Callback metodologies - how to callback a python function (as opposed to js code), without polling?

Question in short

An example from January shows ow to use polling to apply a python callback to a change in a data source. The recent documentation on defining actions and callbacks shows how to call js code on a slider widget.

Is there a way to use the action/callback mechanism to call python functions, or should I refer to the example that uses polling?

Context

For a simplified flask app that does not rely on the bokeh-server, I would like to attach the callback to a requests.post method to communicate with the server

Thanks

As a ref for my own question, Fabio Pilger in this post seems to suggest polling is still the correct approach in 0.9x (short of dipping toes in js)

https://groups.google.com/a/continuum.io/forum/#!topic/bokeh/c4mbWMMrd94