Adding a Flask backend

[KDW1]

Added an alternative setting for the PathView application so that you can either use Pyodide web assembly or send client-side fetch requests to a Flask web server that has execution, evaluation, and traceback routes (the initialization code of the server borrows from the init() function found in worker.ts.

Major Changes:

• Added a backendPreferenceStore which is nearly identical in form to themeStore since it switches between two binary values. It is binded to 'Q' (arbitrarily so), and toggling it calls switchBackend(preference : BackendPreference) from backend.ts
• The backendPreferenceStore is accessed from backend.ts and passed in as an event parameter sent along with the "init" message posted to the worker. This sets the backendPreference for said worker
• Every function within worker.ts that previously called await pyodide.runPythonAsync(...) now has a fetch request counterpart, such that the Flask backend route is shallow copy of what pyodide was doing

Current issues:

• It seems like we're running multiple excess requests due to an error Scope: 'NoneType' object is not iterable which comes from the generateStreamingStartCode() function in pythonHelpers.ts, specifically the find_scopes() funciton

[KDW1]

TODOS

• Review the requirements for handleRun() in the main index page for the SvelteKit application, in terms of whether simulation is running or not, etc...
• Fix the issue where the Flask version doesn't interact with the console's stdout and stderror
• Fix the Scope: 'NoneType' object is not iterable error, which is most likely connected to the excess api calls!
• Finish pull request description

[milanofthe]

@KDW1 good start, basic exec/eval works and the toggle UI is there. But there are some architectural issues preventing this from working properly.

Worker imports stores:
Workers run in a separate thread and can't access main thread state. This is why you hit the localStorage/window errors. The typeof window === 'undefined' check masks it but doesn't fix it. Workers must be self-contained, pass state via postMessage.

Flask inside PyodideBackend:
The Backend interface exists so we can swap implementations cleanly. Flask shouldn't be a flag inside PyodideBackend, it should be its own FlaskBackend class implementing the same interface. No worker needed for Flask, just fetch calls from main thread.

Streaming broken:
This is why Scope returns (None, None) or partial data. The pyodide streaming works because the generator _streamer lives in the worker and maintains state between steps. With Flask each HTTP request is independent - the generator state doesn't persist reliably across requests. For Flask streaming you'd need SSE/WebSocket.

Minor:
Typo reosurces in backend.py CORS config, requirements.txt lists stdlib modules (gc, io, contextlib) which will fail pip install.

Suggestion:
Create a FlaskBackend class, remove store imports from worker.ts. Read the readme for the architecture. Or just copy it into chatgpt.

[KDW1]

Thank you for the help Milan. I appreciate the direction. I'll be sure to work on a separate Flask Backend class in backend.ts and address the rest of the issues. Thank you!

[KDW1]

TODOS (After Review)

• Create FlaskBackend class
• Remove store imports and flask alternatives in worker.ts to be completely removed from the flask backend class
• Work on an api route that focus on the Flask API streaming the data with SSE
• Fix the resources typo