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Builtin helpers

PyScript makes available convenience objects and functions inside Python as well as custom attributes in HTML.

In Python this is done via the pyscript module:

Accessing the document object via the pyscript module
from pyscript import document

In HTML this is done via py-* attributes:

An example of a py-click handler
<button id="foo" py-click="handler_defined_in_python">Click me</button>

Common features

These Python objects / functions are available in both the main thread and in code running on a web worker:

pyscript.window

On the main thread, this object is a direct reference to the import js module which, in turn, is a proxy of the globalThis.

On a worker thread, this object is a proxy for the web page's global window context.

Warning

Please note that in workers, this is still the main window, not the worker's own global context. A worker's global context is always reachable instead via import js (the js object being a proxy for the worker's globalThis).

pyscript.document

On both main and worker threads, this object is a proxy for the the web page's document object. The document is a representation of the DOM and can be used to read or manipulate the content of the web page.

pyscript.display

A function used to display content. The function is intelligent enough to introspect the object[s] it is passed and work out how to correctly display the object[s] in the web page.

The display function takes a list of *values as its first argument, and has two optional named arguments:

  • target=None - the DOM element into which the content should be placed. If not specified, the target will use the current_script() returned id and populate the related dedicated node to show the content.
  • append=True - a flag to indicate if the output is going to be appended to the target.

There are some caveats:

  • When used in the main thread, the display function automatically uses the current <py-script> or <mpy-script> tag as the target into which the content will be displayed.
  • If the <script> tag has the target attribute, and is not a worker, the element on the page with that ID (or which matches that selector) will be used to display the content instead.
  • When used in a worker, the display function needs an explicit target="dom-id" argument to identify where the content will be displayed.
  • In both the main thread and worker, append=True is the default behaviour.
Some display example
<!-- will produce
    <py-script>PyScript</py-script>
-->
<py-script worker>
    from pyscript import display
    display("PyScript", append=False)
</py-script>

<!-- will produce
    <script type="py">...</script>
    <script-py>PyScript</script-py>
-->
<script type="py">
    from pyscript import display
    display("PyScript", append=False)
</script>

<!-- will populate <h1>PyScript</h1> -->
<script type="py" target="my-h1">
    from pyscript import display
    display("PyScript", append=False)
</script>
<h1 id="my-h1"></h1>

<!-- will populate <h2>PyScript</h2> -->
<script type="py" worker>
    from pyscript import display
    display("PyScript", target="my-h2", append=False)
</script>
<h2 id="my-h2"></h2>

pyscript.when

A Python decorator to indicate the decorated function should handle the specified events for selected elements.

The decorator takes two parameters:

  • The event_type should be the name of the browser event to handle as a string (e.g. "click").
  • The selector should be a string containing a valid selector to indicate the target elements in the DOM whose events of event_type are of interest.

The following example has a button with an id of my_button and a decorated function that handles click events dispatched by the button.

The HTML button
<button id="my_button">Click me!</button>
The decorated Python function to handle click events
from pyscript import when, display


@when("click", "#my_button")
def click_handler(event):
    """
    Event handlers get an event object representing the activity that raised
    them.
    """
    display("I've been clicked!")

This functionality is related to the py-* or mpy-* HTML attributes.

pyscript.js_modules

It is possible to define JavaScript modules to use within your Python code.

Such named modules will always then be available under the pyscript.js_modules namespace.

Warning

Please see the documentation (linked above) about restrictions and gotchas when configuring how JavaScript modules are made available to PyScript.

pyscript.fetch

A common task is to fetch data from the web via HTTP requests. The pyscript.fetch function provides a uniform way to achieve this in both Pyodide and MicroPython. It is closely modelled on the Fetch API found in browsers with some important Pythonic differences.

The simple use case is to pass in a URL and await the response. Remember, in order to use await you must have the async attribute in the script tag that references your code. If this request is in a function, that function should also be defined as async.

A simple HTTP GET with pyscript.fetch
from pyscript import fetch


response = await fetch("https://example.com")
if response.ok:
    data = await response.text()
else:
    print(response.status)

The object returned from an await fetch call will have attributes that correspond to the JavaScript response object. This is useful for getting response codes, headers and other metadata before processing the response's data.

Alternatively, rather than using a double await (one to get the response, the other to grab the data), it's possible to chain the calls into a single await like this:

A simple HTTP GET as a single await
from pyscript import fetch

data = await fetch("https://example.com").text()

The following awaitable methods are available to you to access the data returned from the server:

  • arrayBuffer() returns a Python memoryview of the response. This is equivalent to the arrayBuffer() method in the browser based fetch API.
  • blob() returns a JavaScript blob version of the response. This is equivalent to the blob() method in the browser based fetch API.
  • bytearray() returns a Python bytearray version of the response.
  • json() returns a Python datastructure representing a JSON serialised payload in the response.
  • text() returns a Python string version of the response.

The underlying browser fetch API has many request options that you should simply pass in as keyword arguments like this:

Supplying request options.
from pyscript import fetch


response = await fetch("https://example.com", method="POST", body="HELLO").text()

Danger

You may encounter CORS errors (especially with reference to a missing Access-Control-Allow-Origin header.

This is a security feature of modern browsers where the site to which you are making a request will not process a request from a site hosted at another domain.

For example, if your PyScript app is hosted under example.com and you make a request to bbc.co.uk (who don't allow requests from other domains) then you'll encounter this sort of CORS related error.

There is nothing PyScript can do about this problem (it's a feature, not a bug). However, you could use a pass-through proxy service to get around this limitation (i.e. the proxy service makes the call on your behalf).

pyscript.ffi.to_js

A utility function to convert Python references into their JavaScript equivalents. For example, a Python dictionary is converted into a JavaScript object literal (rather than a JavaScript Map), unless a dict_converter is explicitly specified and the runtime is Pyodide.

pyscript.ffi.create_proxy

A utility function explicitly for when a callback function is added via an event listener. It ensures the function still exists beyond the assignment of the function to an event. Should you not create_proxy around the callback function, it will be immediately garbage collected after being bound to the event.

Warning

In Pyodide it's expected that the created proxy is explicitly destroyed when it's not needed / used anymore but that proxy.destroy() method has not been implemented in MicroPython (yet). To try simplifying this dance and automatically destroy proxies, based on JS Garbage Collector heuristics, we have introduced an experimental flag called experimental_create_proxy = "auto" which currently tries to be smart enough to orchestrate the whole proxy creation and destruction dance out of the box. If you'd like to try that flag keep in mind you should never need or care about using explictly create_proxy but like it is with everything experimental, there might be edge cases we have not (yet) tackled.

pyscript.current_target

A utility function to retrieve the unique identifier of the element used to display content. If the element is not a <script> and it has already an id, that id will be returned.

The current_target utility
<!-- current_target(): explicit-id -->
<mpy-script id="explicit-id">
    from pyscript import display, current_target
    display(f"current_target(): {current_target()}")
</mpy-script>

<!-- current_target(): mpy-0 -->
<mpy-script>
    from pyscript import display, current_target
    display(f"current_target(): {current_target()}")
</mpy-script>

<!-- current_target(): mpy-1 -->
<!-- creates right after the <script>:
    <script-py id="mpy-1">
        <div>current_target(): mpy-1</div>
    </script-py>
-->
<script type="mpy">
    from pyscript import display, current_target
    display(f"current_target(): {current_target()}")
</script>

Note

Please note that current_target() points at a visible element on the page, not at the current <script> that is executing the code. If you need to explicitly reach the <script> element, you can always assign an id to it so that at any time, within any listener or functionality, you can document.getElementById(script_id) to reach out that element: <script type="mpy" id="unique-id">...</script>

pyscript.HTML

A class utility able to wrap a generic content and display it on the page. The content can be any of these mime types:

  • text/plain to show the content as text
  • text/html to show the content as HTML
  • image/png to show the content as <img>
  • image/jpeg to show the content as <img>
  • image/svg+xml to show the content as <svg>
  • application/json to show the content as JSON
  • application/javascript to put the content in <script> (discouraged)
The HTML class
<!-- display escaped text:
    &lt;em&gt;em&lt;/em&gt;
-->
<script type="mpy">
    from pyscript import display, HTML
    display("<em>em</em>")
</script>

<!-- display exactly this HTML:
    <em>em</em>
-->
<script type="mpy">
    from pyscript import display, HTML
    display(HTML("<em>em</em>"))
</script>

pyscript.RUNNING_IN_WORKER

This constant indicates when the current code is running within a worker or within the main thread.

It is True when the current code is executing in a worker, False when the code is running on main.

Main-thread only features

pyscript.PyWorker

A class used to instantiate a new worker from within Python.

Note

We currently changed names within the JS module's exports to bootstrap and disambiguate PyWorker from MPWorker and automatically use the right interpreter behind the scene. The Python class currently is always named PyWorker and it requires at least a valid type option, among others, which must be either micropython or pyodide. We will keep this feature alive but in the future this might default to pyodide unless a new MPWorker class is also exported which will default to micropython instead. The explicit type would still exist.

The following fragment demonstrates who to start the Python code in the file worker.py on a new worker from within Python.

Starting a new worker from Python
<script type="mpy">
    from pyscript import PyWorker

    # type can be either `micropython` or `pyodide`
    PyWorker("worker.py", type="micropython")
</script>
<div id="output"></div>
the worker.py content
from pyscript import RUNNING_IN_WORKER, display, sync

display("Hello World", target="output", append=True)

# will log into devtools console
print(RUNNING_IN_WORKER)  # True
print("sleeping")
sync.sleep(1)
print("awake")

Worker only features

pyscript.sync

A function used to pass serializable data from workers to the main thread.

Imagine you have this code on the main thread:

Python code on the main thread
from pyscript import PyWorker

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

worker = PyWorker("./worker.py")
worker.sync.hello = hello

In the code on the worker, you can pass data back to handler functions like this:

Pass data back to the main thread from a worker
from pyscript import sync

sync.hello("PyScript")

HTML attributes

As a convenience, and to ensure backwards compatibility, PyScript allows the use of inline event handlers via custom HTML attributes.

Warning

This classic pattern of coding (inline event handlers) is no longer considered good practice in web development circles.

We include this behaviour for historic reasons, but the folks at Mozilla have a good explanation of why this is currently considered bad practice.

These attributes are expressed as py-* or mpy-* attributes of an HTML element that reference the name of a Python function to run when the event is fired. You should replace the * with the actual name of an event (e.g. py-click or mpy-click). This is similar to how all event handlers on elements start with on in standard HTML (e.g. onclick). The rule of thumb is to simply replace on with py- and then reference the name of a Python function.

A py-click event on an HTML button element.
<button py-click="handle_click" id="my_button">Click me!</button>
The related Python function.
from pyscript import window


def handle_click(event):
    """
    Simply log the click event to the browser's console.
    """
    window.console.log(event)

Under the hood, the pyscript.when decorator is used to enable this behaviour.

Note

In earlier versions of PyScript, the value associated with the attribute was simply evaluated by the Python interpreter. This was unsafe: manipulation of the attribute's value could have resulted in the evaluation of arbitrary code.

This is why we changed to the current behaviour: just supply the name of the Python function to be evaluated, and PyScript will do this safely.