Using Pyodide from Javascript

This document describes using Pyodide directly from Javascript. For information about using Pyodide from Iodide, see Using Pyodide from Iodide.


Include pyodide.js in your project.

The recommended way to include Pyodide in your project is to download a release from here and include the contents in your distribution, and import the pyodide.js file there from a <script> tag.

For prototyping purposes, you may also use the following CDN URL, though doing so is not recommended, since it isn’t versioned and could change or be unstable at any time:

This file has a single Promise object which bootstraps the Python environment: languagePluginLoader. Since this must happen asynchronously, it is a Promise, which you must call then on to complete initialization. When the promise resolves, pyodide will have installed a namespace in global scope: pyodide.

languagePluginLoader.then(() => {
  // pyodide is now ready to use...
  console.log(pyodide.runPython('import sys\nsys.version'));

Running Python code

Python code is run using the pyodide.runPython function. It takes as input a string of Python code. If the code ends in an expression, it returns the result of the expression, converted to Javascript objects (See type conversions).

pyodide.runPython('import sys\nsys.version');

Loading packages

Only the Python standard library and six are available after importing Pyodide. To use other libraries, you’ll need to load their package using pyodide.loadPackage. This downloads the file data over the network (as a .data and .js index file) and installs the files in the virtual filesystem.

Packages can be loaded by name, for those included in the official pyodide repository (e.g. pyodide.loadPackage('numpy')). It is also possible to load packages from custom URLs (e.g. pyodide.loadPackage('https://foo/bar/numpy.js')), in which case the URL must end with <package-name>.js.

When you request a package from the official repository, all of that package’s dependencies are also loaded. Dependency resolution is not yet implemented when loading packages from custom URLs.

Multiple packages can also be loaded in a single call,

pyodide.loadPackage(['cycler', 'pytz'])

pyodide.loadPackage returns a Promise.

pyodide.loadPackage('matplotlib').then(() => {
  // matplotlib is now available

Complete example