You can use the PolyDrawTool as described in this post and then you can respond to the updates it makes to the data source with standard js_on_change or (if this is a Bokeh server app) on_change methods.
You can use the PolyDrawTool as described in this post and then you can respond to the updates it makes to the data source with standard js_on_change or (if this is a Bokeh server app) on_change methods.