Solve missing interpolation method (cubicspline)

By commit 8bb2cc1 scipy.interpolate.CubicSpline method is
referenced in the pandas documentation (see pandas/core/generic.py)
but it is not wrapped by any interpolation method. This commit
solves this adding the corresponding wrapper. SciPy's CubicSpline
is a cubic spline data interpolator that allows explicit control
of the boundary conditions for the interval.

Changes to be committed:
	modified:   ../../../core/missing.py
	modified:   test_interpolate.py

Author: khyox

pandas/core/missing.py

@@ -112,6 +112,7 @@ def clean_interp_method(method, **kwargs):
         "akima",
         "spline",
         "from_derivatives",
+        "cubicspline",
     ]
     if method in ("spline", "polynomial") and order is None:
         raise ValueError("You must specify the order of the spline or polynomial.")
@@ -293,6 +294,7 @@ def interpolate_1d(
         "piecewise_polynomial",
         "pchip",
         "akima",
+        "cubicspline",
     ]
 
     if method in sp_methods:
@@ -349,6 +351,13 @@ def _interpolate_scipy_wrapper(
             ) from err
     elif method == "akima":
         alt_methods["akima"] = _akima_interpolate
+    elif method == "cubicspline":
+        try:
+            alt_methods["cubicspline"] = _cubicspline_interpolate
+        except AttributeError as err:
+            raise ImportError(
+                "Your version of Scipy does not support CubicSpline."
+            ) from err
 
     interp1d_methods = [
         "nearest",
@@ -406,7 +415,7 @@ def _from_derivatives(xi, yi, x, order=None, der=0, extrapolate=False):
     der : int or list
         How many derivatives to extract; None for all potentially nonzero
         derivatives (that is a number equal to the number of points), or a
-        list of derivatives to extract. This numberincludes the function
+        list of derivatives to extract. This number includes the function
         value as 0th derivative.
      extrapolate : bool, optional
         Whether to extrapolate to ouf-of-bounds points based on first and last
@@ -446,8 +455,7 @@ def _akima_interpolate(xi, yi, x, der=0, axis=0):
         A 1-D array of real values.  `yi`'s length along the interpolation
         axis must be equal to the length of `xi`. If N-D array, use axis
         parameter to select correct axis.
-    x : scalar or array_like
-        Of length M.
+    x : scalar or array_like of length M.
     der : int or list, optional
         How many derivatives to extract; None for all potentially
         nonzero derivatives (that is a number equal to the number
@@ -478,6 +486,85 @@ def _akima_interpolate(xi, yi, x, der=0, axis=0):
         return [P(x, nu) for nu in der]
 
 
+def _cubicspline_interpolate(xi, yi, x, axis=0, bc_type="not-a-knot", extrapolate=None):
+    """
+    Convenience function for cubic spline data interpolator.
+
+    See `scipy.interpolate.CubicSpline` for details.
+
+    Parameters
+    ----------
+    xi : array_like, shape (n,)
+        1-d array containing values of the independent variable.
+        Values must be real, finite and in strictly increasing order.
+    yi : array_like
+        Array containing values of the dependent variable. It can have
+        arbitrary number of dimensions, but the length along ``axis``
+        (see below) must match the length of ``x``. Values must be finite.
+    x : scalar or array_like, shape (m,)
+    axis : int, optional
+        Axis along which `y` is assumed to be varying. Meaning that for
+        ``x[i]`` the corresponding values are ``np.take(y, i, axis=axis)``.
+        Default is 0.
+    bc_type : string or 2-tuple, optional
+        Boundary condition type. Two additional equations, given by the
+        boundary conditions, are required to determine all coefficients of
+        polynomials on each segment [2]_.
+        If `bc_type` is a string, then the specified condition will be applied
+        at both ends of a spline. Available conditions are:
+        * 'not-a-knot' (default): The first and second segment at a curve end
+          are the same polynomial. It is a good default when there is no
+          information on boundary conditions.
+        * 'periodic': The interpolated functions is assumed to be periodic
+          of period ``x[-1] - x[0]``. The first and last value of `y` must be
+          identical: ``y[0] == y[-1]``. This boundary condition will result in
+          ``y'[0] == y'[-1]`` and ``y''[0] == y''[-1]``.
+        * 'clamped': The first derivative at curves ends are zero. Assuming
+          a 1D `y`, ``bc_type=((1, 0.0), (1, 0.0))`` is the same condition.
+        * 'natural': The second derivative at curve ends are zero. Assuming
+          a 1D `y`, ``bc_type=((2, 0.0), (2, 0.0))`` is the same condition.
+        If `bc_type` is a 2-tuple, the first and the second value will be
+        applied at the curve start and end respectively. The tuple values can
+        be one of the previously mentioned strings (except 'periodic') or a
+        tuple `(order, deriv_values)` allowing to specify arbitrary
+        derivatives at curve ends:
+        * `order`: the derivative order, 1 or 2.
+        * `deriv_value`: array_like containing derivative values, shape must
+          be the same as `y`, excluding ``axis`` dimension. For example, if
+          `y` is 1D, then `deriv_value` must be a scalar. If `y` is 3D with
+          the shape (n0, n1, n2) and axis=2, then `deriv_value` must be 2D
+          and have the shape (n0, n1).
+    extrapolate : {bool, 'periodic', None}, optional
+        If bool, determines whether to extrapolate to out-of-bounds points
+        based on first and last intervals, or to return NaNs. If 'periodic',
+        periodic extrapolation is used. If None (default), ``extrapolate`` is
+        set to 'periodic' for ``bc_type='periodic'`` and to True otherwise.
+
+    See Also
+    --------
+    scipy.interpolate.CubicHermiteSpline
+
+    Returns
+    -------
+    y : scalar or array_like
+        The result, of shape (m,)
+
+    References
+    ----------
+    .. [1] `Cubic Spline Interpolation
+            <https://en.wikiversity.org/wiki/Cubic_Spline_Interpolation>`_
+            on Wikiversity.
+    .. [2] Carl de Boor, "A Practical Guide to Splines", Springer-Verlag, 1978.
+    """
+    from scipy import interpolate
+
+    P = interpolate.CubicSpline(
+        xi, yi, axis=axis, bc_type=bc_type, extrapolate=extrapolate
+    )
+
+    return P(x)
+
+
 def interpolate_2d(
     values, method="pad", axis=0, limit=None, fill_value=None, dtype=None
 ):

pandas/tests/series/methods/test_interpolate.py

@@ -26,6 +26,7 @@
         "from_derivatives",
         "pchip",
         "akima",
+        "cubicspline",
     ]
 )
 def nontemporal_method(request):
@@ -55,6 +56,7 @@ def nontemporal_method(request):
         "from_derivatives",
         "pchip",
         "akima",
+        "cubicspline",
     ]
 )
 def interp_methods_ind(request):
@@ -97,6 +99,22 @@ def test_interpolate_time_raises_for_non_timeseries(self):
         with pytest.raises(ValueError, match=msg):
             non_ts.interpolate(method="time")
 
+    @td.skip_if_no_scipy
+    def test_interpolate_cubicspline(self):
+
+        ser = Series([10, 11, 12, 13])
+
+        expected = Series(
+            [11.00, 11.25, 11.50, 11.75, 12.00, 12.25, 12.50, 12.75, 13.00],
+            index=Index([1.0, 1.25, 1.5, 1.75, 2.0, 2.25, 2.5, 2.75, 3.0]),
+        )
+        # interpolate at new_index
+        new_index = ser.index.union(Index([1.25, 1.5, 1.75, 2.25, 2.5, 2.75])).astype(
+            float
+        )
+        interp_s = ser.reindex(new_index).interpolate(method="cubicspline")
+        tm.assert_series_equal(interp_s[1:3], expected)
+
     @td.skip_if_no_scipy
     def test_interpolate_pchip(self):