Using Pyodide from a web worker

This document describes how to use pyodide to execute python scripts asynchronously in a web worker.


Setup your project to serve webworker.js. You should also serve pyodide.js, and all its associated .asm.js, .data, .json, and .wasm files as well, though this is not strictly required if pyodide.js is pointing to a site serving current versions of these files.

Update the webworker.js sample so that it has as valid URL for pyodide.js, and sets self.languagePluginUrl to the location of the supporting files.

In your application code create a web worker, and add listeners for onerror and onmessage.

Call postMessage on your web worker, passing an object with the key python containing the script to execute as a string. You may pass other keys in the data object. By default the web worker assigns these to its global scope so that they may be imported from python. The results are returned as the results key, or if an error was encountered, it is returned in the error key.

For example:

var pyodideWorker = new Worker('./webworker.js')

pyodideWorker.onerror = (e) => {
  console.log(`Error in pyodideWorker at ${e.filename}, Line: ${e.lineno}, ${e.message}`)

pyodideWorker.onmessage = (e) => {
  const {results, error} =
  if (results) {
    console.log('pyodideWorker return results: ', results)
  } else if (error) {
    console.log('pyodideWorker error: ', error)

const data = {
  A_rank: [0.8, 0.4, 1.2, 3.7, 2.6, 5.8],
    'import statistics\n' +
    'from js import A_rank\n' +


See Loading Python packages for instructions on how to load packages.

For example, to always load packages numpy and pytz, you would insert the line self.pyodide.loadPackage(['numpy', 'pytz']).then(() => { as shown below:

self.languagePluginUrl = 'http://localhost:8000/'

var onmessage = function(e) { // eslint-disable-line no-unused-vars
  languagePluginLoader.then(() => {
    self.pyodide.loadPackage(['numpy', 'pytz']).then(() => {
      const data =;
      const keys = Object.keys(data);
      for (let key of keys) {
        if (key !== 'python') {
          // Keys other than python must be arguments for the python script.
          // Set them on self, so that `from js import key` works.
          self[key] = data[key];
      self.pyodide.runPythonAsync(data.python, () => {})
          .then((results) => { self.postMessage({results}); })
          .catch((err) => {
            // if you prefer messages with the error
            self.postMessage({error : err.message});
            // if you prefer onerror events
            // setTimeout(() => { throw err; });


Using a web worker is advantageous because the python code is run in a separate thread from your main UI, and hence does not impact your application’s responsiveness. There are some limitations, however. At present, Pyodide does not support sharing the Python interpreter and packages between multiple web workers or with your main thread. Since web workers are each in their own virtual machine, you also cannot share globals between a web worker and your main thread. Finally, although the web worker is separate from your main thread, the web worker is itself single threaded, so only one python script will execute at a time.