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:
In HTML this is done via py-*
attributes:
<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
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 reachable instead
via import js
(the js
object being a proxy for the worker's
globalThis
).
pyscript.document
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 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.append=True
- a flag to indicate if the output is going to be appended to thetarget
.
There are some caveats:
- When used in the main thread, the
display
function automatically uses the current<script>
tag as thetarget
into which the content will be displayed. - If the
<script>
tag has thetarget
attribute, 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 explicittarget="dom-id"
argument to identify where the content will be displayed. - In both the main thread and worker,
append=True
is the default behaviour.
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 ofevent_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.
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 HTML py-* 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
.
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:
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 thearrayBuffer()
method in the browser basedfetch
API.blob()
returns a JavaScriptblob
version of the response. This is equivalent to theblob()
method in the browser basedfetch
API.bytearray()
returns a Pythonbytearray
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:
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.
Main-thread only features
pyscript.PyWorker
A class used to instantiate a new worker from within Python.
Danger
Currently this only works with Pyodide.
The following fragment demonstrates who to start the Python code in the file
worker.py
on a new worker from within Python.
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:
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:
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-*
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
).
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.
<button py-click="handle_click" id="my_button">Click me!</button>
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.