Hint "NiceGUI doesn't work in REPL" doesn't show up anymore

[KrilleGH]

First Check

• I added a very descriptive title here.
• This is not a security issue (those should be reported via the security advisory instead).
• 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

ui.label('Lorem ipsum')

ui.run()

Description

I was busted by NiceGUI script mode not being supported in REPL.

To notice that more quickly #5456 added a message making this obvious. However it seems it doesn't show up anymore.

To reproduce:

• Open a Python REPL
• Type the provided example code

The result is this:

ERROR:    Traceback (most recent call last):
  File "/usr/local/lib/python3.13/site-packages/starlette/routing.py", line 694, in lifespan
    async with self.lifespan_context(app) as maybe_state:
               ~~~~~~~~~~~~~~~~~~~~~^^^^^
  File "/usr/local/lib/python3.13/contextlib.py", line 214, in __aenter__
    return await anext(self.gen)
           ^^^^^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.13/site-packages/nicegui/nicegui.py", line 31, in _lifespan
    await _startup()
  File "/usr/local/lib/python3.13/site-packages/nicegui/nicegui.py", line 122, in _startup
    raise RuntimeError('\n\n'
    ...<5 lines>...
                       'to allow for multiprocessing.')
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 would expect that the message added in that PR appears.

Furthermore, the second suggestion from that message, using a root function, doesn't seem to help either and leads to the same result. To reproduce type this code into the REPL:

from nicegui import ui

def root():
     ui.label('Lorem ipsum')

ui.run(root=root)

I would expect that NiceGUI starts up normally.

NiceGUI Version

3.6.1

Python Version

3.13.11

Browser

Firefox

Operating System

Linux

Additional Context

No response

[evnchn]

Hello @KrilleGH

Sorry but it works on my machine and I cannot recreate it. Any more hints as to what is going on?

[falkoschindler]

@evnchn I can reproduce the problem when running the REPL via uv run python, but not with plain python. And after quite some time thinking, Opus 4.5 found a solution. I verified it successfully. But we should check again more carefully and create a PR.

---

Problem:
In Python 3.13, the new _pyrepl REPL sets __main__.__file__ to an actual file path (/path/to/python3.13/_pyrepl/__main__.py), causing the reload detection to fail. With reload enabled, uvicorn spawns a subprocess that imports nicegui:app directly, where ui.run() was never called, resulting in the confusing error:

RuntimeError: You must call ui.run() to start the server.

Solution:
Changed the reload detection check from __main__.__file__ to sys.argv[0], which is consistently empty ('') in REPL/interactive contexts across all Python versions:

# Before:
if reload and not hasattr(__main__, '__file__'):

# After:
if reload and not helpers.is_file(sys.argv[0] if sys.argv else None):

Result:

• REPL/stdin/-c: Auto-reload is disabled, server starts correctly, proper REPL hint is shown
• Running from file: Auto-reload remains enabled as expected

Also fixed a typo in the warning message: "is is only" → "it is only"

[evnchn]

@falkoschindler but then there is the python -i my_script.py which runs the script then drop into interactive mode. Should the user then want to launch NiceGUI, this would foil the REPL-detection...

[evnchn]

Wait no. Where did Opus came up with __main__.__file__. The issue came with sys.argv parsing all along!

nicegui/nicegui/ui_run.py

Line 144 in 0cde546

if not sys.argv or not sys.argv[0] or not helpers.is_file(sys.argv[0]):

So we need to expand our check to cover the uv run python case, I believe

[evnchn]

https://stackoverflow.com/a/64523765

https://stackoverflow.com/a/2356427

I like bool(getattr(sys, 'ps1', sys.flags.interactive))

[KrilleGH]

Sorry I didn't mention how to start the interpreter, I didn't imagine it's so important.

I use the official Python Docker image, Trixie-Slim to be precise. I have my own Dockerfile which installs NiceGUI system-wide with pip. I entered the container with docker exec -it nicegui bash and just started python there.

There's no uv involved on my side, didn't find the time for that, but it seems it's another way to trigger that behaviour.

[KrilleGH]

And I'm a bit puzzled by __main__.__file__, I think it doesn't exist. At least not until there's an actual __main__.py file in the code, which NiceGUI doesn't seem to have. Is that an AI hallucination?

[falkoschindler]

__main__.__file__ should be defined when Python executes a script file. It is currently used by ui.run:

nicegui/nicegui/ui_run.py

Lines 216 to 218 in 0cde546

	if reload and not hasattr(__main__, '__file__'):
	log.warning('disabling auto-reloading because is is only supported when running from a file')
	core.app.config.reload = reload = False

But I'll have to look into this problem and Opus' and @evnchn's suggestions with a bit more time and focus next week. Yesterday I basically dumped my findings so that you can pick up from there if you want.