Examples

Complete, runnable examples live in the examples/ directory of the repository. This page walks through what they demonstrate.

Qt (observe_qt.py)

examples/observe_qt.py shows the unidirectional data flow that observ enables in a classic widget UI:

• A single state = reactive({"clicked": 0, "progress": 0}) is shared between two independent widgets.

• The Display widget declares watchers that keep a label and a progress bar in sync with the state — note the immediate=True so the widgets are also initialized from the watchers:

  self.watchers["label"] = watch(label_text, self.label.setText, immediate=True)
  self.watchers["progress_visible"] = watch(
      lambda: state["progress"] > 0, self.progress.setVisible, immediate=True
  )
  

• The Controls widget never talks to Display directly — it only mutates the state (from a worker thread's signals), and the watchers take care of the rest.

• The scheduler is hooked up with init("asyncio") and the app runs under QtAsyncio.run(), so watcher callbacks are batched per event loop iteration.

Run it with:

uv run --group qt examples/observe_qt.py

Rendercanvas (observe_rc.py)

examples/observe_rc.py demonstrates observ driving an interactive canvas application:

• The positions of three draggable blocks live in reactive state.
• Two @watch-decorated effects keep the blocks at a consistent distance from each other: dragging one block updates the state, which triggers the effects, which update the other blocks' positions in turn.
• The scheduler is integrated with the rendercanvas loop via scheduler.register_rendercanvas(loop).

This example also shows the scheduler's cycle handling at work: the two effects write to state the other one depends on, and the scheduler's deduplication is what lets them settle instead of ping-ponging forever.

Run it with:

uv run --group pygfx --group numpy examples/observe_rc.py