disallow boolean coordinates?

Today I stumbled over a small pitfall, which I think could be avoided:

I am working with arrays that have axes labeled with categorical values and I ended up using True/False as labels for some binary categories:

test = xarray.DataArray(
     numpy.ones((3,2)), 
     dims=["binary","ternary"],
     coords={"ternary":[3,7,9],"binary":[False,True]}
)

now came the big surprise, when I wanted to reduce over selections of the data:

test.sel(ternary=[9,3,7]) # does exactly what I expect and gives me the correctly permuted 3x2 array
test.sel(binary=[True,False]) # does not do what I expect

Instead of using the coordinate values like with the ternary category, it uses the list as boolean mask and hence I get a 3x1 array at the binary=False coordinate.

I assume that this behavior is reasonable in most cases - And I for sure will stop using bools as binary category labels.
That said in the above case the conceptually identical call results in completely different outcome.

My (radical) proposal would be: forbid binary coordinates in general to avoid such confusion.

Curious about your thoughts! Hth,

Marti

Originally posted by @martinitus in https://github.com/pydata/xarray/discussions/4861

Start by examining DataArray.sel and how list-like boolean indexers are interpreted for coordinates. Compare the behavior of boolean and numeric coordinate selections, then determine the intended API change before implementing it. Done means the project has a decided, tested behavior for boolean coordinates that avoids the reported ambiguity.