Running Panel in the browser with WASM#

Panel lets you write dashboards and other applications in Python that are accessed through a web browser. Normally, the Python interpreter runs as a separate Jupyter or Bokeh server process and communicates with the JavaScript code running in the client browser. However, Python can also be run directly in the browser using WASM, without the need for a separate server.

Panel uses Pyodide for this and PyScript for rendering.

Converting panel applications#

Future versions of Panel can convert your Panel application from one or more Python scripts or Notebook files, including Templates, into an HTML file using panel convert. The only requirements are:

they only import global modules and packages and no relative imports from other scripts or modules
the libraries have been compiled for Pyodide or are available as Python wheels <Wheel> on the Python Package Index (PyPI).

Example#

In the following example we will convert the Overview notebook into a standalone HTML page with

$ panel convert overview.ipynb --out pyodide
Column
    [0] Column
        [0] FloatSlider(end=3.0, name='frequency', start=-1.0, value=1.0)
        [1] FloatSlider(end=3.0, name='amplitude', start=-1.0, value=1.0)
        [2] IntSlider(end=100, name='n', start=5, value=200)
    [1] Row
        [0] Matplotlib(Figure, name='interactive00114')
Launching server at http://localhost:40405

Now you can open http://localhost:40405 in your browser and try out the app:

You can now add the pyodide/overview.html file to your Github pages or similar – no separate server is required.

Options#

In the following I explain some of the options of panel convert.

--to

The format to convert to. There are three options, each with different advantages and disadvantages:

pyodide (default): The application is run with pyodide in the main thread. This option is less performant than pyodide-worker, but produces a fully self-contained HTML file that does not need to be hosted on a static file server, such as Github Pages.
pyodide-worker: generates HTML and JS files, but includes a web worker that runs in a separate thread. This is the most powerful option, but the files must be hosted on a static file server.
pyscript: creates an HTML file that uses PyScript <https://pyscript.net>. This creates standalone HTML files with <py-env> and <py-script> tags containing the dependencies and application code. This output is the most readable and should have the same performance as the pyodide option.

-out

The directory to write the files to.

--pwa

adds files that make the application a Progressive Web App.

Progressive Web Apps (PWAs) provide a way for your web apps to behave almost like a native app, both on mobile devices and on the desktop. panel convert has a --pwa option that generates the files necessary to turn your panel and pyodide app into a PWA.

--skip-embed

skips embedding pre-rendered content in the converted file.

Panel embeds pre-rendered content in the HTML page and replaces it with live components once the page is loaded. However, this can take a long time. If you want to disable this behaviour and render a blank page first, use the --skip-embed option.

--index

creates an index when you convert several applications at once, so you can easily navigate between them.

--requirements

Explicit requirements to add to the converted file or to a requirements.txt file.

By default, requirements are derived from code.

If a library uses an optional import that cannot be derived from your application’s list of imports, you must specify an explicit list of dependencies.

Note

panel and its dependencies, including NumPy and Bokeh, are loaded automatically, which means that the explicit requirements for the above application would be as follows:

$ panel convert overview.ipynb --out pyodide --requirements pandas matplotlib

AAlternatively, you can provide a requirements.txt file:

$ panel convert overview.ipynb --out pyodide --requirements requirements.txt

--watch

Observe the source files.

You can get a complete overview with panel convert -u.

Tip

If the converted application does not work as expected, you can usually find the errors in the browser console, see Finding Your Browser’s Developer Console.