Simon Willison’s Weblog

Bringing Python to Workers using Pyodide and WebAssembly (via) Cloudflare Workers is Cloudflare’s serverless hosting tool for deploying server-side functions to edge locations in their CDN.

They just released Python support, accompanied by an extremely thorough technical explanation of how they got that to work. The details are fascinating.

Workers runs on V8 isolates, and the new Python support was implemented using Pyodide (CPython compiled to WebAssembly) running inside V8.

Getting this to work performantly and ergonomically took a huge amount of work.

There are too many details in here to effectively summarize, but my favorite detail is this one:

“We scan the Worker’s code for import statements, execute them, and then take a snapshot of the Worker’s WebAssembly linear memory. Effectively, we perform the expensive work of importing packages at deploy time, rather than at runtime.”