)
        self.panel_sizer = wx.BoxSizer(wx.VERTICAL)
        self.panel.SetSizer(self.panel_sizer)
        self.label = wx.StaticText(self.panel, label='not running')
        self.panel_sizer.Add(self.label, 0, wx.ALL|wx.CENTER, 5)
        self.listbox = wx.ListBox(self.panel)
        self.panel_sizer.Add(self.listbox, 1, wx.ALL|wx.CENTER, 5)
        self.button = wx.Button(self.panel, label='blocking task')
        self.button.Bind(wx.EVT_BUTTON, self.on_button)
        self.panel_sizer.Add(self.button, 0, wx.ALL|wx.CENTER, 5)
        self.Layout()
        self.Show()
 
    def on_button(self, event):
        print('Button clicked')
        self.blocking_code()
 
    def blocking_code(self):
        self.label.SetLabel('running')
 
        for number in range(5):
            self.listbox.Append(str(number))
            print(number)
            time.sleep(1)
 
        self.label.SetLabel('not running')
 
 
if __name__ == '__main__':
    app = wx.App(False)
    main_frame = MainFrame(None)
    app.MainLoop()
Output:
Button clicked 0 1 2 3 4


Example of adding a thread but still getting a error

To get around this locking up problem we can use threads.

If we use a thread on its own the gui will become responsive but if the button is pressed a few times in a row, the number sequence will get jumbled up because each time the button is pressed a new thread is started.
There will also be an error if the gui is closed while the threads are still working, the gui loop doesn't like separate threads calling it.
Error:
RuntimeError: main thread is not in main loop
import threading

class MainFrame(wx.Frame):
 
    def __init__(self, *args, **kwargs):
        super().__init__(*args, **kwargs)
        ....
        ....

    def on_button(self, event):
        print('Button clicked')
        thread = threading.Thread(target=self.blocking_code)
        thread.start()

    def blocking_code(self):
        self.label.SetLabel('running')
Output:
Button clicked 0 Button clicked 0 1 1 2 Button clicked 0 2 3 1
Error:
Exception in thread Thread-2: Traceback (most recent call last): File "C:\Users\Dave\AppData\Local\Programs\Python\Python37\lib\threading.py", line 917, in _bootstrap_inner self.run() File "C:\Users\Dave\AppData\Local\Programs\Python\Python37\lib\threading.py", line 865, in run self._target(*self._args, **self._kwargs) File "C:\Users\Dave\Documents\Eclipse Workspace\Test\forum\wx_blocking.py", line 37, in blocking_code self.listbox.Append(str(number)) RuntimeError: wrapped C/C++ object of type ListBox has been deleted


Example of a solution to the problem

To get this working correctly we can use wx's CallAfter to make the gui changes happen in the gui thread and so only one thread is running and multiple clicks are queued up we can use concurrent futures ThreadPoolExecutor.

import wx
from concurrent import futures
import time
 
thread_pool_executor = futures.ThreadPoolExecutor(max_workers=1)

 
class MainFrame(wx.Frame):
 
    def __init__(self, *args, **kwargs):
        super().__init__(*args, **kwargs)
        self.panel = wx.Panel(self)
        self.panel_sizer = wx.BoxSizer(wx.VERTICAL)
        self.panel.SetSizer(self.panel_sizer)
        self.label = wx.StaticText(self.panel, label='not running')
        self.panel_sizer.Add(self.label, 0, wx.ALL | wx.CENTER, 5)
        self.listbox = wx.ListBox(self.panel)
        self.panel_sizer.Add(self.listbox, 1, wx.ALL | wx.CENTER, 5)
        self.button = wx.Button(self.panel, label='blocking task')
        self.button.Bind(wx.EVT_BUTTON, self.on_button)
        self.panel_sizer.Add(self.button, 0, wx.ALL | wx.CENTER, 5)
        self.Layout()
        self.Show()
 
    def on_button(self, event):
        print('Button clicked')
        thread_pool_executor.submit(self.blocking_code)
        
    def set_label_text(self, text=''):
        self.label.SetLabel(text)
 
    def listbox_insert(self, item):
        self.listbox.Append(item)
        print(item)
 
    def blocking_code(self):
        wx.CallAfter(self.set_label_text, 'running')
 
        for number in range(5):
            wx.CallAfter(self.listbox_insert, str(number))
            time.sleep(1)
 
        wx.CallAfter(self.set_label_text, 'not running')
 
 
if __name__ == '__main__':
    app = wx.App(False)
    main_frame = MainFrame(None)
    app.MainLoop()

Output:
Button clicked 0 Button clicked Button clicked 1 2 3 4 0 1 2 3 4 0 1 2 3 4


Example of a solution to the problem using decorators

Decorators of CallAfter and ThreadPoolExecutor can be used as shown below, by using these any blocking method just needs to be decorated by submit_to_pool_executor and any methods called from a separate thread just need decorating with wx_call_after.

import wx
from concurrent import futures
import time
import functools

thread_pool_executor = futures.ThreadPoolExecutor(max_workers=1)


def wx_call_after(target):

    @functools.wraps(target)
    def wrapper(self, *args, **kwargs):
        args = (self,) + args
        wx.CallAfter(target, *args, **kwargs)

    return wrapper


def submit_to_pool_executor(executor):

    def decorator(target):

        @functools.wraps(target)
        def wrapper(*args, **kwargs):
            return executor.submit(target, *args, **kwargs)

        return wrapper

    return decorator


class MainFrame(wx.Frame):

    def __init__(self, *args, **kwargs):
        super().__init__(*args, **kwargs)
        self.panel = wx.Panel(self)
        self.panel_sizer = wx.BoxSizer(wx.VERTICAL)
        self.panel.SetSizer(self.panel_sizer)
        self.label = wx.StaticText(self.panel, label='not running')
        self.panel_sizer.Add(self.label, 0, wx.ALL | wx.CENTER, 5)
        self.listbox = wx.ListBox(self.panel)
        self.panel_sizer.Add(self.listbox, 1, wx.ALL | wx.CENTER, 5)
        self.button = wx.Button(self.panel, label='blocking task')
        self.button.Bind(wx.EVT_BUTTON, self.on_button)
        self.panel_sizer.Add(self.button, 0, wx.ALL | wx.CENTER, 5)
        self.Layout()
        self.Show()

    def on_button(self, event):
        print('Button clicked')
        self.blocking_code()

    @wx_call_after
    def set_label_text(self, text=''):
        self.label.SetLabel(text)

    @wx_call_after
    def listbox_insert(self, item):
        self.listbox.Append(item)
        print(item)

    @submit_to_pool_executor(thread_pool_executor)
    def blocking_code(self):
        self.set_label_text('running')

        for number in range(5):
            self.listbox_insert(str(number))
            time.sleep(1)

        self.set_label_text('not running')


if __name__ == '__main__':
    app = wx.App(False)
    main_frame = MainFrame(None)
    app.MainLoop()
Quote
#2
If you just want to delay something, the first argument to after is how long to wait in millisecs before calling the passed in callback.
import wx


class MainFrame(wx.Frame):

    def __init__(self, *args, **kwargs):
        super().__init__(*args, **kwargs)
        self.panel = wx.Panel(self)
        self.panel_sizer = wx.BoxSizer(wx.VERTICAL)
        self.panel.SetSizer(self.panel_sizer)
        self.label = wx.StaticText(self.panel, label='Player 1 turn')
        self.panel_sizer.Add(self.label, 0, wx.ALL | wx.CENTER, 5)
        self.button = wx.Button(self.panel, label='Player 1 Move')
        self.button.Bind(wx.EVT_BUTTON, self.on_button)
        self.panel_sizer.Add(self.button, 0, wx.ALL | wx.CENTER, 5)
        self.Layout()
        self.Show()

    def on_button(self, event):
        print('Button clicked')
        self.label.SetLabel('player 2 thinking')
        self.button.Disable()
        wx.CallLater(3000, self.delayed_player_2)

    def delayed_player_2(self):
        self.label.SetLabel('player 2 moving')
        wx.CallLater(1000, self.player_2_finsihed)

    def player_2_finsihed(self):
        self.label.SetLabel('player 1 turn')
        self.button.Enable()


if __name__ == '__main__':
    app = wx.App(False)
    main_frame = MainFrame(None)
    app.MainLoop()
Quote

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