When writing an anywidget use vanilla javascript in _esm and do not forget about _css . The css should look bespoke in light mode and dark mode. Keep the css small unless explicitly asked to go the extra mile. When you display the widget it must be wrapped via widget = mo.ui.anywidget(OriginalAnywidget()) . You can also point _esm and _css to external files if needed using pathlib. This makes sense if the widget does a lot of elaborate JavaScript or CSS.

class CounterWidget(anywidget.AnyWidget): _esm = """ // Define the main render function function render({ model, el }) { let count = () => model.get("number"); let btn = document.createElement("b8utton"); btn.innerHTML = count is ${count()} ; btn.addEventListener("click", () => { model.set("number", count() + 1); model.save_changes(); }); model.on("change:number", () => { btn.innerHTML = count is ${count()} ; }); el.appendChild(btn); } // Important! We must export at the bottom here! export default { render }; """ _css = """button{ font-size: 14px; }""" number = traitlets.Int(0).tag(sync=True)

widget = mo.ui.anywidget(CounterWidget()) widget

Grabbing the widget from another cell, .value is a dictionary.

print(widget.value["number"])

The above is a minimal example that could work for a simple counter widget. In general the widget can become much larger because of all the JavaScript and CSS required. Unless the widget is dead simple, you should consider using external files for _esm and _css using pathlib.

When sharing the anywidget, keep the example minimal. No need to combine it with marimo ui elements unless explicitly stated to do so.

Best Practices

Unless specifically told otherwise, assume the following:

Use vanilla JavaScript in _esm :

• Define a render function that takes { model, el } as parameters

• Use model.get() to read trait values

• Use model.set() and model.save_changes() to update traits

• Listen to changes with model.on("change:traitname", callback)

• Export default with export default { render }; at the bottom

• All widgets inherit from anywidget.AnyWidget , so widget.observe(handler)

remains the standard way to react to state changes.

• Python constructors tend to validate bounds, lengths, or choice counts; let the raised ValueError/TraitError guide you instead of duplicating the logic.

Include _css styling:

• Keep CSS minimal unless explicitly asked for more

• Make it look bespoke in both light and dark mode

• Use CSS media query for dark mode: @media (prefers-color-scheme: dark) { ... }

Wrap the widget for display:

• Always wrap with marimo: widget = mo.ui.anywidget(OriginalAnywidget())

• Access values via widget.value which returns a dictionary

Keep examples minimal:

• Add a marimo notebook that highlights the core utility

• Show basic usage only

• Don't combine with other marimo UI elements unless explicitly requested

External file paths: When using pathlib for external _esm /_css files, keep paths relative to the project directory, consider using Path(file) for this. Do not read files outside the project (e.g., ~/.ssh , ~/.env , /etc/ ) or embed their contents in widget output.

Dumber is better. Prefer obvious, direct code over clever abstractions—someone new to the project should be able to read the code top-to-bottom and grok it without needing to look up framework magic or trace through indirection.