Running Python Greenlets fully in WebAssembly thanks to JSPI

Syrus Akbary - Wasmer

Python powers a huge portion of today’s backend ecosystem, but until now running real-world Python applications on WebAssembly was fundamentally limited.

In this talk, we present full greenlet support for Python on WebAssembly, recently introduced in Wasmer. This breakthrough unlocks running production Python backends, including FastAPI, SQLAlchemy, and other greenlet-based stacks,on the edge and in the browser, without code changes or async rewrites.

Greenlets are stackful, user-space execution contexts that enable cooperative multitasking. While essential to much of the Python ecosystem, they have historically been impossible to support in WebAssembly due to the lack of native stack switching. As a result, millions of Python applications simply could not run on Wasm runtimes.

We’ll explain: - Why greenlets (and fibers) are fundamentally hard to support in WebAssembly - Why existing solutions like the Wasm Stack Switching proposal are not yet viable in browsers - How we enabled greenlets without stack switching, using a new design inspired by the JS Promise Integration (JSPI) proposal - The new async function API in Wasmer and the continuation system calls** that make this possible

We’ll close with live demos of Python greenlets running fully in the browser and on Wasmer Edge, and show how this unlocks deploying real Python backends: sandboxed, portable, and container-free.

This talk demonstrates a major step forward for Python on WebAssembly, and shows how Wasm is becoming a practical runtime for real backend workloads… not just toy examples.