Running Python ASGI apps in the browser via Pyodide + a service worker

# Research: Running Python ASGI apps in the browser via Pyodide + a service worker Source: [https://simonwillison.net/2026/May/30/pyodide-asgi-browser/](https://simonwillison.net/2026/May/30/pyodide-asgi-browser/) [Research](https://simonwillison.net/elsewhere/research/)[Running Python ASGI apps in the browser via Pyodide \+ a service worker](https://github.com/simonw/research/tree/main/pyodide-asgi-browser#readme)— By running Python ASGI web applications entirely in the browser using Pyodide and a dedicated service worker, this project intercepts all same\-origin requests under \`/app/\` and executes them against the Python app via the ASGI protocol—removing the need for a backend server except for static files\. The mechanism is demonstrated with both a FastAPI demo and the full Datasette app, confirming its generality across ASGI apps\. [Datasette Lite](https://lite.datasette.io/)is my version of Datasette that runs entirely in the browser using Pyodide in WebAssembly\. When I first built it[four years ago](https://simonwillison.net/2022/May/4/datasette-lite/)I used Web Workers and code that intercepts navigation operations and fetches the generated HTML by running the Python app\. This worked, but had the disadvantage that any JavaScript in`<script\>`tags would not be executed \- breaking some Datasette functionality and a whole lot of Datasette plugins\. This morning I[set Claude Opus 4\.8 the task](https://github.com/simonw/research/pull/112)\(in Claude Code for web\) of figuring out how to run Python ASGI apps in Pyodide using Service Workers instead, and it seems to work\! Here's a[basic ASGI FastCGI demo](https://simonw.github.io/research/pyodide-asgi-browser/)and here's[a demo that runs Datasette 1\.0a31](https://simonw.github.io/research/pyodide-asgi-browser/datasette.html)\. I'm still getting my head around exactly how it works, but once I've done that I plan to upgrade Datasette Lite itself\.