UxDataArray.isel() on edge-centered data crashes with confusing KeyError: No variable named '_subgrid_edge_indices'... if connectivity isn't populated #1758

Version

v2026.09.1

How did you install UXarray?

Source

What happened?

Calling UxDataArray.isel(n_face=...) or .isel(n_node=...) on edge-centered data crashes with a very confusing error (see title). I suspect this is related to the lines in uxarray.grid.slice._slice_face_indices commented as "Only slice edge dimension if we have the face edge connectivity", which hide the ds["_subgrid_edge_indices"] = xr.DataArray(edge_indices, dims=["n_edge"]) inside a check of if "face_edge_connectivity" in ds, and when that check gives False it silently does something else instead of crashing or raising a warning.

This problem currently only affects UxDataArray, not also UxDataset, because UxDataset simply ignores the command to slice by anything (see #1713) in these cases.

What did you expect to happen?

I expected a clearer error message telling me why I can't do this, or suggesting that I should call arr.uxgrid.face_edge_connectivity to populate the connectivity and then try again, or for the connectivity to just be automatically populated at this stage because it was required for an operation I was attempting (most/all other uxarray grid vars get populated automatically as soon as they are needed…).

Can you provide a MCVE to repoduce the bug?

import uxarray as ux
arr = ux.tutorial.open_dataset('quad-hexagon-random-edge')['random_data_edge']
arr.isel(n_face=0)  # crashes
arr.isel(n_node=0)  # crashes

# but, if running the following before calling isel():
arr.uxgrid.face_edge_connectivity

# then the isel() commands do not crash:
arr.isel(n_face=0)
arr.isel(n_node=0)