Simple Component Registry#

The Python side needs to tell the browser side about registered custom elements, aka components. In this step, a Python function will return some data to the JS worker script.

Components in Python#

We want coding in Python, then running in browser. But what is the unit of work that ties the two sides together? And what is the unit of sharing?


Stated differently, custom elements. With this, a web page can have HTML such as <todo-list>My Items</todo-list> that comes from Python code.

:::{note} Why not Web Components? Web components are a superset of custom elements, adding things such as a Shadow DOM. But this brings in sharp edges when used in practice. It isn’t yet clear the gain is worth the pain. :::

We’ll talk more about components later.

Python Registry#

Let’s write our registry.

In tests/, add a failing test to get the registry. This test will have an import which fails While we’re at it, we’ll delete test_hello as we no longer need a placeholder.

from pyodide_components import get_registry

def test_get_registry():
    assert get_registry() == [1, 2, 3]

Our tests fail, as expected. Let’s now put in an implementation in

def get_registry():
    return [1, 2, 3]

We’ll similarly delete the PYODIDE_COMPONENTS flag, as it isn’t needed to demonstrate testing.

:::{note} Why in It would be nicer to put this in, say, But Pyodide doesn’t yet have a good story for packages. You have to fetch every file yourself. Thus, we’ll put all the Python in one file for now. :::

Tests now pass, let’s hook this up to the JS side and do a test.

Fetch src/pyodide_components/

When working with Pyodide – Python in the browser – you first have to fetch the Python code and put it in the virtual filesystem. You can then import it. Later, we’ll have a wheel in PyPI and Pyodide can handle the installation.

The initialize function in worker.js gets some lines added to it. We’re using fetch to make the HTTP request:

let loaderContent;
const response = await fetch("./");
if (response.ok) {
    loaderContent = await response.text();
} else {
    console.log("Failed to fetch");

And this…fails badly:

TypeError: Failed to parse URL from ./
    at Object.fetch (node:internal/deps/undici/undici:11118:11)

If you’re using a NodeJS before 18.3.0 (when native fetch landed), you might get a different error.

What’s going on? If you think about it, this code makes no sense from a NodeJS test. There isn’t an HTTP server (and we really don’t want one for testing.)

We need a way to intercept fetch and mock its responses. That’s the topic for the next section.