Multiprocessing issues with uv tool operations

[mrasore98]

First Check

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Example Code

# pyproject.toml
[tool.poetry.scripts]
myscript = "myapp.ui.app:run"


# myapp.ui.app
def run():
    configure_ui()
    configure_file_logging()
    logger.info("STARTING APPLICATION", internal=True)
    ui.run(
        native=True,  # have also tried with False
        dark=False,
        binding_refresh_interval=0.08,
        reload=False,  # have also tried with default (True)
    )
    logger.info("APPLICATION CLOSED", internal=True)

Description

I am trying to use the uv tool install command as a means of distributing the application, instead of using pyinstaller, but am running into issues with multiprocessing. To make sure it runs with uv, I've been testing by running the following steps:

1. poetry build
2. uvx --from dist/myapp-....-.whl myscript

When running with poetry run myscript all is well, but with uvx I get the "STARTING APPLICATION" log message (indicating the entry point has been called successfully) before I get the following error.

RuntimeError:

You must call ui.run() to start the server.
If ui.run() is behind a main guard
   if __name__ == "__main__":
remove the guard or replace it with
   if __name__ in {"__main__", "__mp_main__"}:
to allow for multiprocessing.

ERROR:    Application startup failed. Exiting.

I believe this is because uv tool spawns a new process, which might interfere with this logic from nicegui.ui_run.py lines 161-162:

    if multiprocessing.current_process().name != 'MainProcess':
        return

Q1: Can someone please explain why these lines are necessary?
Q2: Is my assessment correct that these are the offending lines causing the incompatibility issues between nicegui and uv?
Q3: Any solutions come to mind to fix this?

NiceGUI Version

2.16.1

Python Version

3.12.0

Browser

Chrome, Other

Operating System

Windows

Additional Context

$ uv --version
uv 0.6.2 (6d3614eec 2025-02-19)

$ poetry --version
Poetry (version 1.7.1)

[rodja]

Interesting. This might be related to #4613.

[mrasore98]

Seems like it. I'll try the fix in that PR and continue the discussion there.

[falkoschindler]

The question remains, why multiprocessing.current_process().name is not "MainProcess" in your situation. Usually uvx shouldn't change the fact that the process is named "MainProcess", as you can verify with this line:

uvx python -c "import multiprocessing,sys; print(multiprocessing.current_process().name)"

[mrasore98]

I was able to verify uvx does run as the MainProcess using this logic from within my app. Since opening this discussion, I updated uv to 0.7.2 and refreshed the cache and that seemed to fix my issue (still using NiceGUI 2.16.1)