"No response returned" when too many video sources are loaded in native browser

[MaidScientistIzutsumiMarin]

First Check

• I added a very descriptive title here.
• This is not a Q&A. I am sure something is wrong with NiceGUI or its documentation.
• I used the GitHub search to find a similar issue and came up empty.

Example Code

from nicegui import ui

for _ in range(2):
    ui.video("test.mp4")

ui.run(native=True, reload=False)

Description

Start the app.
A RuntimeError occurs with the message "No response returned."

NiceGUI Version

2.21.1

Python Version

Python 3.13.5

Browser

Other

Operating System

Windows

Additional Context

This does not happen if I launch the program in Firefox. I've attached test.mp4 for reproducibility, but this happens with any videos if enough are loaded. It is also related to the length of the videos, where shorter videos need more to be embedded before the error occurs. The number of videos can be increased if this isn't causing an error. The error doesn't seem to cause any issues with the actual functionality of the program besides lagging.

The error also occurs when the event loop is set to WindowsSelectorEventLoopPolicy.

test.mp4

[falkoschindler]

Hi @MaidScientistIzutsumiMarin,

Am I right to assume that the problem already occurs with 2x test.mp4?
I just tested with 100x test.mp4 and didn't get any RuntimeError.

Can anyone else reproduce the problem?

[MaidScientistIzutsumiMarin]

Hi @MaidScientistIzutsumiMarin,

Am I right to assume that the problem already occurs with 2x test.mp4? I just tested with 100x test.mp4 and didn't get any RuntimeError.

Can anyone else reproduce the problem?

For me it happens with just 2 of them. Also, I've included the console output. If this is too long, I can move it to a Pastebin or something if there is a standard way to include long errors here.

NiceGUI ready to go on http://127.0.0.1:8000
No response returned.
  + Exception Group Traceback (most recent call last):
  |   File "C:\Users\Cookie\AppData\Local\Programs\Python\Python313\Lib\site-packages\starlette\_utils.py", line 77, in collapse_excgroups
  |     yield
  |   File "C:\Users\Cookie\AppData\Local\Programs\Python\Python313\Lib\site-packages\starlette\middleware\base.py", line 183, in __call__
  |     async with anyio.create_task_group() as task_group:
  |                ~~~~~~~~~~~~~~~~~~~~~~~^^
  |   File "C:\Users\Cookie\AppData\Local\Programs\Python\Python313\Lib\site-packages\anyio\_backends\_asyncio.py", line 772, in __aexit__
  |     raise BaseExceptionGroup(
  |         "unhandled errors in a TaskGroup", self._exceptions
  |     ) from None
  | ExceptionGroup: unhandled errors in a TaskGroup (1 sub-exception)
  +-+---------------- 1 ----------------
    | Traceback (most recent call last):
    |   File "C:\Users\Cookie\AppData\Local\Programs\Python\Python313\Lib\site-packages\starlette\middleware\errors.py", line 164, in __call__
    |     await self.app(scope, receive, _send)
    |   File "C:\Users\Cookie\AppData\Local\Programs\Python\Python313\Lib\site-packages\starlette\middleware\base.py", line 182, in __call__
    |     with recv_stream, send_stream, collapse_excgroups():
    |                                    ~~~~~~~~~~~~~~~~~~^^
    |   File "C:\Users\Cookie\AppData\Local\Programs\Python\Python313\Lib\contextlib.py", line 162, in __exit__
    |     self.gen.throw(value)
    |     ~~~~~~~~~~~~~~^^^^^^^
    |   File "C:\Users\Cookie\AppData\Local\Programs\Python\Python313\Lib\site-packages\starlette\_utils.py", line 83, in collapse_excgroups
    |     raise exc
    |   File "C:\Users\Cookie\AppData\Local\Programs\Python\Python313\Lib\site-packages\starlette\middleware\base.py", line 184, in __call__
    |     response = await self.dispatch_func(request, call_next)
    |                ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |   File "C:\Users\Cookie\AppData\Local\Programs\Python\Python313\Lib\site-packages\nicegui\middlewares.py", line 23, in dispatch
    |     response = await call_next(request)
    |                ^^^^^^^^^^^^^^^^^^^^^^^^
    |   File "C:\Users\Cookie\AppData\Local\Programs\Python\Python313\Lib\site-packages\starlette\middleware\base.py", line 159, in call_next
    |     raise app_exc
    |   File "C:\Users\Cookie\AppData\Local\Programs\Python\Python313\Lib\site-packages\starlette\middleware\base.py", line 144, in coro
    |     await self.app(scope, receive_or_disconnect, send_no_error)
    |   File "C:\Users\Cookie\AppData\Local\Programs\Python\Python313\Lib\site-packages\starlette\middleware\base.py", line 182, in __call__
    |     with recv_stream, send_stream, collapse_excgroups():
    |                                    ~~~~~~~~~~~~~~~~~~^^
    |   File "C:\Users\Cookie\AppData\Local\Programs\Python\Python313\Lib\contextlib.py", line 162, in __exit__
    |     self.gen.throw(value)
    |     ~~~~~~~~~~~~~~^^^^^^^
    |   File "C:\Users\Cookie\AppData\Local\Programs\Python\Python313\Lib\site-packages\starlette\_utils.py", line 83, in collapse_excgroups
    |     raise exc
    |   File "C:\Users\Cookie\AppData\Local\Programs\Python\Python313\Lib\site-packages\starlette\middleware\base.py", line 184, in __call__
    |     response = await self.dispatch_func(request, call_next)
    |                ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |   File "C:\Users\Cookie\AppData\Local\Programs\Python\Python313\Lib\site-packages\nicegui\middlewares.py", line 13, in dispatch
    |     response = await call_next(request)
    |                ^^^^^^^^^^^^^^^^^^^^^^^^
    |   File "C:\Users\Cookie\AppData\Local\Programs\Python\Python313\Lib\site-packages\starlette\middleware\base.py", line 160, in call_next
    |     raise RuntimeError("No response returned.")
    | RuntimeError: No response returned.
    +------------------------------------

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "C:\Users\Cookie\AppData\Local\Programs\Python\Python313\Lib\site-packages\starlette\middleware\errors.py", line 164, in __call__
    await self.app(scope, receive, _send)
  File "C:\Users\Cookie\AppData\Local\Programs\Python\Python313\Lib\site-packages\starlette\middleware\base.py", line 182, in __call__
    with recv_stream, send_stream, collapse_excgroups():
                                   ~~~~~~~~~~~~~~~~~~^^
  File "C:\Users\Cookie\AppData\Local\Programs\Python\Python313\Lib\contextlib.py", line 162, in __exit__
    self.gen.throw(value)
    ~~~~~~~~~~~~~~^^^^^^^
  File "C:\Users\Cookie\AppData\Local\Programs\Python\Python313\Lib\site-packages\starlette\_utils.py", line 83, in collapse_excgroups
    raise exc
  File "C:\Users\Cookie\AppData\Local\Programs\Python\Python313\Lib\site-packages\starlette\middleware\base.py", line 184, in __call__
    response = await self.dispatch_func(request, call_next)
               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "C:\Users\Cookie\AppData\Local\Programs\Python\Python313\Lib\site-packages\nicegui\middlewares.py", line 23, in dispatch
    response = await call_next(request)
               ^^^^^^^^^^^^^^^^^^^^^^^^
  File "C:\Users\Cookie\AppData\Local\Programs\Python\Python313\Lib\site-packages\starlette\middleware\base.py", line 159, in call_next
    raise app_exc
  File "C:\Users\Cookie\AppData\Local\Programs\Python\Python313\Lib\site-packages\starlette\middleware\base.py", line 144, in coro
    await self.app(scope, receive_or_disconnect, send_no_error)
  File "C:\Users\Cookie\AppData\Local\Programs\Python\Python313\Lib\site-packages\starlette\middleware\base.py", line 182, in __call__
    with recv_stream, send_stream, collapse_excgroups():
                                   ~~~~~~~~~~~~~~~~~~^^
  File "C:\Users\Cookie\AppData\Local\Programs\Python\Python313\Lib\contextlib.py", line 162, in __exit__
    self.gen.throw(value)
    ~~~~~~~~~~~~~~^^^^^^^
  File "C:\Users\Cookie\AppData\Local\Programs\Python\Python313\Lib\site-packages\starlette\_utils.py", line 83, in collapse_excgroups
    raise exc
  File "C:\Users\Cookie\AppData\Local\Programs\Python\Python313\Lib\site-packages\starlette\middleware\base.py", line 184, in __call__
    response = await self.dispatch_func(request, call_next)
               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "C:\Users\Cookie\AppData\Local\Programs\Python\Python313\Lib\site-packages\nicegui\middlewares.py", line 13, in dispatch
    response = await call_next(request)
               ^^^^^^^^^^^^^^^^^^^^^^^^
  File "C:\Users\Cookie\AppData\Local\Programs\Python\Python313\Lib\site-packages\starlette\middleware\base.py", line 160, in call_next
    raise RuntimeError("No response returned.")
RuntimeError: No response returned.
ERROR:    Exception in ASGI application
  + Exception Group Traceback (most recent call last):
  |   File "C:\Users\Cookie\AppData\Local\Programs\Python\Python313\Lib\site-packages\starlette\_utils.py", line 77, in collapse_excgroups
  |     yield
  |   File "C:\Users\Cookie\AppData\Local\Programs\Python\Python313\Lib\site-packages\starlette\middleware\base.py", line 183, in __call__
  |     async with anyio.create_task_group() as task_group:
  |                ~~~~~~~~~~~~~~~~~~~~~~~^^
  |   File "C:\Users\Cookie\AppData\Local\Programs\Python\Python313\Lib\site-packages\anyio\_backends\_asyncio.py", line 772, in __aexit__
  |     raise BaseExceptionGroup(
  |         "unhandled errors in a TaskGroup", self._exceptions
  |     ) from None
  | ExceptionGroup: unhandled errors in a TaskGroup (1 sub-exception)
  +-+---------------- 1 ----------------
    | Traceback (most recent call last):
    |   File "C:\Users\Cookie\AppData\Local\Programs\Python\Python313\Lib\site-packages\uvicorn\protocols\http\httptools_impl.py", line 409, in run_asgi
    |     result = await app(  # type: ignore[func-returns-value]
    |              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |         self.scope, self.receive, self.send
    |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |     )
    |     ^
    |   File "C:\Users\Cookie\AppData\Local\Programs\Python\Python313\Lib\site-packages\uvicorn\middleware\proxy_headers.py", line 60, in __call__
    |     return await self.app(scope, receive, send)
    |            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |   File "C:\Users\Cookie\AppData\Local\Programs\Python\Python313\Lib\site-packages\fastapi\applications.py", line 1054, in __call__
    |     await super().__call__(scope, receive, send)
    |   File "C:\Users\Cookie\AppData\Local\Programs\Python\Python313\Lib\site-packages\starlette\applications.py", line 113, in __call__
    |     await self.middleware_stack(scope, receive, send)
    |   File "C:\Users\Cookie\AppData\Local\Programs\Python\Python313\Lib\site-packages\starlette\middleware\errors.py", line 186, in __call__
    |     raise exc
    |   File "C:\Users\Cookie\AppData\Local\Programs\Python\Python313\Lib\site-packages\starlette\middleware\errors.py", line 164, in __call__
    |     await self.app(scope, receive, _send)
    |   File "C:\Users\Cookie\AppData\Local\Programs\Python\Python313\Lib\site-packages\starlette\middleware\base.py", line 182, in __call__
    |     with recv_stream, send_stream, collapse_excgroups():
    |                                    ~~~~~~~~~~~~~~~~~~^^
    |   File "C:\Users\Cookie\AppData\Local\Programs\Python\Python313\Lib\contextlib.py", line 162, in __exit__
    |     self.gen.throw(value)
    |     ~~~~~~~~~~~~~~^^^^^^^
    |   File "C:\Users\Cookie\AppData\Local\Programs\Python\Python313\Lib\site-packages\starlette\_utils.py", line 83, in collapse_excgroups
    |     raise exc
    |   File "C:\Users\Cookie\AppData\Local\Programs\Python\Python313\Lib\site-packages\starlette\middleware\base.py", line 184, in __call__
    |     response = await self.dispatch_func(request, call_next)
    |                ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |   File "C:\Users\Cookie\AppData\Local\Programs\Python\Python313\Lib\site-packages\nicegui\middlewares.py", line 23, in dispatch
    |     response = await call_next(request)
    |                ^^^^^^^^^^^^^^^^^^^^^^^^
    |   File "C:\Users\Cookie\AppData\Local\Programs\Python\Python313\Lib\site-packages\starlette\middleware\base.py", line 159, in call_next
    |     raise app_exc
    |   File "C:\Users\Cookie\AppData\Local\Programs\Python\Python313\Lib\site-packages\starlette\middleware\base.py", line 144, in coro
    |     await self.app(scope, receive_or_disconnect, send_no_error)
    |   File "C:\Users\Cookie\AppData\Local\Programs\Python\Python313\Lib\site-packages\starlette\middleware\base.py", line 182, in __call__
    |     with recv_stream, send_stream, collapse_excgroups():
    |                                    ~~~~~~~~~~~~~~~~~~^^
    |   File "C:\Users\Cookie\AppData\Local\Programs\Python\Python313\Lib\contextlib.py", line 162, in __exit__
    |     self.gen.throw(value)
    |     ~~~~~~~~~~~~~~^^^^^^^
    |   File "C:\Users\Cookie\AppData\Local\Programs\Python\Python313\Lib\site-packages\starlette\_utils.py", line 83, in collapse_excgroups
    |     raise exc
    |   File "C:\Users\Cookie\AppData\Local\Programs\Python\Python313\Lib\site-packages\starlette\middleware\base.py", line 184, in __call__
    |     response = await self.dispatch_func(request, call_next)
    |                ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |   File "C:\Users\Cookie\AppData\Local\Programs\Python\Python313\Lib\site-packages\nicegui\middlewares.py", line 13, in dispatch
    |     response = await call_next(request)
    |                ^^^^^^^^^^^^^^^^^^^^^^^^
    |   File "C:\Users\Cookie\AppData\Local\Programs\Python\Python313\Lib\site-packages\starlette\middleware\base.py", line 160, in call_next
    |     raise RuntimeError("No response returned.")
    | RuntimeError: No response returned.
    +------------------------------------

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "C:\Users\Cookie\AppData\Local\Programs\Python\Python313\Lib\site-packages\uvicorn\protocols\http\httptools_impl.py", line 409, in run_asgi
    result = await app(  # type: ignore[func-returns-value]
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
        self.scope, self.receive, self.send
        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    )
    ^
  File "C:\Users\Cookie\AppData\Local\Programs\Python\Python313\Lib\site-packages\uvicorn\middleware\proxy_headers.py", line 60, in __call__
    return await self.app(scope, receive, send)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "C:\Users\Cookie\AppData\Local\Programs\Python\Python313\Lib\site-packages\fastapi\applications.py", line 1054, in __call__
    await super().__call__(scope, receive, send)
  File "C:\Users\Cookie\AppData\Local\Programs\Python\Python313\Lib\site-packages\starlette\applications.py", line 113, in __call__
    await self.middleware_stack(scope, receive, send)
  File "C:\Users\Cookie\AppData\Local\Programs\Python\Python313\Lib\site-packages\starlette\middleware\errors.py", line 186, in __call__
    raise exc
  File "C:\Users\Cookie\AppData\Local\Programs\Python\Python313\Lib\site-packages\starlette\middleware\errors.py", line 164, in __call__
    await self.app(scope, receive, _send)
  File "C:\Users\Cookie\AppData\Local\Programs\Python\Python313\Lib\site-packages\starlette\middleware\base.py", line 182, in __call__
    with recv_stream, send_stream, collapse_excgroups():
                                   ~~~~~~~~~~~~~~~~~~^^
  File "C:\Users\Cookie\AppData\Local\Programs\Python\Python313\Lib\contextlib.py", line 162, in __exit__
    self.gen.throw(value)
    ~~~~~~~~~~~~~~^^^^^^^
  File "C:\Users\Cookie\AppData\Local\Programs\Python\Python313\Lib\site-packages\starlette\_utils.py", line 83, in collapse_excgroups
    raise exc
  File "C:\Users\Cookie\AppData\Local\Programs\Python\Python313\Lib\site-packages\starlette\middleware\base.py", line 184, in __call__
    response = await self.dispatch_func(request, call_next)
               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "C:\Users\Cookie\AppData\Local\Programs\Python\Python313\Lib\site-packages\nicegui\middlewares.py", line 23, in dispatch
    response = await call_next(request)
               ^^^^^^^^^^^^^^^^^^^^^^^^
  File "C:\Users\Cookie\AppData\Local\Programs\Python\Python313\Lib\site-packages\starlette\middleware\base.py", line 159, in call_next
    raise app_exc
  File "C:\Users\Cookie\AppData\Local\Programs\Python\Python313\Lib\site-packages\starlette\middleware\base.py", line 144, in coro
    await self.app(scope, receive_or_disconnect, send_no_error)
  File "C:\Users\Cookie\AppData\Local\Programs\Python\Python313\Lib\site-packages\starlette\middleware\base.py", line 182, in __call__
    with recv_stream, send_stream, collapse_excgroups():
                                   ~~~~~~~~~~~~~~~~~~^^
  File "C:\Users\Cookie\AppData\Local\Programs\Python\Python313\Lib\contextlib.py", line 162, in __exit__
    self.gen.throw(value)
    ~~~~~~~~~~~~~~^^^^^^^
  File "C:\Users\Cookie\AppData\Local\Programs\Python\Python313\Lib\site-packages\starlette\_utils.py", line 83, in collapse_excgroups
    raise exc
  File "C:\Users\Cookie\AppData\Local\Programs\Python\Python313\Lib\site-packages\starlette\middleware\base.py", line 184, in __call__
    response = await self.dispatch_func(request, call_next)
               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "C:\Users\Cookie\AppData\Local\Programs\Python\Python313\Lib\site-packages\nicegui\middlewares.py", line 13, in dispatch
    response = await call_next(request)
               ^^^^^^^^^^^^^^^^^^^^^^^^
  File "C:\Users\Cookie\AppData\Local\Programs\Python\Python313\Lib\site-packages\starlette\middleware\base.py", line 160, in call_next
    raise RuntimeError("No response returned.")
RuntimeError: No response returned.

[python-and-novella]

@falkoschindler I reproduce it, I use windows pc:

from nicegui import ui

ui.video('test.mp4')
ui.video('test.mp4')

ui.run(native=True,reload=False)

[falkoschindler]

Thanks everyone! There are a few things we can try to narrow it down:

1. Can you, please, try the following pywebview app (requires test.mp4 in the same directory):

   import threading
   
   import uvicorn
   import webview
   from fastapi import FastAPI
   from fastapi.responses import FileResponse, HTMLResponse
   
   app = FastAPI()
   
   @app.get('/', response_class=HTMLResponse)
   def index():
       return HTMLResponse('''
           <html>
               <body>
                   <video controls width="100%"><source src="/media/test.mp4" type="video/mp4"></video>
                   <video controls width="100%"><source src="/media/test.mp4" type="video/mp4"></video>
               </body>
           </html>
       ''')
   
   @app.get('/media/{filename}')
   def media(filename: str):
       return FileResponse(filename, media_type='video/mp4')
   
   if __name__ == '__main__':
       threading.Thread(target=lambda: uvicorn.run(app, host='0.0.0.0', port=8000), daemon=True).start()
   
       webview.create_window('Test', 'http://localhost:8000')
       webview.start()

2. What happens if you set ui.video.SOURCE_IS_MEDIA_FILE = False at the top of the NiceGUI example?

3. Does adding a main guard if __name__ == '__main__': or if __name__ in {'__main__', '__mp_main__'}: help?

[python-and-novella]

For 1, it works well, no errros.
For 2, it reports this:

Exception in callback _ProactorBasePipeTransport._call_connection_lost(None)
handle: <Handle _ProactorBasePipeTransport._call_connection_lost(None)>
Traceback (most recent call last):
  File "D:\Programs\Python\Python312\Lib\asyncio\events.py", line 88, in _run
    self._context.run(self._callback, *self._args)
  File "D:\Programs\Python\Python312\Lib\asyncio\proactor_events.py", line 165, in _call_connection_lost
    self._sock.shutdown(socket.SHUT_RDWR)
ConnectionResetError: [WinError 10054] 远程主机强迫关闭了一个现有的连接。
Exception in callback _ProactorBasePipeTransport._call_connection_lost(None)
handle: <Handle _ProactorBasePipeTransport._call_connection_lost(None)>
Traceback (most recent call last):
  File "D:\Programs\Python\Python312\Lib\asyncio\events.py", line 88, in _run
    self._context.run(self._callback, *self._args)
  File "D:\Programs\Python\Python312\Lib\asyncio\proactor_events.py", line 165, in _call_connection_lost
    self._sock.shutdown(socket.SHUT_RDWR)
ConnectionResetError: [WinError 10054] 远程主机强迫关闭了一个现有的连接。

For 3, it has no effects.

[python-and-novella]

BTW, run the following code, it works well in browser.

from nicegui import ui

ui.video('test.mp4')
ui.video('test.mp4')

ui.run(native=False,reload=False)

But use the pywebview to view it, it will report error:

import webview
webview.create_window('test', 'http://localhost:8080')
webview.start()

[MaidScientistIzutsumiMarin]

1. It works, although I do inconsistently get the following error. That said, I believe this is a problem with the default Windows event loop policy and not NiceGUI. It appears 7 out of 10 test launches; 1 had it twice.

Exception in callback _ProactorBasePipeTransport._call_connection_lost()
handle: <Handle _ProactorBasePipeTransport._call_connection_lost()>
Traceback (most recent call last):
  File "C:\Program Files\WindowsApps\PythonSoftwareFoundation.Python.3.13_3.13.1520.0_x64__qbz5n2kfra8p0\Lib\asyncio\events.py", line 89, in _run
    self._context.run(self._callback, *self._args)
    ~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "C:\Program Files\WindowsApps\PythonSoftwareFoundation.Python.3.13_3.13.1520.0_x64__qbz5n2kfra8p0\Lib\asyncio\proactor_events.py", line 165, in _call_connection_lost
    self._sock.shutdown(socket.SHUT_RDWR)
    ~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^
ConnectionResetError: [WinError 10054] An existing connection was forcibly closed by the remote host

2. No changes in behavior or console. 8 out of 10 test launches had the error; 4 had it twice.
3. No changes in behavior or console.
   • With if __name__ == '__main__':, 8 out of 10 test launches had the error; 2 had it twice.
   • With if __name__ in {'__main__', '__mp_main__'}:, 7 out of 10 test launches had the error; 0 had it twice.

I don't think the chance of getting the error actually changed at all between any of the versions. The error only happened after one of these lines:

INFO:     127.0.0.1:51551 - "GET /media/test.mp4 HTTP/1.1" 206 Partial Content

Here is the full output from one of the runs that had two errors:

INFO:     Started server process [2476]
INFO:     Waiting for application startup.
INFO:     Application startup complete.
INFO:     Uvicorn running on http://0.0.0.0:8000 (Press CTRL+C to quit)
INFO:     127.0.0.1:51622 - "GET / HTTP/1.1" 200 OK
INFO:     127.0.0.1:51625 - "GET /media/test.mp4 HTTP/1.1" 206 Partial Content
Exception in callback _ProactorBasePipeTransport._call_connection_lost()
handle: <Handle _ProactorBasePipeTransport._call_connection_lost()>
Traceback (most recent call last):
  File "C:\Program Files\WindowsApps\PythonSoftwareFoundation.Python.3.13_3.13.1520.0_x64__qbz5n2kfra8p0\Lib\asyncio\events.py", line 89, in _run
    self._context.run(self._callback, *self._args)
    ~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "C:\Program Files\WindowsApps\PythonSoftwareFoundation.Python.3.13_3.13.1520.0_x64__qbz5n2kfra8p0\Lib\asyncio\proactor_events.py", line 165, in _call_connection_lost
    self._sock.shutdown(socket.SHUT_RDWR)
    ~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^
ConnectionResetError: [WinError 10054] An existing connection was forcibly closed by the remote host
INFO:     127.0.0.1:51627 - "GET /media/test.mp4 HTTP/1.1" 206 Partial Content
Exception in callback _ProactorBasePipeTransport._call_connection_lost()
handle: <Handle _ProactorBasePipeTransport._call_connection_lost()>
Traceback (most recent call last):
  File "C:\Program Files\WindowsApps\PythonSoftwareFoundation.Python.3.13_3.13.1520.0_x64__qbz5n2kfra8p0\Lib\asyncio\events.py", line 89, in _run
    self._context.run(self._callback, *self._args)
    ~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "C:\Program Files\WindowsApps\PythonSoftwareFoundation.Python.3.13_3.13.1520.0_x64__qbz5n2kfra8p0\Lib\asyncio\proactor_events.py", line 165, in _call_connection_lost
    self._sock.shutdown(socket.SHUT_RDWR)
    ~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^
ConnectionResetError: [WinError 10054] An existing connection was forcibly closed by the remote host
INFO:     127.0.0.1:51629 - "GET /media/test.mp4 HTTP/1.1" 206 Partial Content
INFO:     127.0.0.1:51629 - "GET /media/test.mp4 HTTP/1.1" 206 Partial Content
INFO:     127.0.0.1:51629 - "GET /favicon.ico HTTP/1.1" 404 Not Found

[falkoschindler]

Thanks for the update!

I've been chatting with ChatGPT about the problem and it keeps coming up with a race condition in Starlette which has been fixed in version 0.38.3. Do you, by any chance, have an older Starlette version installed?

[python-and-novella]

I installed starlette == 0.47.2 ,
The environment is:

Package            Version
------------------ -----------
aiofiles           24.1.0
aiohappyeyeballs   2.6.1
aiohttp            3.12.14
aiosignal          1.4.0
annotated-types    0.7.0
anyio              4.9.0
attrs              25.3.0
bidict             0.23.1
bottle             0.13.4
certifi            2025.7.14
cffi               1.17.1
click              8.2.1
clr-loader         0.2.7.post0
colorama           0.4.6
contourpy          1.3.2
cycler             0.12.1
docutils           0.20.1
fastapi            0.116.1
fonttools          4.59.0
frozenlist         1.7.0
h11                0.16.0
httpcore           1.0.9
httptools          0.6.4
httpx              0.28.1
idna               3.10
ifaddr             0.2.0
iniconfig          2.1.0
itsdangerous       2.2.0
jinja2             3.1.6
kiwisolver         1.4.8
libsass            0.23.0
markdown-it-py     3.0.0
markdown2          2.5.3
markupsafe         3.0.2
matplotlib         3.10.3
mdurl              0.1.2
multidict          6.6.3
narwhals           1.48.0
nicegui            2.21.1
nicegui-highcharts 2.1.0
numpy              2.3.1
orjson             3.11.0
packaging          25.0
pillow             11.3.0
pip                25.1.1
plotly             6.2.0
pluggy             1.6.0
propcache          0.3.2
proxy-tools        0.1.0
pscript            0.7.7
pycparser          2.22
pydantic           2.11.7
pydantic-core      2.33.2
pygments           2.19.2
pyparsing          3.2.3
pytest             8.4.1
python-dateutil    2.9.0.post0
python-dotenv      1.1.1
python-engineio    4.12.2
python-multipart   0.0.20
python-socketio    5.13.0
pythonnet          3.0.5
pywebview          5.4
pyyaml             6.0.2
redis              6.2.0
rich               14.0.0
setuptools         80.9.0
simple-websocket   1.1.0
six                1.17.0
sniffio            1.3.1
starlette          0.47.2
typing-extensions  4.14.1
typing-inspection  0.4.1
uvicorn            0.35.0
vbuild             0.8.2
watchfiles         1.1.0
websockets         15.0.1
wsproto            1.2.0
yarl               1.20.1

I hope it will help you.

[MaidScientistIzutsumiMarin]

My version of starlette is also 0.47.2:

Package               Version
--------------------- -----------
aiofiles              24.1.0
aiohappyeyeballs      2.6.1
aiohttp               3.12.14
aiosignal             1.4.0
annotated-types       0.7.0
anyio                 4.9.0
attrs                 25.3.0
bidict                0.23.1
bottle                0.13.4
certifi               2025.7.14
cffi                  1.17.1
charset-normalizer    3.4.2
click                 8.2.1
clr_loader            0.2.7.post0
colorama              0.4.6
colorlog              6.9.0
distro                1.9.0
docutils              0.21.2
fastapi               0.116.1
ffmpeg_normalize      1.32.5
ffmpeg_progress_yield 1.0.1
filetype              1.2.0
flit                  3.12.0
flit_core             3.12.0
fontra                0.5.2
freetype-py           2.5.1
frozenlist            1.7.0
h11                   0.16.0
httpcore              1.0.9
httptools             0.6.4
httpx                 0.28.1
idna                  3.10
ifaddr                0.2.0
itsdangerous          2.2.0
jaraco.classes        3.4.0
jaraco.context        6.0.1
jaraco.functools      4.2.1
Jinja2                3.1.6
jiter                 0.10.0
keyring               25.6.0
markdown-it-py        3.0.0
markdown2             2.5.3
MarkupSafe            3.0.2
mdurl                 0.1.2
more-itertools        10.7.0
multidict             6.6.3
mutagen               1.47.0
nicegui               2.21.1
oauthlib              3.3.1
openai                1.97.1
orjson                3.11.0
pip                   25.1.1
platformdirs          4.3.8
propcache             0.3.2
proxy_tools           0.1.0
pscript               0.7.7
pwinput               1.0.3
pycparser             2.22
pydantic              2.11.7
pydantic_core         2.33.2
pydantic-extra-types  2.10.5
Pygments              2.19.2
python-dotenv         1.1.1
python-engineio       4.12.2
python-multipart      0.0.20
python-socketio       5.13.0
pythonnet             3.0.5
pytimeparse2          1.7.1
pywebview             5.4
pywin32-ctypes        0.2.3
PyYAML                6.0.2
regex                 2024.11.6
requests              2.32.4
requests-oauthlib     2.0.0
rich                  14.0.0
shellingham           1.5.4
simple-websocket      1.1.0
sniffio               1.3.1
starlette             0.47.2
tiktoken              0.9.0
tomli_w               1.2.0
tomlkit               0.13.3
tqdm                  4.67.1
tumblr-backup         1.0.7
typed-ffmpeg          3.6
typer                 0.16.0
typing_extensions     4.14.1
typing-inspection     0.4.1
urllib3               2.5.0
uvicorn               0.35.0
vbuild                0.8.2
watchfiles            1.1.0
websockets            15.0.1
wsproto               1.2.0
yarl                  1.20.1
yt-dlp                2025.7.21

[falkoschindler]

Ok, it looks like the RuntimeError originates from multiple range requests sent and cancelled by the browser. This is ChatGPT's summary:

RuntimeError("No response returned.") is still raised in Starlette when the client drops the connection before a response body is produced. Browsers do this with <video> elements by firing several Range requests and cancelling most of them immediately. You only need a couple of videos for the pattern to appear—size and duration are irrelevant. Two 10‑second videos can easily generate half a dozen short‑lived requests that get aborted, triggering the exception.

So: latest Starlette doesn’t “fix” it; it still bubbles up that disconnect as a RuntimeError. Whether that’s a bug or just noisy logging is debatable, but either way you can safely ignore or intercept it.

It suggests adding a middleware to "swallow the disconnect exception":

import anyio
from starlette.responses import Response
from starlette.middleware.base import BaseHTTPMiddleware
from nicegui import app

class IgnoreClientDisconnect(BaseHTTPMiddleware):
    async def dispatch(self, request, call_next):
        try:
            return await call_next(request)
        except RuntimeError as exc:
            if str(exc) == "No response returned.":
                # client closed the connection; return a 499-like response
                return Response(status_code=499)
            raise
        except anyio.EndOfStream:
            return Response(status_code=499)

app.add_middleware(IgnoreClientDisconnect)

Can you check if this helps? Of course it is still debatable how the ultimate solution might look like. But this might give us a hint if we are on the right track. Thanks!

[python-and-novella]

Yes, it works well, no more errors after I restart it cleanly.

[MaidScientistIzutsumiMarin]

That works for us, too. There is still noticeable lag in the UI when loading a lot of videos, but at least the error log is clean.