How to get the window object from another thread

[xpierroz]

Hello everyone,
I have a NiceGUI application used with pywebview.
However, my app is running from another thread, here's the code:

class UvicornServer(multiprocessing.Process):

    def __init__(self, config: Config):
        super().__init__()
        self.server = Server(config=config)
        self.config = config

    def stop(self):
        self.terminate()

    def run(self, *args, **kwargs):
        self.server.run()

def start_window(shared_dict, url_to_load):
    def on_closed():
        shared_dict['window'] = 'closed'

    win = webview.create_window('Demo', url=url_to_load)
    win.events.closed += on_closed
    shared_dict['window'] = win
    webview.start(storage_path=tempfile.mkdtemp())
    
app = FastAPI()

@app.get('/')
def read_root():
    return {'Hello': 'World'}

@ui.page('/minimize')
def minimize_window():
    # Access the shared window object
    window = shared_dict['window']
    print(window)
    if window:
        window.minimize()

@ui.page('/show')
def show():
    ui.image('https://picsum.photos/id/377/640/360')

ui.run_with(app)

if __name__ == '__main__':
    server_ip = "127.0.0.1"
    server_port = 8081

    manager = multiprocessing.Manager()
    shared_dict = manager.dict()

    windowsp = multiprocessing.Process(target=start_window, args=(shared_dict, f'http://{server_ip}:{server_port}/show'))
    windowsp.start()

    config = Config("main:app", host=server_ip, port=server_port, log_level="debug")
    instance = UvicornServer(config=config)
    instance.start()

    while True:
        pass
    
    instance.stop()

My question is, how to get the window object from the @ui.page('/minimize'), I did search about this and I discovered multiprocessing.Manager(), however, in the start_window() func, this gets me an error:
shared_dict['window'] = win

Full traceback:

Process Process-2:
Traceback (most recent call last):
  File "C:\Users\pierr\AppData\Local\Programs\Python\Python311\Lib\multiprocessing\process.py", line 314, in _bootstrap
    self.run()
  File "C:\Users\pierr\AppData\Local\Programs\Python\Python311\Lib\multiprocessing\process.py", line 108, in run
    self._target(*self._args, **self._kwargs)
  File "c:\Users\pierr\Documents\GitHub\DPresence\ttest.py", line 32, in start_window
    shared_dict['window'] = win
    ~~~~~~~~~~~^^^^^^^^^^
  File "<string>", line 2, in __setitem__
  File "C:\Users\pierr\AppData\Local\Programs\Python\Python311\Lib\multiprocessing\managers.py", line 821, in _callmethod
    conn.send((self._id, methodname, args, kwds))
  File "C:\Users\pierr\AppData\Local\Programs\Python\Python311\Lib\multiprocessing\connection.py", line 205, in send
    self._send_bytes(_ForkingPickler.dumps(obj))
                     ^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "C:\Users\pierr\AppData\Local\Programs\Python\Python311\Lib\multiprocessing\reduction.py", line 51, in dumps
    cls(buf, protocol).dump(obj)
AttributeError: Can't pickle local object 'start_window.<locals>.on_closed'

I have no idea if this is the right method to get the window object, I tried webview.windows but it is an empty list and webview.active_window() returns None.

So is using multiprocessing.Manager() the right way, if yes, what should I change to get it to work?

[r0x0r]

You are confusing threads with processes. Getting a window in a multi-threaded environment should not be a problem, but sharing data between processes is rather tricky. The error you get means that window object cannot be pickled as required by multiprocessing. You may have luck with an alternative pickling library like dill, see https://stackoverflow.com/questions/19984152/what-can-multiprocessing-and-dill-do-together for details. I have not tested any of this, but from my experience with passing data in a multiprocessing environment in Python is rather painful.