Workers
Workers run code that won't block the "main thread" controlling the user interface. If you block the main thread, your web page becomes annoyingly unresponsive. You should never block the main thread.
Happily, PyScript makes it very easy to use workers and uses a feature recently added to web standards called Atomics. You don't need to know about Atomics to use web workers, but it's useful to know that the underlying coincident library uses it under the hood.
Info
Sometimes you only need to await
in the main thread on a method in a
worker when neither window
nor document
are referenced in the code
running on the worker.
In these cases, you don't need any special header or service worker as long as the method exposed from the worker returns a serializable result.
HTTP headers
To use the window
and document
objects from within a worker (i.e. use
synchronous Atomics) you must ensure your web server enables the following
headers (this is the default behavior for
pyscript.com):
Access-Control-Allow-Origin: *
Cross-Origin-Opener-Policy: same-origin
Cross-Origin-Embedder-Policy: require-corp
Cross-Origin-Resource-Policy: cross-origin
If you're unable to configure your server's headers, you have two options:
- Use the mini-coi project to enforce headers.
- Use the
service-worker
attribute with thescript
element.
Option 1: mini-coi
For performance reasons, this is the preferred option so Atomics works at native speed.
The simplest way to use mini-coi is to copy the
mini-coi.js
file content and save it in the root of your website (i.e. /
), and reference
it as the first child tag in the <head>
of your HTML documents:
Option 2: service-worker
attribute
This allows you to slot in a custom service worker to handle requirements for synchronous operations.
Each <script type="m/py">
or <m/py-script>
may optionally have
a service-worker
attribute pointing to a locally served file (the
same way mini-coi.js
needs to be served).
- You can chose
mini-coi.js
itself or any other custom service worker, as long as it provides either the right headers to enable synchronous operations via Atomics, or it enables sabayon polyfill events. - Alternatively, you can copy and paste the
sabayon Service Worker
into your local project and point at that in the attribute. This will
not change the original behavior of your project, it will not interfere with
all default or pre-defined headers your application uses already but it will
fallback to a (slower but working) synchronous operation that allows
both
window
anddocument
access in your worker logic.
<html>
<head>
<!-- PyScript link and script -->
</head>
<body>
<script type="py" service-worker="./sw.js" worker>
from pyscript import window, document
document.body.append("Hello PyScript!")
</script>
</body>
</html>
Warning
Using sabayon as the fallback for synchronous operations via Atomics should be the last solution to consider. It is inevitably slower than using native Atomics.
If you must use sabayon, always reduce the amount of synchronous operations by caching references from the main thread.
# ❌ THIS IS UNNECESSARILY SLOWER
from pyscript import document
# add a data-test="not ideal attribute"
document.body.dataset.test = "not ideal"
# read a data-test attribute
print(document.body.dataset.test)
# - - - - - - - - - - - - - - - - - - - - -
# ✔️ THIS IS FINE
from pyscript import document
# if needed elsewhere, reach it once
body = document.body
dataset = body.dataset
# add a data-test="not ideal attribute"
dataset.test = "not ideal"
# read a data-test attribute
print(dataset.test)
In latter example the number of operations has been reduced from six to just four. The rule of thumb is: if you ever need a DOM reference more than once, cache it. 👍
Start working
To start your code in a worker, simply ensure the <script>
, <py-script>
or
<mpy-script>
tag pointing to the code you want to run has a worker
attribute flag:
You may also want to add a name
attribute to the tag, so you can use
pyscript.workers
in the main thread to retrieve a reference to the worker:
Alternatively, to launch a worker from within Python running on the main thread use the pyscript.PyWorker class and you must reference both the target Python script and interpreter type:
from pyscript import PyWorker
# The type MUST be given and can be either `micropython` or `pyodide`
my_worker = PyWorker("my-worker-code.py", type="micropython")
Worker interactions
Code running in the worker needs to be able to interact with code running in the main thread and perhaps have access to the web page. This is achieved via some helpful builtin APIs.
Note
For ease of use, the worker related functionality in PyScript is a simpler presentation of more sophisticated and powerful behaviour available via PolyScript.
If you are a confident advanced user, please consult the XWorker related documentation from the PolyScript project for how to make use of these features.
To synchronise serializable data between the worker and the main thread use
the sync
function in the worker to reference a
function registered on the main thread:
from pyscript import PyWorker
def hello(name="world"):
return(f"Hello, {name}")
# Create the worker.
worker = PyWorker("./worker.py", type="micropython")
# Register the hello function as callable from the worker.
worker.sync.hello = hello
from pyscript import sync, window
greeting = sync.hello("PyScript")
window.console.log(greeting)
Alternatively, for the main thread to call functions in a worker, specify the
functions in a __export__
list:
import sys
def version():
return sys.version
# Define what to export to the main thread.
__export__ = ["version", ]
Then ensure you have a reference to the worker in the main thread (for
instance, by using the pyscript.workers
):
<script type="py" src="./my-worker-code.py" worker name="my-worker"></script>
from pyscript import workers
my_worker = await workers["my-worker"]
print(await my_worker.version())
The values passed between the main thread and the worker must be serializable. Try the example given above via this project on PyScript.com.
No matter if your code is running on the main thread or in a web worker,
both the pyscript.window
(representing the main
thread's global window context) and
pyscript.document
(representing the web
page's
document object)
will be available and work in the same way. As a result, a worker can reach
into the DOM and access some window
based APIs.
Warning
Access to the window
and document
objects is a powerful feature. Please
remember that:
- Arguments to and the results from such calls, when used in a worker, must be serializable, otherwise they won't work.
- If you manipulate the DOM via the
document
object, and other workers or code on the main thread does so too, they may interfere with each other and produce unforeseen problematic results. Remember, with great power comes great responsibility... and we've given you a bazooka (so please remember not to shoot yourself in the foot with it).
Common Use Case
While it is possible to start a MicroPython or Pyodide worker from either MicroPython or Pyodide running on the main thread, the most common use case we have encountered is MicroPython on the main thread starting a Pyodide worker.
Here's how:
index.html
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="utf-8">
<meta name="viewport" content="width=device-width,initial-scale=1">
<!-- PyScript CSS -->
<link rel="stylesheet" href="https://pyscript.net/releases/2024.8.2/core.css">
<!-- This script tag bootstraps PyScript -->
<script type="module" src="https://pyscript.net/releases/2024.8.2/core.js"></script>
<title>PyWorker - mpy bootstrapping pyodide example</title>
<script type="mpy" src="main.py"></script>
</head>
</html>
main.py
from pyscript import PyWorker, document
# Bootstrap the Pyodide worker, with optional config too.
# The worker is:
# * Owned by this script, no JS or Pyodide code in the same page can access
# it.
# * It allows pre-sync methods to be exposed.
# * It has a ready Promise to await for when Pyodide is ready in the worker.
# * It allows the use of post-sync (methods exposed by Pyodide in the
# worker).
worker = PyWorker("worker.py", type="pyodide")
# Expose a utility that can be immediately invoked in the worker.
worker.sync.greetings = lambda: print("Pyodide bootstrapped")
print("before ready")
# Await until Pyodide has completed its bootstrap, and is ready.
await worker.ready
print("after ready")
# Await any exposed methods exposed via Pyodide in the worker.
result = await worker.sync.heavy_computation()
print(result)
# Show the result at the end of the body.
document.body.append(result)
# Free memory and get rid of everything in the worker.
worker.terminate()
worker.py
from pyscript import sync
# Use any methods from main.py on the main thread.
sync.greetings()
# Expose any methods meant to be used from main.
sync.heavy_computation = lambda: 6 * 7
Save these files in a tmp
folder, ensure your headers (just
use npx mini-coi ./tmp
to serve via localhost) then see the following
outcome in the browser's devtools.