Currently, opening the Qt figureoptions UI for an image whose cmap is not registered in the colormap registry, has a name not matching any registry entry, but is actually equal (==, i.e. has the same LUT and colorbar-extension attributes) to a registry entry, leads to an error. A typical example would be

import cmap, pylab as p  # third-party
p.imshow([[0, 1]], cmap=cmap.Colormap("bids:magma").to_mpl())

and opening the qt figure options; this leads to the error "index 'bids:magma' is invalid ...".

Note that if the cmap is different from any registered cmap then we already add it to the UI combobox (try e.g. cmap=cmap.Colormap("imagej:fire")); the only problem was if it was equal to a registered cmap (this arises because when the code was originally written, cmap instance equality was by identity, not by comparing LUTs, so the cmap not in cm._colormaps.values() check behaved differently).

Fix that by checking whether the colormap name is registered. The behavior is still ill-defined in the opposite (theoretical) case of an unregistered cmap different from any registered cmap but with a matching name, but I'd argue that case is more pathological.

Test by running the above code and opening the qt figureoptions.

(aka how some code I wrote 9 years ago (#5469, a bit scary it's been that long) got broken by some code written 5.5 years later (#20227), and how to fix that with a one-line patch with 20 lines of explanation. I guess that's technically a regression :-))

PR summary

PR checklist

• "closes #0000" is in the body of the PR description to link the related issue
• new and changed code is tested
• Plotting related features are demonstrated in an example
• New Features and API Changes are noted with a directive and release note
• Documentation complies with general and docstring guidelines