Running Python ASGI apps in the browser via Pyodide + a service worker
Datasette Lite is a browser-based version of Datasette that utilizes Pyodide in WebAssembly. The new implementation uses Service Workers to run Python ASGI apps, overcoming limitations of the previous method that did not execute JavaScript in <script> tags. The author plans to upgrade Datasette Lite after further understanding the new approach.
- ▪Datasette Lite runs entirely in the browser using Pyodide in WebAssembly.
- ▪The previous implementation used Web Workers, which had limitations with JavaScript execution.
- ▪The new method employs Service Workers to run Python ASGI apps successfully.
Simon Willison files mainly under blogs. We currently carry 54 of its stories.
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Record
| Original publisher | Simon Willison |
| Canonical URL | https://simonwillison.net/2026/May/30/pyodide-asgi-browser/#atom-everything |
| Publication time | 2026-05-30T21:02:16+00:00 |
| Retrieval time | 2026-05-30T21:27:40.352Z |
| Last seen | 2026-05-30T21:27:40.352Z |
| Headline source | Publisher (no WeSearch rewrite) |
| Excerpt source | publisher body |
| Excerpt method | First ~120 words (~800 chars) of extracted publisher body, fair-use limited. |
| Summary | WeSearch · cerebras-chat (WeSearch summarizer) |
| Summary source text | contentText |
| Citation coverage | Summary is a WeSearch-generated derivative; primary citation is the original publisher URL. |
| Cluster | L1ovFqYi6xfU |
| Cluster logic | Grouped by semantic title/content similarity across sources within a rolling window. Same-publisher template collisions are excluded from coverage comparison. |
| Ranking reason | Story pages are not engagement-ranked. Hub feeds use recency, with optional source-diversified chronological ordering (cap consecutive stories per source). No personalized ranking. |
| Publisher visit | Yes — open original |
| Substitutes article? | No — link-out required for full text |
Rights status (four layers)
WeSearch handling by dimension
| Indexing | May the item be indexed (stored, ranked, made findable)? | Allowed |
| Snippet | May a short excerpt of the publisher's text be shown? | Allowed |
| AI summary | May WeSearch generate its own short summary of the article? | Limited |
| Retrieval / RAG | May the content be exposed for third-party retrieval-augmented generation? | Not asserted |
| Model training | May the content be used to train AI models? | Not asserted |
| Commercial reuse | May the content be reused commercially? | Not permitted |
Basis: Derived from the published RSS/Atom feed. Contact: [email protected]. Reviewed: 2026-07-24.
Opening excerpt (first ~120 words) tap to expand
Datasette Lite is my version of Datasette that runs entirely in the browser using Pyodide in WebAssembly. When I first built it four years ago 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 (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 and here's a demo that runs Datasette 1.0a31.
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Excerpt limited to ~120 words for fair-use compliance. The full article is at Simon Willison.
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