add `flag_values` and `flag_meanings` to 2D masks?

This would allow to indicate which region corresponds to which number via attrs and would work nicely with cf_xarray.

import cf_xarray
import xarray as xr

da = xr.DataArray(
    [[1, 2, 1], [1, 2, np.NaN]],
    dims=("lat", "lon"),
    attrs={
        "flag_values": [1, 2],
        "flag_meanings": "atlantic_ocean pacific_ocean",
        "standard_name": "region",
    },
  name="mask",
)

print(da.cf)

print(da.cf == "atlantic_ocean")

print(da.cf.isin(["indian_ocean", "pacific_ocean"]))

See

• https://cf-xarray.readthedocs.io/en/latest/flags.html
• xarray-contrib/cf-xarray#199
• http://cfconventions.org/Data/cf-conventions/cf-conventions-1.8/cf-conventions.html#flags

Open questions:

• do we add abbrevs or names (probably abbrevs)
• does it work for float arrays (as I encode “not belonging to any region” with NaN)?

[mathause]

Also I am not 100% clear on what abbrevs vs names means in this context?

Each region has 3 “indices” - a number, an abbreviation, and a name, e.g.:

Regions:
0    Asi          Asia

But we can only attach one of the “abbreviation” and “name” to flag_meanings - but which one should it be?

Currently I am tempted to just assign the abbreviation.

Alternatively we could make it an option:

    def mask(
        self,
        lon_or_obj,
        lat=None,
        lon_name="lon",
        lat_name="lat",
        method=None,
        wrap_lon=None,
+       flag_meanings: Literal["abbrevs", "names"] ="abbrevs",
    ):

---

The code would then (more or less) just be:

    # abbrevs should not have spaces
    abbrevs = " ".join(abbrevs)
    
    attrs={
        "flag_values": numbers,
        "flag_meanings": abrevs,
        "standard_name": "region",
    },

    mask_2D.attrs = attrs

---

Another open question - if some regions are missing from the mask - do we still list them under "flag_values" and "flag_meanings"? I’d also need to test this with cf_xarray.

But overall I think this whole thing should be relatively straight-forward…

[jbusecke]

As long as we have some set way to do it, that seems totally fine to me. But there might be some spec that I am missing here?