Loading packages

Only the Python standard library is available after importing Pyodide. To use other packages, you’ll need to load them using either:


micropip can also be used to load packages built in Pyodide (in which case it relies on pyodide.loadPackage).

If you use pyodide.runPythonAsync Pyodide will automatically download all packages that the code snippet imports. This is particularly useful for making a repl since users might import unexpected packages. At present, runPythonAsync will not download packages from PyPi, it will only download packages included in the Pyodide distribution. Internally, pyodide.runPythonAsync uses pyodide.loadPackagesFromImports which in turn uses pyodide.find_imports, so if you need more control over automatic package loading you can use these more basic APIs.

Loading packages with pyodide.loadPackage

Packages included in the official Pyodide repository can be loaded using pyodide.loadPackage:


It is also possible to load packages from custom URLs:


The file name in the URL must be /<package-name>.js and there must be an accompanying .data file in the same directory.

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.

In general, loading a package twice is not permitted. However, one can override a dependency by loading a custom URL with the same package name before loading the dependent package.

Multiple packages can also be loaded at the same time by passing a list to `loadPackage.

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

pyodide.loadPackage returns a Promise which resolves when all of the packages are finished loading:

async function main(){
  await loadPyodide({ 'indexURL' : '<some-url>' });
  await pyodide.loadPackage('matplotlib');
  // matplotlib is now available


Installing packages from PyPI

Pyodide supports installing pure Python wheels from PyPI with micropip. micropip.install() returns a Python Future <https://docs.python.org/3/library/asyncio-future.html>_ so you can await the future or otherwise use the Python future API to do work once the packages have finished loading:

  import micropip
  await micropip.install('snowballstemmer')
  import snowballstemmer
  stemmer = snowballstemmer.stemmer('english')
  print(stemmer.stemWords('go goes going gone'.split()))

Micropip implements file integrity validation by checking the hash of the downloaded wheel against pre-recorded hash digests from the PyPi JSON API.

Installing wheels from arbitrary URLs

Pure Python wheels can also be installed from any URL with micropip,

import micropip

Micropip decides whether a file is a URL based on whether it ends in “.whl” or not. The wheel name in the URL must follow PEP 427 naming convention, which will be the case if the wheels is made using standard Python tools (pip wheel, setup.py bdist_wheel).

All required dependencies must have been previously installed with micropip or pyodide.loadPackage.

If the file is on a remote server, the server must set Cross-Origin Resource Sharing (CORS) headers to allow access. Otherwise, you can prepend a CORS proxy to the URL. Note however that using third-party CORS proxies has security implications, particularly since we are not able to check the file integrity, unlike with installs from PyPi.


  <meta charset="utf-8">
  <script type="text/javascript" src="https://cdn.jsdelivr.net/pyodide/v0.17.0/full/pyodide.js"></script>
  <script type="text/javascript">
    async function main(){
      await loadPyodide({ indexURL : 'https://cdn.jsdelivr.net/pyodide/v0.17.0/full/' });
      await pyodide.runPythonAsync(`
        import micropip # runPythonAsync will load micropip automatically
        await micropip.install('snowballstemmer')
        import snowballstemmer
        stemmer = snowballstemmer.stemmer('english')
        print(stemmer.stemWords('go goes going gone'.split()))