Efficiently update patch colors

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1

Hello,
I would like to know if there exists an easy color update for a single patch that I might be missing…

When using a circle glyph, one can update its color conveniently via a ColumnDataSource:

cds_circle = ColumnDataSource(data=dict(x=[5], y=[2], c=["#111aaa"]))
fig.circle(x='x', y='y', color='c', source=cds_circle, radius=2)

def change_circle_color():
    new_color = ...
    cds_circle.data['c'] = [new_color]

However for fig.patch(x='x', y='y', color='c', source=cds_patch) we get a value error. Ok, x and y have a different column length compared to c, but also if c is of the same length, the Value Error still appears. It makes sense since defining multiple colors for one patch has no clear meaning.

So I tried around a bit, and came to the conclusion, that there exist these different options:

  1. Using fig.patches (with one patch entry only)
  2. storing the GlyphRenderer object r = fig.patch(...) and change the color in the callback function via r.glyph.fill_color and r.glyph.line_color
  3. Using a tag in the patch (e.g. fig.patch(..., tags=["some_tag"]) ) and find the right renderer in the callback function by using this tag by searching through the list of fig.renderers.

The approach #2 was not available in my code, since that would have led to re-writing a lot of functions to return r over multiple levels.
#3 would do it, but always searching through all renderers of a figure seems like an over kill.
#1 seems to be the approach which would require the least changes. However I don’t feel very well using patches which is actually meant for multiple patches, while patch for a single patch would suffice. The only problem with the single patch is the color update issue.

TLDR:
Can I somehow update the color of a single patch via ColumnDataSource similar to the circle code example above?

2

You can’t. The Patch glyph only draws one patch, it is not “vectorized” in the same way most other Bokeh glyphs. As a result, it can’t accept a vector of colors like most of other Bokeh glyphs. The correct way to update the color of a patch is your option 2: set the (singular) r.glyph.fill_color and r.glyph.line_color.

Also, just trying to spare any future confusion:

Ok, x and y have a different column length compared to c,

That’s never OK :slight_smile: All columns in a CDS must always all have the same length at all times. Think of it as cheap version of a Pandas DataFrame (which also always has same-length columns). Mis-matched column lengths should usually generate an error, but in case it does not, just know any result is explicitly undefined behavior.

3

Yes, I know that I must avoid different column length, that’s why I tried it with just multiplying the color value list of length 1 in order to get a list of the length of x and y :sweat_smile:

So I will keep going with option #1 then, since option #2 would require some time consuming re-coding. But I will keep in mind the other way for the future :slightly_smiling_face:

Is there btw. any benefit in using Patch instead of Patches? :thinking:
Patches seem to be more flexible and if one uses a single Patch, a bit of additional computation time shouldn’t matter anyway.

4

If you only need to draw a single patch, it really comes down to preference and where you want to the differences due to the connected topology to surface:

  • in the data, having to provide a “list of list” vector for a single patch with Patches
  • in the visual properties, which cannot be vectorized and must be set on the glyph for Patch
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