Viewing LOD status in python

samr85 · July 27, 2020, 12:09pm

Hi,
I’m working with massive data sets, so my data needs to be cut down significantly in python before the data is sent to the browser. As such, when the user zooms in, I need to send through a new block of data, calculating which might take up to about half a second.

Currently if someone zooms in with the mousewheel, this then runs the update code many times, so after zooming it continues to calculate all the old updates, change the data, then throw it away again for several seconds. I believe that respecting the LODStart and LODEnd events I should be able to fix this trivially, however I can’t work out how to find the LOD status in the python side - the only way I can find to get it is to subscribe to the LODStart and LODEnd in the JS side, and send that back through something like a dummy CDS or non-rendered checkbox. Is there any nicer way?
Thanks
Sam

Bryan · July 27, 2020, 4:21pm

The events_app.py example shows how register for all the available events in both Python and JS, including the LOD events:

github.com

bokeh/bokeh/blob/branch-3.0/examples/howto/events_app.py

""" Demonstration Bokeh app of how to register event callbacks in both
Javascript and Python using an adaptation of the color_scatter example
from the bokeh gallery. This example extends the js_events.py example
with corresponding Python event callbacks.
"""

import numpy as np

from bokeh import events
from bokeh.io import curdoc
from bokeh.layouts import column, row
from bokeh.models import Button, CustomJS, Div
from bokeh.plotting import figure


def display_event(div, attributes=[]):
    """
    Function to build a suitable CustomJS to display the current event
    in the div model.
    """

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samr85 · July 28, 2020, 11:18am

Thank you! Sorry for asking such a basic question - for some reason auto-complete in VSCode was showing me on_change, js_on_change and js_on_event, but wasn’t showing that on_event was a function!

samr85 · July 28, 2020, 3:03pm

Ah, turns out that LODEnd doesn’t fire for my graphs anyway, due to this bug still being open (LODEnd not firing when there are linked x_ranges) Linking the ranges of plots will break the LODEnd event · Issue #7707 · bokeh/bokeh · GitHub…

Bryan · July 29, 2020, 5:46pm

@samr85 Well that issue clearly fell off the radar. I’d suggest pinging it with a link back to this discussion and I will at least look enough to see if it seems like there is any quick/easy fix

samr85 · July 29, 2020, 5:51pm

Thank you! not 100% sure what you mean, but have put a link here in the issue. I’d also updated it yesterday with my guess at what the issue was, and some possible fixes - I’m happy to do a pull request to fix it myself if you’d let me know if any of my suggestions are acceptable.