Add a `side_effect` option to data properties.

[julianstirling]

Is your feature request related to a problem? Please describe.
Data properties are a lot shorter and cleaner to write than functional properties when a property is simple. However, if I need something to happen when the property is set (forwarding it to hardware, re-generating cached data, etc) then a functional property is needed.

Describe the solution you'd like

A side_effect argument that can accept a callable.

frame_rate: float = lt.property(default=25, side_effect=self.update_camera_properties)

I am not sure however, if it is possible to use self here? As it is within a class but not within a classes function where self is defined?

[rwb27]

It's definitely not possible to use self there, unfortunately. My thought on the syntax was we could do something like:

class MyThing(lt.Thing):
    frame_rate: float = lt.property(...)

    @lt.on_set("frame_rate"):
    def actually_set_frame_rate(self):
        ...

It's possible that, if we wrote a MyPy plugin, we could replace the @lt.on_set("name") with @frame_rate.on_set which is probably nicer.

Being able to add a side effect of setting data properties was always the intention - but I didn't implement it when I overhauled properties, as it was relatively separable into its own block of work. If you've hit the point where this would be useful, it's definitely worth doing something.