Reloader is breaking the execution if using Poetry scripts

[delucca]

The Flask reloader is apparently using the executed command instead of the actual Python package. When I try running with Poetry scripts, it throws an error (only if the reloader is enabled)

To replicate the behavior you can create a new Poetry Python package, install Flask and create the following Poetry script:

[tool.poetry.scripts]
dev = 'my_package.main:run'

(as you can imagine, you need to have a run function inside a file called main.py within a folder called my_package that will create the app and run it)

Now, you can set FLASK_DEBUG=0 and run it with poetry run dev. It will work (as expected).

After this, you can set FLASK_DEBUG=1 and run the same command, it will fail saying that it wasn't able to find the dev file. I've explored it a little bit, and the problem isn't the debugger, but the reloader (if you launch with the debugger, but without the reloader it works)

It seems the reloader somehow uses the executed command (dev) as the expected Python file to be used upon reloading and, because of that, it can't find the file and throws an error.

Environment:

• Python version: 3.10
• Flask version: 2.2.2

[davidism]

If I modify Werkzeug's reloader to print out sys.argv and sys.orig_argv, it shows this:

sys.argv = ['dev']
sys.orig_argv = ['/home/david/.cache/pypoetry/virtualenvs/hello-y1FJtfO2-py3.10/bin/python', '-c', "import sys; from importlib import import_module; sys.argv = ['dev']; sys.exit(import_module('hello').run())"]

This points to an issue with Poetry, it is breaking the documented behavior of sys.argv by setting sys.argv = ["dev"]; it should either set it to ["-c"] or to the filename of the module it imported. Please report that to Poetry.

That said, Werkzeug still wouldn't support this, as the python -c 'code' form of invoking Python does not put enough information in sys.argv to recreate the command. I was already looking at sys.orig_argv, but that was only added in Python 3.10.

Since you're using Flask, you should use the flask command to run the development server, rather than creating your own entry point.

[davidism]

I opened #2589 to better describe what I was working on already. It will fix this issue as well. The sys.argv issue I described above should still be reported to Poetry.

[delucca]

thanks @davidism for the detailed explanation. Tomorrow I'll open an issue on the Poetry repository 😁

Regarding using Flask CLI, I'm already doing that. I found that issue because I wanted to experiment with Poetry Scripts and accidentally found it 😁