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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 {js:func}~globalThis.loadPyodide.

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

See the {ref}quickstart for a walk-through tutorial as well as {ref}loading_packages and {ref}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:
 - `cdn.jsdelivr.net/pyodide/` distributes Python packages built with Pyodide as well as `pyodide.js`
 - `cdn.jsdelivr.net/npm/pyodide@0.19.0/` is a mirror of the Pyodide NPM package, which includes none of the WASM files

Supported browsers

Webassembly support in browsers is evolving very rapidly, and we recommend using the latest browsers whenever possible to take full advantage of Pyodide and its webassembly features. If you are using an older browser, some features may not work properly.

Currently, Pyodide is being tested against the following browser versions, so we recommend using a browser version at least equal to or higher than these.

Browser	Version	Release date
Firefox	112	11 April 2023
Chrome	112	29 March 2023
Safari	16.4	27 March 2023

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 {ref}Pyodide in a WebWorker <using_from_webworker>.

It's also possible to run {ref}Pyodide in a Service Worker <using_from_service_worker>.

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

Node.js

Starting with Pyodide 0.25.0, Node.js < 18 is no longer officially supported.
Older versions of Node.js might still work, but they are not tested or guaranteed to work.

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
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");
undefined
> let pyodide = await loadPyodide();
undefined
> await pyodide.runPythonAsync("1+1");
2

.. toctree::
   :hidden:

   loading-custom-python-code.md
   file-system.md
   accessing-files.md
   webworker.md
   service-worker.md
   working-with-bundlers.md

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