ui.log raise error : "Client has been deleted but is still being used" on first page refresh ....

[zak-45]

Description

Info : Win 11, Python 3.12, NiceGUI 2.12.1

I have made a script to capture stdin, stderr and put the messages into ui.log().
All is working well, except on first page reload (...no more after), I receive this error message:

Client has been deleted but is still being used. This is most likely a bug in your application code. See https://github.com/zauberzeug/nicegui/issues/3028 for more information.
Stack (most recent call last):
  File "C:\Program Files\Python311\Lib\threading.py", line 1002, in _bootstrap
    self._bootstrap_inner()
  File "C:\Program Files\Python311\Lib\threading.py", line 1045, in _bootstrap_inner
    self.run()
  File "C:\Program Files\Python311\Lib\threading.py", line 982, in run
    self._target(*self._args, **self._kwargs)
  File "C:\Users\zak-4\PycharmProjects\WLEDVideoSync\src\utl\console.py", line 74, in read_queue
    self.log_ui.push(log_message)
  File "C:\Users\zak-4\PycharmProjects\WLEDVideoSync\.venv\Lib\site-packages\nicegui\elements\log.py", line 26, in push
    Label(text)
  File "C:\Users\zak-4\PycharmProjects\WLEDVideoSync\.venv\Lib\site-packages\nicegui\elements\label.py", line 13, in __init__
    super().__init__(tag='div', text=text)
  File "C:\Users\zak-4\PycharmProjects\WLEDVideoSync\.venv\Lib\site-packages\nicegui\elements\mixins\text_element.py", line 14, in __init__
    super().__init__(**kwargs)
  File "C:\Users\zak-4\PycharmProjects\WLEDVideoSync\.venv\Lib\site-packages\nicegui\element.py", line 77, in __init__
    self.client.outbox.enqueue_update(self)
  File "C:\Users\zak-4\PycharmProjects\WLEDVideoSync\.venv\Lib\site-packages\nicegui\outbox.py", line 50, in enqueue_update
    self.client.check_existence()
  File "C:\Users\zak-4\PycharmProjects\WLEDVideoSync\.venv\Lib\site-packages\nicegui\client.py", line 346, in check_existence
    helpers.warn_once('Client has been deleted but is still being used. '
  File "C:\Users\zak-4\PycharmProjects\WLEDVideoSync\.venv\Lib\site-packages\nicegui\helpers.py", line 20, in warn_once
    log.warning(message, stack_info=stack_info)

Had take a look to the provided link, but not really related for my case ..??!!

Here is the script :

from nicegui import ui
import sys
import multiprocessing
import threading
import time

class ConsoleCapture:
    """Captures and displays console output in a NiceGUI UI.

    Redirects stdout and stderr to a queue, which is then displayed
    in an ui.log element.  Provides options for UI customization and restoring
    the original console output streams.
    """
    def __init__(self, show_console=False, text_color='text-white', bg_color='bg-black'):
        """Initialize the ConsoleCapture.

        Sets up console capturing by redirecting stdout and stderr, initializing the UI
        if requested, and starting a background thread to process captured output.
        """
        self.original_stdout = sys.stdout
        self.original_stderr = sys.stderr
        self.text_color = text_color
        self.bg_color = bg_color
        if show_console:
            self.setup_ui()
        else:
            self.log_ui = None
        self.log_queue = multiprocessing.Queue()

        sys.stdout = self
        sys.stderr = self

        # Start a background thread to read from the queue
        self.running = True
        threading.Thread(target=self.read_queue, daemon=True).start()

    def setup_ui(self):
        """Set up the UI for the console output.

        Creates and configures an ui.log element to display captured console output,
        applying specified styling classes.
        """
        self.log_ui = ui.log()
        self.log_ui.classes(f'console-output w-full h-30 {self.bg_color} {self.text_color}')

    def write(self, text):
        """Override sys.stdout and sys.stderr to send output to the queue and original streams."""
        if text.strip():
            self.log_queue.put(text.strip())  # Send to the queue
            # Write to the original stdout or stderr
            if "Error" in text:
                self.original_stderr.write(text)
            else:
                self.original_stdout.write(text)

    def flush(self):
        """Flush method for compatibility."""
        pass

    def restore(self):
        """Restore original stdout and stderr."""
        sys.stdout = self.original_stdout
        sys.stderr = self.original_stderr
        self.running = False

    def read_queue(self):
        """Continuously read from the queue and update the UI log."""
        while self.running:
            try:
                while not self.log_queue.empty():
                    log_message = self.log_queue.get()
                    if self.log_ui is not None:
                        self.log_ui.push(log_message)
            except Exception as e:
                self.original_stderr.write(f"Queue reading error: {e}\n")
            time.sleep(0.1)  # Prevent busy waiting


if __name__ in "__main__":
    # NiceGUI app
    @ui.page('/')
    async def main_page():

        capture = ConsoleCapture(show_console=False)

        with ui.column():
            ui.button('Print Message', on_click=lambda: print('Hello from stdout!'))
            ui.button('Raise Exception', on_click=lambda: 1 / 0)
            ui.button('Send Custom Log', on_click=lambda: capture.log_queue.put("[INFO] Custom log message"))
            ui.button('Restore Console', on_click=capture.restore)

        capture.setup_ui()

        ui.context.client.on_disconnect(lambda: print('client disconnected'))
        ui.context.client.on_connect(lambda: print('client connected'))

    ui.run(reload=False)

Thanks

[falkoschindler]

Hi @zak-45,

With self.log_ui = ui.log() you're keeping a reference to the ui.log element. This is exactly the problem discussed in #3028.
Because we wanted to avoid spamming the console, the warning is only shown once.

[zak-45]

Thanks a lot for the feedback and your clarification. So, this should explain why this occur only once, but to be honnest, I do not found a way to disregard this first warning on my case, any advice from your side.

[falkoschindler]

There is a demo showcasing how to attach to a logger and detach on disconnect:
https://nicegui.io/documentation/log#attach_to_a_logger

[zak-45]

Thanks, have seen this example. yes, no error appear.... but even with this one, some trouble happend. Just refresh the page 3 times and no more 'time' displayed into NiceGUI page.???

[falkoschindler]

You're right: The demo is broken! The first disconnect removes the handler and the ui.log element receives no log messages anymore. I'll look into it.

[falkoschindler]

I fixed it in dd1fa04.