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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 the underlying coincident library uses it under the hood.


Sometimes you only need to await in the main thread the result of a call to a method exposed in a worker.

In such a limited case, and on the understanding that code in the worker will not be able to reach back into the main thread, you should use the sync_main_only flag in your configuration.

While this eliminates the need for the Atomics related header configuration (see below), the only possible use case is to return a serialisable result from the method called on the worker.

HTTP headers

For Atomics to work you must ensure your web server enables the following headers (this is the default behaviour for

Access-Control-Allow-Origin: *
Cross-Origin-Opener-Policy: same-origin
Cross-Origin-Embedder-Policy: require-corp
Cross-Origin-Resource-Policy: cross-origin

If you are not able to configure your server's headers, use the mini-coi project to achieve the same end.


The simplest way to use mini-coi is to place the mini-coi.js file in the root of your website (i.e. /), and reference it as the first child tag in the <head> of your HTML documents:

    <script src="/mini-coi.js" scope="./"></script> 
    <!-- etc -->
  <!-- etc --> 

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:

Evaluating code in a worker
<script type="py" src="./" worker></script>

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:

Launch a worker from within Python
from pyscript import PyWorker

# The type MUST be given and can be either `micropython` or `pyodide`
PyWorker("", 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 utilities.


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:

Python code running on the main thread.
from pyscript import PyWorker

def hello(name="world"):
    return(f"Hello, {name}")

# Create the worker.
worker = PyWorker("./", type="micropython")

# Register the hello function as callable from the worker.
worker.sync.hello = hello
Python code in the resulting worker.
from pyscript import sync, window

greeting = sync.hello("PyScript")

The values passed between the main thread and the worker must be serializable. Try the example given above via this project on

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.


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:


Evaluate via MicroPython on the main thread
<!DOCTYPE html>
<html lang="en">
    <meta charset="utf-8">
    <meta name="viewport" content="width=device-width,initial-scale=1">
    <!-- PyScript CSS -->
    <link rel="stylesheet" href="">
    <!-- This script tag bootstraps PyScript -->
    <script type="module" src=""></script>
    <title>PyWorker - mpy bootstrapping pyodide example</title>
    <!-- the async attribute is useful but not mandatory -->
    <script type="mpy" src="" async></script>

MicroPython's bootstrapping a Pyodide worker.
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("", 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()

# Show the result at the end of the body.

# Free memory and get rid of everything in the worker.

The script runs in the Pyodide worker.
from pyscript import sync

# Use any methods from on the main thread.

# 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.

before ready
Pyodide bootstrapped
after ready