`NDPointIndex` should fail early and throw nice error if scipy not installed

[ianhi]

What happened?

NDPointIndex has a hard dependency on scipy but when you try to use it with a default xarray install with no extras

you get this error:

➜  nd-scipy git:(main) ✗ uv run nd_scipy.py
Traceback (most recent call last):
  File "/Users/ian/Documents/dev/testing/nd-scipy/nd_scipy.py", line 19, in <module>
    ds_index = ds.set_xindex(("xx", "yy"), xr.indexes.NDPointIndex)
  File "/Users/ian/.cache/uv/environments-v2/nd-scipy-363973e27d29e760/lib/python3.14/site-packages/xarray/core/dataset.py", line 5019, in set_xindex
    index = index_cls.from_variables(coord_vars, options=options)
  File "/Users/ian/.cache/uv/environments-v2/nd-scipy-363973e27d29e760/lib/python3.14/site-packages/xarray/indexes/nd_point_index.py", line 276, in from_variables
    tree_adapter_cls(points, options=opts),
    ~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/ian/.cache/uv/environments-v2/nd-scipy-363973e27d29e760/lib/python3.14/site-packages/xarray/indexes/nd_point_index.py", line 78, in __init__
    from scipy.spatial import KDTree
ModuleNotFoundError: No module named 'scipy'

which could be confusing as a user.

What did you expect to happen?

• An error raised on import of the Index rather only once I try to create one
• More informative error message saying something like "to use NDPointIndex please install scipy"

Similar to how there is a nice message if you are missing a backend:

ValueError: unrecognized engine 'zarr' must be one of your download engines: ['store']. To install additional dependencies, see:
https://docs.xarray.dev/en/stable/user-guide/io.html
https://docs.xarray.dev/en/stable/getting-started-guide/installing.html

Minimal Complete Verifiable Example

# /// script
# requires-python = ">=3.14"
# dependencies = [
#     "xarray",
# ]
# ///


import numpy as np
import xarray as xr
from xarray.indexes import NDPointIndex

shape = (5, 10)
xx = xr.DataArray(np.random.uniform(0, 10, size=shape), dims=("y", "x"))
yy = xr.DataArray(np.random.uniform(0, 5, size=shape), dims=("y", "x"))
data = (xx - 5) ** 2 + (yy - 2.5) ** 2

ds = xr.Dataset(data_vars={"data": data}, coords={"xx": xx, "yy": yy})
ds_index = ds.set_xindex(("xx", "yy"), NDPointIndex)

Steps to reproduce

uv run

MVCE confirmation

• Minimal example — the example is as focused as reasonably possible to demonstrate the underlying issue in xarray.
• Complete example — the example is self-contained, including all data and the text of any traceback.
• Verifiable example — the example copy & pastes into an IPython prompt or Binder notebook, returning the result.
• New issue — a search of GitHub Issues suggests this is not a duplicate.
• Recent environment — the issue occurs with the latest version of xarray and its dependencies.

Relevant log output

Anything else we need to know?

No response

Environment

Details