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.install() (Python) for pure Python packages with wheels as well as Pyodide packages (including Emscripten/wasm32 binary wheels). It can install packages from PyPI, the JsDelivr CDN or from other URLs.

  • pyodide.loadPackage() (Javascript) for packages built with Pyodide. This is a function with less overhead but also more limited functionality. micropip uses this function to load Pyodide packages. In most cases you should be using micropip.

In some cases, and in particular in the REPL, packages are installed implicitly from imports. The Pyodide REPL uses pyodide.loadPackagesFromImports() to automatically download all packages that the code snippet imports. This is useful since users might import unexpected packages in REPL. At present, loadPackagesFromImports() will not download packages from PyPI, it will only download packages included in the Pyodide distribution. See Packages built in Pyodide to check the full list of packages included in Pyodide.

How to chose between micropip.install and pyodide.loadPackage?#

While micropip.install() is written in Python and pyodide.loadPackage() in Javascript this has no incidence on when to use each of these functions. Indeed, you can easily switch languages using the Type translations with,

  • from Javascript,

    let micropip = pyodide.pyimport(package_name);
  • from Python,

    import pyodide_js
    await pyodide_js.loadPackage('package_name')

Instead, the general advice is to use micropip.install() for everything except in the following cases where pyodide.loadPackage() might be more appropriate,

  • to load micropip itself,

  • when you are optimizing for size, do not want to install the micropip package, and do not need to install packages from PyPI with dependency resolution.


Installing packages#

Pyodide supports installing following types of packages with micropip,

  • pure Python wheels from PyPI with micropip.

  • pure Python and binary wasm32/emscripten wheels (also informally known as “Pyodide packages” or “packages built by Pyodide”) from the JsDelivr CDN and custom URLs. micropip.install() is an async Python function which returns a coroutine, so it need to be called with an await clause to run.

await pyodide.loadPackage("micropip");
const micropip = pyodide.pyimport("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). Micropip will also install the dependencies of the wheel. If dependency resolution is not desired, you may pass deps=False.

Cross-Origin Resource Sharing (CORS)

If the file is on a remote server, the server must set Cross-Origin Resource Sharing (CORS) headers to allow access. If the server doesn’t set CORS headers, you can use a CORS proxy. Note 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. See this stack overflow answer for more information about CORS.


    <meta charset="utf-8" />
    <script type="text/javascript" src="https://cdn.jsdelivr.net/pyodide/v0.26.1/full/pyodide.js"></script>
    <script type="text/javascript">
      async function main() {
        let pyodide = await loadPyodide();
        await pyodide.loadPackage("micropip");
        const micropip = pyodide.pyimport("micropip");
        await micropip.install("snowballstemmer");
        await pyodide.runPython(`
        import snowballstemmer
        stemmer = snowballstemmer.stemmer('english')
        print(stemmer.stemWords('go goes going gone'.split()))

Loading packages with pyodide.loadPackage()#

Packages included in the official Pyodide repository can be loaded using loadPackage():

await pyodide.loadPackage("numpy");

It is also possible to load packages from custom URLs:

await pyodide.loadPackage(

The file name must be a valid wheel name.

When you request a package from the official repository, all of the package’s dependencies are also loaded. There is no dependency resolution when loading packages from custom URLs. If you want dependency resolution for custom URLs, use micropip.

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().

await pyodide.loadPackage(["cycler", "pytz"]);

loadPackage() returns a Promise which resolves when all the packages are finished loading:

let pyodide;
async function main() {
  pyodide = await loadPyodide();
  await pyodide.loadPackage("matplotlib");
  // matplotlib is now available