We don’t want to download Pyodide every time we run a test or open a page. Let’s write a little downloader script and register it as a console app.

The Theory#

We want Pyodide locally. It’s cumbersome to download and extract, so we’ll automate it with a script.

We’ll store this in src/pyodide_components/pyodide with an entry in .gitignore to ensure it doesn’t get checked in. Why under src? We’ll explain in a bit.

The Code#

We add the code to src/pyodide_components/

"""Downloads Pyodide and extracts to correct place."""
from pyodide_components import HERE

"""Automation scripts for getting setup."""
import os
import tarfile
from pathlib import Path
from shutil import copytree, rmtree
from tempfile import TemporaryDirectory

from urllib3 import PoolManager

def get_pyodide():
    print("Getting Pyodide")
    base_url = ""
    url = f"{base_url}/0.22.0a1/pyodide-0.22.0a1.tar.bz2"
    http = PoolManager()
    r = http.request('GET', url)
    with TemporaryDirectory() as tmp_dir_name:
        tmp_dir = Path(tmp_dir_name)
        temp_file = tmp_dir / "pyodide.tar.bz2"
        tar =
        target = HERE / "pyodide"
        if target.exists():
        copytree(tmp_dir / "pyodide", target)

if __name__ == '__main__':

That code depends on HERE. Let’s add it to src/pyodide_components/

from pathlib import Path

HERE = Path(__file__).parent

Running it#

It’s a function that is run from a __main__ block when you execute this in a virtual environment:

$ python -m pyodide_components.downloader

When run, it creates a directory at src/pyodide_components/pyodide. We should also add that directory to our .gitignore:


Add node-fetch dependency#

If you fire up the dev server in package.json, you’ll see a warning:

The following dependencies are imported but could not be resolved:

  node-fetch (imported by /somepath/pyodide-components/src/pyodide_components/pyodide/pyodide.mjs)

Vite is doing some static analysis and notices that Pyodide’s JS depends on a package called node-fetch. We can silence this by installing it:

$ npm install -S node-fetch

Places for improvement#

We won’t spend too much time “hardening” this, as it is a just a means to an end. We could though:

  • Add these instructions to a README

  • Write a test with some mocks to prove the logic works

  • Not hard-coding the URL path and version