issues

At SciPy, I had the realization that it would be relatively straightforward to make element-wise math between xarray objects lazy. This would let us support lazy coordinate arrays, a feature that has quite a few use-cases, e.g., for both geoscience and astronomy.

The trick would be to write a lazy array class that holds an element-wise vectorized function and passes indexers on to its arguments. I haven't thought too hard about this yet for vectorized indexing, but it could be quite efficient for outer indexing. I have some prototype code but no tests yet.

The question is how to hook this into xarray operations. In particular, supposing that the inputs to a function do no hold dask arrays: - Should we try to make every element-wise operation with vectorized functions (ufuncs) lazy by default? This might have negative performance implications and would be a little tricky to implement with xarray's current code, since we still implement binary operations like + with separate logic from apply_ufunc. - Should we make every element-wise operation that explicitly uses apply_ufunc() lazy by default? - Or should we only make element-wise operations lazy with apply_ufunc() if you use some special flag, e.g., apply_ufunc(..., lazy=True)?

I am leaning towards the last option for now but would welcome other opinions.