Using Pyodide from Javascript

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


To include Pyodide in your project you can use the following CDN URL,

You can also download a release from Github releases (or build it yourself), include its contents in your distribution, and import the pyodide.js file there from a <script> tag. See the following section on serving pyodide files for more details.

The pyodide.js 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).

import sys

Complete example

Create and save a test index.html page with the following contents:

<!DOCTYPE html>
      <script type="text/javascript">
          // set the pyodide files URL (packages.json, etc)
          window.languagePluginUrl = '';
      <script src=""></script>
    Pyodide test page <br>
    Open your browser console to see pyodide output
    <script type="text/javascript">
          languagePluginLoader.then(function () {
                  import sys
              console.log(pyodide.runPython('print(1 + 2)'));

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

Serving pyodide files

If you built your pyodide distribution or downloaded the release tarball you need to serve pyodide files with a appropriate headers.

Because browsers require WebAssembly files to have mimetype of application/wasm we’re unable to serve our files using Python’s built-in SimpleHTTPServer module.

Let’s wrap Python’s Simple HTTP Server and provide the appropiate mimetype for WebAssembly files into a file (in the pyodide_local directory):

import sys
import socketserver
from http.server import SimpleHTTPRequestHandler

class Handler(SimpleHTTPRequestHandler):

    def end_headers(self):
        # Enable Cross-Origin Resource Sharing (CORS)
        self.send_header('Access-Control-Allow-Origin', '*')

if sys.version_info < (3, 7, 5):
    # Fix for WASM MIME type for older Python versions
    Handler.extensions_map['.wasm'] = 'application/wasm'

if __name__ == '__main__':
    port = 8000
    with socketserver.TCPServer(("", port), Handler) as httpd:
        print("Serving at:{}".format(port))

Let’s test it out. In your favourite shell, let’s start our WebAssembly aware web server:


Point your WebAssembly aware browser to http://localhost:8000/index.html and open your browser console to see the output from python via pyodide!