17-dynamic-py-py

17-dynamic-py-py — Python Worker loads a Python Dynamic Worker

This example shows how a Python Worker can use the Worker Loader binding to dynamically load and execute another Python Worker at runtime.

The parent Python Worker keeps the source of the dynamic Worker as a string (for demonstration purposes — in a real application you would load it from KV, R2, or any other storage). Each request is forwarded to the dynamically-loaded Worker, which runs in its own isolated sandbox.

Run it

uv run pywrangler dev

Then press b to open a browser, or:

curl http://localhost:8787/

You should see:

Hello from a dynamically-loaded Python Worker!

How it works

The parent Worker declares a worker_loaders binding in wrangler.jsonc:

"worker_loaders": [
  { "binding": "LOADER" }
]

In src/entry.py, the parent calls self.env.LOADER.get(id, callback). The callback returns a JS object describing the Worker to load — its compatibilityDate, compatibilityFlags, mainModule, and modules.

worker = self.env.LOADER.get(
    "hello-python-v1",
    create_proxy(_worker_code),
)
return await worker.getEntrypoint().fetch(request)

The runtime caches loaded isolates by ID, so the callback typically only runs the first time a given id is seen. To load updated code, change the id (for example, "hello-python-v2").

create_proxy() from pyodide.ffi is required to expose the Python callback to the JavaScript runtime. to_js(..., dict_converter=Object.fromEntries) converts the Python dict into a plain JS object so the loader receives the shape it expects.