hv.Image(group="") no longer works

[jbednar]

Until HoloViews 1.13.5 it was legal to use a group name of "" for an Image, but starting with v1.14.0 that gives an error:

import numpy as np, holoviews as hv
hv.extension()

ls = np.linspace(0, 10, 200)
xx, yy = np.meshgrid(ls, ls)
hv.Image(np.sin(xx)*np.cos(yy), group="").opts(cmap="greys")

In a brief investigation I couldn't see what code had changed, but apparently something has. In any case, searching for "capitalize" in https://github.com/holoviz/holoviews/blob/master/holoviews/core/util.py reveals various code paths that implicitly depend on the length of a string being nonzero, which doesn't seem like a safe assumption in general and is problematic in this case.

Here, obviously group could simply be omitted, but this came up for existing code in colorcet that accepted a user argument for the group, and when the user wants a blank group they now have to set the group to the very non-intuitive value "Image", whereas the much more reasonable group="" was sufficient to suppress printing the group name before.

[hoxbro]

A git bisect shows that this was introduced in #4567

[jbednar]

Thanks; that helps! Alas, #4567 is a squash of 98 commits, with changes to 51 files, so we'll probably have to git bisect within that PR.

[hoxbro]

Done. Commit 969aa17 seem to be the culprit.

You can get the desired plot by adding nodata=True in opts.

import numpy as np
import holoviews as hv
import panel as pn

hv.extension()

ls = np.linspace(0, 10, 200)
xx, yy = np.meshgrid(ls, ls)
image = hv.Image(np.sin(xx) * np.cos(yy)).opts(cmap="greys", nodata=True)

[jbednar]

Thanks! That commit doesn't seem to change any of this behavior directly, but it must have somehow triggered it, e.g. by defining a new Compositor for Images. #5131 seems to fix it by addressing the underlying assumption that a string will always have a first character, which is not true in this case.