Using Pyodide#

Pyodide may be used in a web browser or a backend JavaScript environment.

Web browsers#

To use Pyodide in a web page you need to load pyodide.js and initialize Pyodide with loadPyodide().

<!doctype html>
      <script src=""></script>
    <script type="text/javascript">
      async function main(){
        let pyodide = await loadPyodide();
        console.log(pyodide.runPython("1 + 2"));

See the Getting started for a walk-through tutorial as well as Loading packages and Type translations for a more in depth discussion about existing capabilities.

You can also use the Pyodide NPM package to integrate Pyodide into your application.


To avoid confusion, note that:

  • distributes Python packages built with Pyodide as well as pyodide.js

  • is a mirror of the Pyodide NPM package, which includes none of the WASM files

Supported browsers#

Pyodide works in any modern web browser with WebAssembly support.

Tier 1 browsers are tested as part of the test suite with continuous integration,


Minimal supported version

Release date



22 October 2019



4 December 2018

Chrome 89 and 90 have bugs in the webassembly compiler which makes using Pyodide with them unstable. Known problems occur in numpy and have been observed occasionally in other packages. See #1384.


Latest browser versions generally provide more reliable WebAssembly support and will run Pyodide faster, so their use is recommended.

Tier 2 browsers are known to work, but they are not systematically tested in Pyodide,


Minimal supported version

Release date



15 September 2020



26 February 2020

Other browsers with WebAssembly support might also work however they are not officially supported.

Web Workers#

By default, WebAssembly runs in the main browser thread, and it can make UI non-responsive for long-running computations.

To avoid this situation, one solution is to run Pyodide in a WebWorker.

It’s also possible to run Pyodide in a Service Worker.

If you’re not sure whether you need web workers or service workers, here’s an overview and comparison of the two.



The following instructions have been tested with Node.js 18.5.0. To use Pyodide with older versions of Node, you might need to use additional command line arguments, see below.

It is now possible to install the Pyodide npm package in Node.js. To follow these instructions you need at least Pyodide 0.21.0. You can explicitly ask npm to use the alpha version:

$ npm install "pyodide@>=0.21.0-alpha.2"

Once installed, you can run the following simple script:

// hello_python.js
const { loadPyodide } = require("pyodide");

async function hello_python() {
  let pyodide = await loadPyodide();
  return pyodide.runPythonAsync("1+1");

hello_python().then((result) => {
  console.log("Python says that 1+1 =", result);
$ node hello_python.js
Loading distutils
Loaded distutils
Python says that 1+1= 2

Or you can use the REPL. To start the Node.js REPL with support for top level await, use node --experimental-repl-await:

$ node --experimental-repl-await
Welcome to Node.js v18.5.0.
Type ".help" for more information.
> const { loadPyodide } = require("pyodide");
> let pyodide = await loadPyodide();
Loading distutils
Loaded distutils
> await pyodide.runPythonAsync("1+1");

Node.js versions <0.17#

  • Node.js versions 14.x and 16.x: to use certain features of Pyodide you need to manually install node-fetch, e.g. by doing npm install node-fetch.

  • Node.js v14.x: you need to pass the option --experimental-wasm-bigint when starting Node. Note that this flag is not documented by node --help and moreover, if you pass --experimental-wasm-bigint to node >14 it is an error:

$ node -v

$ node --experimental-wasm-bigint hello_python.js
warning: no blob constructor, cannot create blobs with mimetypes
warning: no BlobBuilder
Loading distutils
Loaded distutils
Python says that 1+1= 2

If you wish to suppress the blob constructor warnings which appear in node <18, the following code works in node >= v14.18 (do this before calling loadPyodide):

globalThis.Blob = require("node:buffer").Blob;

However, in versions of node < v16.17, this will print an ExperimentalWarning. If you also don’t want the ExperimentalWarning, you can use cross-blob to polyfill Blob.