Support vectorized interpolation with more scipy interpolators

doc/whats-new.rst

@@ -32,6 +32,9 @@ New Features
   `Tom Nicholas <https://github.com/TomNicholas>`_.
 - Added zarr backends for :py:func:`open_groups` (:issue:`9430`, :pull:`9469`).
   By `Eni Awowale <https://github.com/eni-awowale>`_.
+- Added support for vectorized interpolation using additional interpolators
+  from the ``scipy.interpolate`` module (:issue:`9049`, :pull:`9526`).
+  By `Holly Mandel <https://github.com/hollymandel>`_.
 
 Breaking changes
 ~~~~~~~~~~~~~~~~

xarray/core/dataarray.py

@@ -2224,12 +2224,12 @@ def interp(
 
         Performs univariate or multivariate interpolation of a DataArray onto
         new coordinates using scipy's interpolation routines. If interpolating
-        along an existing dimension, :py:class:`scipy.interpolate.interp1d` is
-        called. When interpolating along multiple existing dimensions, an
+        along an existing dimension,  either :py:class:`scipy.interpolate.interp1d`
+        or a 1-dimensional scipy interpolator (e.g. :py:class:`scipy.interpolate.KroghInterpolator`)
+        is called. When interpolating along multiple existing dimensions, an
         attempt is made to decompose the interpolation into multiple
-        1-dimensional interpolations. If this is possible,
-        :py:class:`scipy.interpolate.interp1d` is called. Otherwise,
-        :py:func:`scipy.interpolate.interpn` is called.
+        1-dimensional interpolations. If this is possible, the 1-dimensional interpolator is called.
+        Otherwise, :py:func:`scipy.interpolate.interpn` is called.
 
         Parameters
         ----------

xarray/core/dataset.py

@@ -3885,12 +3885,12 @@ def interp(
 
         Performs univariate or multivariate interpolation of a Dataset onto
         new coordinates using scipy's interpolation routines. If interpolating
-        along an existing dimension, :py:class:`scipy.interpolate.interp1d` is
-        called.  When interpolating along multiple existing dimensions, an
+        along an existing dimension, either :py:class:`scipy.interpolate.interp1d`
+        or a 1-dimensional scipy interpolator (e.g. :py:class:`scipy.interpolate.KroghInterpolator`)
+        is called.  When interpolating along multiple existing dimensions, an
         attempt is made to decompose the interpolation into multiple
-        1-dimensional interpolations. If this is possible,
-        :py:class:`scipy.interpolate.interp1d` is called. Otherwise,
-        :py:func:`scipy.interpolate.interpn` is called.
+        1-dimensional interpolations. If this is possible, the 1-dimensional interpolator
+        is called. Otherwise, :py:func:`scipy.interpolate.interpn` is called.
 
         Parameters
         ----------

xarray/core/missing.py

@@ -138,6 +138,7 @@ def __init__(
         copy=False,
         bounds_error=False,
         order=None,
+        axis=-1,
         **kwargs,
     ):
         from scipy.interpolate import interp1d
@@ -173,6 +174,7 @@ def __init__(
             bounds_error=bounds_error,
             assume_sorted=assume_sorted,
             copy=copy,
+            axis=axis,
             **self.cons_kwargs,
         )
 
@@ -479,7 +481,8 @@ def _get_interpolator(
     interp1d_methods = get_args(Interp1dOptions)
     valid_methods = tuple(vv for v in get_args(InterpOptions) for vv in get_args(v))
 
-    # prioritize scipy.interpolate
+    # prefer numpy.interp for 1d linear interpolation. This function cannot
+    # take higher dimensional data but scipy.interp1d can.
     if (
         method == "linear"
         and not kwargs.get("fill_value", None) == "extrapolate"
@@ -492,21 +495,33 @@ def _get_interpolator(
         if method in interp1d_methods:
             kwargs.update(method=method)
             interp_class = ScipyInterpolator
-        elif vectorizeable_only:
-            raise ValueError(
-                f"{method} is not a vectorizeable interpolator. "
-                f"Available methods are {interp1d_methods}"
-            )
         elif method == "barycentric":
+            kwargs.update(axis=-1)
             interp_class = _import_interpolant("BarycentricInterpolator", method)
         elif method in ["krogh", "krog"]:
+            kwargs.update(axis=-1)
             interp_class = _import_interpolant("KroghInterpolator", method)
         elif method == "pchip":
+            kwargs.update(axis=-1)
             interp_class = _import_interpolant("PchipInterpolator", method)
         elif method == "spline":
+            utils.emit_user_level_warning(
+                "The 1d SplineInterpolator class is performing an incorrect calculation and "
+                "is being deprecated. Please use `method=polynomial` for 1D Spline Interpolation.",
+                PendingDeprecationWarning,
+            )
+            if vectorizeable_only:
+                raise ValueError(f"{method} is not a vectorizeable interpolator. ")
             kwargs.update(method=method)
             interp_class = SplineInterpolator
         elif method == "akima":
+            kwargs.update(axis=-1)
+            interp_class = _import_interpolant("Akima1DInterpolator", method)
+        elif method == "makima":
+            kwargs.update(method="makima", axis=-1)
+            interp_class = _import_interpolant("Akima1DInterpolator", method)
+        elif method == "makima":
+            kwargs.update(method="makima", axis=-1)
             interp_class = _import_interpolant("Akima1DInterpolator", method)
         else:
             raise ValueError(f"{method} is not a valid scipy interpolator")
@@ -525,6 +540,7 @@ def _get_interpolator_nd(method, **kwargs):
 
     if method in valid_methods:
         kwargs.update(method=method)
+        kwargs.setdefault("bounds_error", False)
         interp_class = _import_interpolant("interpn", method)
     else:
         raise ValueError(
@@ -614,9 +630,6 @@ def interp(var, indexes_coords, method: InterpOptions, **kwargs):
     if not indexes_coords:
         return var.copy()
 
-    # default behavior
-    kwargs["bounds_error"] = kwargs.get("bounds_error", False)
-
     result = var
     # decompose the interpolation into a succession of independent interpolation
     for indep_indexes_coords in decompose_interp(indexes_coords):
@@ -663,8 +676,8 @@ def interp_func(var, x, new_x, method: InterpOptions, kwargs):
     new_x : a list of 1d array
         New coordinates. Should not contain NaN.
     method : string
-        {'linear', 'nearest', 'zero', 'slinear', 'quadratic', 'cubic'} for
-        1-dimensional interpolation.
+        {'linear', 'nearest', 'zero', 'slinear', 'quadratic', 'cubic', 'pchip', 'akima',
+            'makima', 'barycentric', 'krogh'} for 1-dimensional interpolation.
         {'linear', 'nearest'} for multidimensional interpolation
     **kwargs
         Optional keyword arguments to be passed to scipy.interpolator
@@ -756,7 +769,7 @@ def interp_func(var, x, new_x, method: InterpOptions, kwargs):
 def _interp1d(var, x, new_x, func, kwargs):
     # x, new_x are tuples of size 1.
     x, new_x = x[0], new_x[0]
-    rslt = func(x, var, assume_sorted=True, **kwargs)(np.ravel(new_x))
+    rslt = func(x, var, **kwargs)(np.ravel(new_x))
     if new_x.ndim > 1:
         return reshape(rslt, (var.shape[:-1] + new_x.shape))
     if new_x.ndim == 0:

xarray/core/types.py

@@ -228,7 +228,9 @@ def copy(
 Interp1dOptions = Literal[
     "linear", "nearest", "zero", "slinear", "quadratic", "cubic", "polynomial"
 ]
-InterpolantOptions = Literal["barycentric", "krogh", "pchip", "spline", "akima"]
+InterpolantOptions = Literal[
+    "barycentric", "krogh", "pchip", "spline", "akima", "makima"
+]
 InterpOptions = Union[Interp1dOptions, InterpolantOptions]
 
 DatetimeUnitOptions = Literal[

xarray/tests/__init__.py

@@ -87,6 +87,7 @@ def _importorskip(
 
 has_matplotlib, requires_matplotlib = _importorskip("matplotlib")
 has_scipy, requires_scipy = _importorskip("scipy")
+has_scipy_ge_1_13, requires_scipy_ge_1_13 = _importorskip("scipy", "1.13")
 with warnings.catch_warnings():
     warnings.filterwarnings(
         "ignore",

xarray/tests/test_interp.py

@@ -16,6 +16,7 @@
     assert_identical,
     has_dask,
     has_scipy,
+    has_scipy_ge_1_13,
     requires_cftime,
     requires_dask,
     requires_scipy,
@@ -132,29 +133,66 @@ def func(obj, new_x):
     assert_allclose(actual, expected)
 
 
-@pytest.mark.parametrize("use_dask", [False, True])
-def test_interpolate_vectorize(use_dask: bool) -> None:
-    if not has_scipy:
-        pytest.skip("scipy is not installed.")
-
-    if not has_dask and use_dask:
-        pytest.skip("dask is not installed in the environment.")
-
+@requires_scipy
+@pytest.mark.parametrize(
+    "use_dask, method",
+    (
+        (False, "linear"),
+        (False, "akima"),
+        pytest.param(
+            False,
+            "makima",
+            marks=pytest.mark.skipif(not has_scipy_ge_1_13, reason="scipy too old"),
+        ),
+        pytest.param(
+            True,
+            "linear",
+            marks=pytest.mark.skipif(not has_dask, reason="dask not available"),
+        ),
+        pytest.param(
+            True,
+            "akima",
+            marks=pytest.mark.skipif(not has_dask, reason="dask not available"),
+        ),
+    ),
+)
+def test_interpolate_vectorize(use_dask: bool, method: InterpOptions) -> None:
     # scipy interpolation for the reference
-    def func(obj, dim, new_x):
+    def func(obj, dim, new_x, method):
+        scipy_kwargs = {}
+        interpolant_options = {
+            "barycentric": scipy.interpolate.BarycentricInterpolator,
+            "krogh": scipy.interpolate.KroghInterpolator,
+            "pchip": scipy.interpolate.PchipInterpolator,
+            "akima": scipy.interpolate.Akima1DInterpolator,
+            "makima": scipy.interpolate.Akima1DInterpolator,
+        }
+
         shape = [s for i, s in enumerate(obj.shape) if i != obj.get_axis_num(dim)]
         for s in new_x.shape[::-1]:
             shape.insert(obj.get_axis_num(dim), s)
 
-        return scipy.interpolate.interp1d(
-            da[dim],
-            obj.data,
-            axis=obj.get_axis_num(dim),
-            bounds_error=False,
-            fill_value=np.nan,
-        )(new_x).reshape(shape)
+        if method in interpolant_options:
+            interpolant = interpolant_options[method]
+            if method == "makima":
+                scipy_kwargs["method"] = method
+            return interpolant(
+                da[dim], obj.data, axis=obj.get_axis_num(dim), **scipy_kwargs
+            )(new_x).reshape(shape)
+        else:
+
+            return scipy.interpolate.interp1d(
+                da[dim],
+                obj.data,
+                axis=obj.get_axis_num(dim),
+                kind=method,
+                bounds_error=False,
+                fill_value=np.nan,
+                **scipy_kwargs,
+            )(new_x).reshape(shape)
 
     da = get_example_data(0)
+
     if use_dask:
         da = da.chunk({"y": 5})
 
@@ -165,17 +203,17 @@ def func(obj, dim, new_x):
         coords={"z": np.random.randn(30), "z2": ("z", np.random.randn(30))},
     )
 
-    actual = da.interp(x=xdest, method="linear")
+    actual = da.interp(x=xdest, method=method)
 
     expected = xr.DataArray(
-        func(da, "x", xdest),
+        func(da, "x", xdest, method),
         dims=["z", "y"],
         coords={
             "z": xdest["z"],
             "z2": xdest["z2"],
             "y": da["y"],
             "x": ("z", xdest.values),
-            "x2": ("z", func(da["x2"], "x", xdest)),
+            "x2": ("z", func(da["x2"], "x", xdest, method)),
         },
     )
     assert_allclose(actual, expected.transpose("z", "y", transpose_coords=True))
@@ -191,18 +229,18 @@ def func(obj, dim, new_x):
         },
     )
 
-    actual = da.interp(x=xdest, method="linear")
+    actual = da.interp(x=xdest, method=method)
 
     expected = xr.DataArray(
-        func(da, "x", xdest),
+        func(da, "x", xdest, method),
         dims=["z", "w", "y"],
         coords={
             "z": xdest["z"],
             "w": xdest["w"],
             "z2": xdest["z2"],
             "y": da["y"],
             "x": (("z", "w"), xdest.data),
-            "x2": (("z", "w"), func(da["x2"], "x", xdest)),
+            "x2": (("z", "w"), func(da["x2"], "x", xdest, method)),
         },
     )
     assert_allclose(actual, expected.transpose("z", "w", "y", transpose_coords=True))
@@ -393,19 +431,17 @@ def test_nans(use_dask: bool) -> None:
     assert actual.count() > 0
 
 
+@requires_scipy
 @pytest.mark.parametrize("use_dask", [True, False])
 def test_errors(use_dask: bool) -> None:
-    if not has_scipy:
-        pytest.skip("scipy is not installed.")
-
-    # akima and spline are unavailable
+    # spline is unavailable
     da = xr.DataArray([0, 1, np.nan, 2], dims="x", coords={"x": range(4)})
     if not has_dask and use_dask:
         pytest.skip("dask is not installed in the environment.")
         da = da.chunk()
 
-    for method in ["akima", "spline"]:
-        with pytest.raises(ValueError):
+    for method in ["spline"]:
+        with pytest.raises(ValueError), pytest.warns(PendingDeprecationWarning):
             da.interp(x=[0.5, 1.5], method=method)  # type: ignore[arg-type]
 
     # not sorted
@@ -922,7 +958,10 @@ def test_interp1d_bounds_error() -> None:
         (("x", np.array([0, 0.5, 1, 2]), dict(unit="s")), False),
     ],
 )
-def test_coord_attrs(x, expect_same_attrs: bool) -> None:
+def test_coord_attrs(
+    x,
+    expect_same_attrs: bool,
+) -> None:
     base_attrs = dict(foo="bar")
     ds = xr.Dataset(
         data_vars=dict(a=2 * np.arange(5)),

xarray/tests/test_missing.py

@@ -137,7 +137,11 @@ def test_scipy_methods_function(method) -> None:
     # Note: Pandas does some wacky things with these methods and the full
     # integration tests won't work.
     da, _ = make_interpolate_example_data((25, 25), 0.4, non_uniform=True)
-    actual = da.interpolate_na(method=method, dim="time")
+    if method == "spline":
+        with pytest.warns(PendingDeprecationWarning):
+            actual = da.interpolate_na(method=method, dim="time")
+    else:
+        actual = da.interpolate_na(method=method, dim="time")
     assert (da.count("time") <= actual.count("time")).all()