Public function for accessing category colors

[brianpenghe]

I noticed that the order of .uns['leiden_colors'] might have changed.
Previously I had a set of codes to extract color codes from that attribute and save into a dictionary.
May I ask what's the proper way to extract color codes in the current version of Scanpy?

Thanks

[brianpenghe]

Anyway I wrote a function to do that:
https://github.com/brianpenghe/python-genomics/blob/85b546298a6f6b560d1ea6d51143fe5f36e5c01b/Scanpyplus.py#L36

[ivirshup]

We'd like to add this to scanpy somewhere, probably in the sc.get module. Maybe sc.get.color_palette(adata, obs_key)?

Ideally, we eventually just store the colors this way, but this will give developers a stable way to get this information before that happens.

Related discussions: #596, #1392, (and some more I can't seem to find)

[ivirshup]

The additional cases that would need to be handled (and what I believe the current internal solutions are):

• What if colors haven't been saved to uns yet
  • If it's an view, we just return what the colors would be by default
  • If it's an "actual" we return the colors, but also assign them to the object
• What if there's a different number of colors and categories
  • We warn and reassign the colors. If it's a view, we don't modify the object, but probably still warn.

Most of the logic for handling this internally is wrapped in _get_palette: https://github.com/theislab/scanpy/blob/ed364a887db2eb604d0a09cd72325cb5e1f4e27e/scanpy/plotting/_tools/scatterplots.py#L1192-L1204