No attribute 'session_context' with redim range and datashade

[mmorys]

Software version info

Ubuntu 18.04
python 3.7.6
bokeh 2.0.1
holoviews 1.13.1
notebook 6.0.3
browser: Chromium 80.0.3987.149

Description of expected behavior and the observed behavior

If a plot, such as Scatter, has its range modified using redim prior to being datashaded
(e.g.datashade(hv.Scatter(df, 'x', 'y').redim(y=hv.Dimension('y', range=(-0.5,1.5)))) ),
an error is thrown. This should not cause an error, and worked prior to holoviews 1.13.0.

Conditions for this error behavior:

• holoviews>=1.13.0
• Deployed using bokeh serve (Same error when deployed using panel.show() if using panel library)
• redim used to modify range values

This error does not occur if any of the following are true:

• holoviews<1.13.0
• Run in a Jupyter notebook
• redim does not modify range (e.g. hv.Scatter(df, 'x', 'y').redim(y=hv.Dimension('y2')))

Complete, minimal, self-contained example code that reproduces the issue

# minimal_example.py

import holoviews as hv
import numpy as np
import pandas as pd
from holoviews.operation.datashader import datashade

hv.extension('bokeh')
renderer = hv.renderer('bokeh')

# Create dummy dataset
num_pts = 1000
df = pd.DataFrame({'x':np.linspace(0,100,num_pts), 'y':np.random.random(num_pts)})

# Create scatterplot
hv_plot = hv.Scatter(df, 'x', 'y')

# Redim range (THIS IS THE LINE THAT CAUSES AN ERROR)
hv_plot = hv_plot.redim(y=hv.Dimension('y', range=(-0.5,1.5)))

# Datashade the scatter plot
datashaded_plot = datashade(hv_plot)

# Create bokeh doc
doc = renderer.server_doc(datashaded_plot)

# Run example using 'bokeh serve --show minimal_example.py' from console

Stack traceback and/or browser JavaScript console output

bokeh.application.application - ERROR - Error running application handler <bokeh.application.handlers.script.ScriptHandler object at 0x7f58c595dcd0>: 'NoneType' object has no attribute 'session_context'
File "plot.py", line 218, in refresh:
if self.document.session_context: Traceback (most recent call last):
  File "/home/user/anaconda3/envs/bokeh-dev/lib/python3.7/site-packages/bokeh/application/handlers/code_runner.py", line 197, in run
    exec(self._code, module.__dict__)
  File "/home/user/Documents/panel_viz/bin/minimal_example.py", line 25, in <module>
    doc = renderer.server_doc(datashaded_plot)
  File "/home/user/anaconda3/envs/bokeh-dev/lib/python3.7/site-packages/holoviews/plotting/renderer.py", line 466, in server_doc
    return obj.layout.server_doc(doc)
  File "/home/user/anaconda3/envs/bokeh-dev/lib/python3.7/site-packages/panel/viewable.py", line 600, in server_doc
    model = self.get_root(doc)
  File "/home/user/anaconda3/envs/bokeh-dev/lib/python3.7/site-packages/panel/viewable.py", line 536, in get_root
    root = self._get_model(doc, comm=comm)
  File "/home/user/anaconda3/envs/bokeh-dev/lib/python3.7/site-packages/panel/layout.py", line 136, in _get_model
    objects = self._get_objects(model, [], doc, root, comm)
  File "/home/user/anaconda3/envs/bokeh-dev/lib/python3.7/site-packages/panel/layout.py", line 126, in _get_objects
    child = pane._get_model(doc, root, model, comm)
  File "/home/user/anaconda3/envs/bokeh-dev/lib/python3.7/site-packages/panel/pane/holoviews.py", line 227, in _get_model
    plot = self._render(doc, comm, root)
  File "/home/user/anaconda3/envs/bokeh-dev/lib/python3.7/site-packages/panel/pane/holoviews.py", line 286, in _render
    return renderer.get_plot(self.object, **kwargs)
  File "/home/user/anaconda3/envs/bokeh-dev/lib/python3.7/site-packages/holoviews/plotting/bokeh/renderer.py", line 73, in get_plot
    plot = super(BokehRenderer, self_or_cls).get_plot(obj, doc, renderer, **kwargs)
  File "/home/user/anaconda3/envs/bokeh-dev/lib/python3.7/site-packages/holoviews/plotting/renderer.py", line 223, in get_plot
    plot.update(init_key)
  File "/home/user/anaconda3/envs/bokeh-dev/lib/python3.7/site-packages/holoviews/plotting/plot.py", line 878, in update
    return self.initialize_plot()
  File "/home/user/anaconda3/envs/bokeh-dev/lib/python3.7/site-packages/holoviews/plotting/bokeh/element.py", line 1334, in initialize_plot
    Stream.trigger(trigger)
  File "/home/user/anaconda3/envs/bokeh-dev/lib/python3.7/site-packages/holoviews/streams.py", line 169, in trigger
    subscriber(**dict(union))
  File "/home/user/anaconda3/envs/bokeh-dev/lib/python3.7/site-packages/holoviews/plotting/plot.py", line 218, in refresh
    if self.document.session_context:
AttributeError: 'NoneType' object has no attribute 'session_context'

[philippjfr]

We'll have to update the docs, I would not recommend using renderer.server_doc at this point (although it should be fixed for backward compatibility). Instead I'd use panel:

import panel as pn
pn.panel(datashaded_plot).servable()

[mmorys]

I will be using panel as you mentioned. However, the same bug still exists when launching the code using panel serve --show minimal_example.py.

[philippjfr]

Okay, thanks, I can indeed reproduce.

[philippjfr]

Will hopefully find a fix today and then release 1.13.2.

[github-actions]

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