Faster Pyodide Testing#

We’re going to do a lot of “sit in JS, execute Python.” We want to speed it up. Let’s change our test strategy to re-use a single Pyodide across many tests.

Clean up worker tests#

We have a test that asserts the JS initialize function returns some flag. Actually, it’s going to return the Pyodide instance.

Let’s re-organize our worker tests:

import {beforeEach, expect, test} from "vitest";
import {initialize} from "../src/pyodide_components/worker.js";

// Make an interpreter and capture its startup state
const thisPyodide = await initialize();
const initialPyodideState = thisPyodide.pyodide_py._state.save_state();

beforeEach(async () => {
    // On each test, reset to an "empty" interpreter

test("Load and initialize Pyodide", () => {
    expect(typeof thisPyodide.runPythonAsync).to.equal("function");

test("Confirm valid and running Pyodide", async () => {
    const result = await thisPyodide.runPythonAsync("1+1");

Lots of changes here. First, the two lines about thisPyodide. We make a Pyodide instance a module scope, then immediately capture its default startup state.

In beforeEach, we speed up our test runs. Instead of making a new interpreter all the time, we re-use the existing one. But we reset it to its initial state.

We then split our tests into two parts: did we actually get a Pyodide, and does it run Python code?

Worker returns a pyodide#

Over in our initialize function, for now, just return Pyodide:

import {loadPyodide} from "./pyodide/pyodide.mjs";

export async function initialize() {
    return await loadPyodide();


We’re already in a neat spot. We can sit in a test, in a debugger, in our smart editor – and evaluate Python from JS.