Rewrite interp to use apply_ufunc (#9881)

* Don't eagerly compute dask arrays in localize

* Clean up test

* Clean up Variable handling

* Silence test warning

* Use apply_ufunc instead

* Add test for #4463

Closes #4463

* complete tests

* Add comments

* Clear up broadcasting

* typing

* try a different warning filter

* one more fix

* types + more duck_array_ops

* fixes

* [pre-commit.ci] auto fixes from pre-commit.com hooks

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* Apply suggestions from code review

Co-authored-by: Michael Niklas  <[email protected]>

* Apply suggestions from code review

Co-authored-by: Illviljan <[email protected]>

* [pre-commit.ci] auto fixes from pre-commit.com hooks

for more information, see https://pre-commit.ci

* Apply suggestions from code review

Co-authored-by: Illviljan <[email protected]>

* [pre-commit.ci] auto fixes from pre-commit.com hooks

for more information, see https://pre-commit.ci

* fix

* Revert "Apply suggestions from code review"

This reverts commit 1b9845d.

---------

Co-authored-by: Illviljan <[email protected]>
Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>
Co-authored-by: Michael Niklas <[email protected]>

xarray/core/dataset.py

@@ -2921,19 +2921,11 @@ def _validate_interp_indexers(
         """Variant of _validate_indexers to be used for interpolation"""
         for k, v in self._validate_indexers(indexers):
             if isinstance(v, Variable):
-                if v.ndim == 1:
-                    yield k, v.to_index_variable()
-                else:
-                    yield k, v
-            elif isinstance(v, int):
+                yield k, v
+            elif is_scalar(v):
                 yield k, Variable((), v, attrs=self.coords[k].attrs)
             elif isinstance(v, np.ndarray):
-                if v.ndim == 0:
-                    yield k, Variable((), v, attrs=self.coords[k].attrs)
-                elif v.ndim == 1:
-                    yield k, IndexVariable((k,), v, attrs=self.coords[k].attrs)
-                else:
-                    raise AssertionError()  # Already tested by _validate_indexers
+                yield k, Variable(dims=(k,), data=v, attrs=self.coords[k].attrs)
             else:
                 raise TypeError(type(v))
 
@@ -4127,18 +4119,6 @@ def interp(
 
         coords = either_dict_or_kwargs(coords, coords_kwargs, "interp")
         indexers = dict(self._validate_interp_indexers(coords))
-
-        if coords:
-            # This avoids broadcasting over coordinates that are both in
-            # the original array AND in the indexing array. It essentially
-            # forces interpolation along the shared coordinates.
-            sdims = (
-                set(self.dims)
-                .intersection(*[set(nx.dims) for nx in indexers.values()])
-                .difference(coords.keys())
-            )
-            indexers.update({d: self.variables[d] for d in sdims})
-
         obj = self if assume_sorted else self.sortby(list(coords))
 
         def maybe_variable(obj, k):
@@ -4169,16 +4149,18 @@ def _validate_interp_indexer(x, new_x):
             for k, v in indexers.items()
         }
 
-        # optimization: subset to coordinate range of the target index
-        if method in ["linear", "nearest"]:
-            for k, v in validated_indexers.items():
-                obj, newidx = missing._localize(obj, {k: v})
-                validated_indexers[k] = newidx[k]
-
-        # optimization: create dask coordinate arrays once per Dataset
-        # rather than once per Variable when dask.array.unify_chunks is called later
-        # GH4739
-        if obj.__dask_graph__():
+        has_chunked_array = bool(
+            any(is_chunked_array(v._data) for v in obj._variables.values())
+        )
+        if has_chunked_array:
+            # optimization: subset to coordinate range of the target index
+            if method in ["linear", "nearest"]:
+                for k, v in validated_indexers.items():
+                    obj, newidx = missing._localize(obj, {k: v})
+                    validated_indexers[k] = newidx[k]
+            # optimization: create dask coordinate arrays once per Dataset
+            # rather than once per Variable when dask.array.unify_chunks is called later
+            # GH4739
             dask_indexers = {
                 k: (index.to_base_variable().chunk(), dest.to_base_variable().chunk())
                 for k, (index, dest) in validated_indexers.items()
@@ -4190,10 +4172,9 @@ def _validate_interp_indexer(x, new_x):
             if name in indexers:
                 continue
 
-            if is_duck_dask_array(var.data):
-                use_indexers = dask_indexers
-            else:
-                use_indexers = validated_indexers
+            use_indexers = (
+                dask_indexers if is_duck_dask_array(var.data) else validated_indexers
+            )
 
             dtype_kind = var.dtype.kind
             if dtype_kind in "uifc":