BUG: GH10365 in interpolate_1d when method is piecewise_polynomial

doc/source/whatsnew/v0.17.0.txt

@@ -78,3 +78,4 @@ Bug Fixes
 - Bug in ``ExcelReader`` when worksheet is empty (:issue:`6403`)
 - Bug in ``Table.select_column`` where name is not preserved (:issue:`10392`)
 - Bug in ``DataFrame.interpolate`` with ``axis=1`` and ``inplace=True`` (:issue:`10395`)
+- Bug in ``interpolate_1d`` with ``method='piecewise_polynomial`` (:issue:`10365`)

pandas/core/common.py

@@ -1682,8 +1682,6 @@ def _interp_limit(invalid, limit):
                   'piecewise_polynomial', 'pchip']
     if method in sp_methods:
         new_x = new_x[firstIndex:]
-        xvalues = xvalues[firstIndex:]
-
         result[firstIndex:][invalid] = _interpolate_scipy_wrapper(
             valid_x, valid_y, new_x, method=method, fill_value=fill_value,
             bounds_error=bounds_error, order=order)
@@ -1745,6 +1743,8 @@ def _interpolate_scipy_wrapper(x, y, new_x, method, fill_value=None,
             y = y.copy()
         if not new_x.flags.writeable:
             new_x = new_x.copy()
+        if method == "piecewise_polynomial":
+            y = y.reshape((-1, 1))
         method = alt_methods[method]
         new_y = method(x, y, new_x)
     return new_y

pandas/tests/test_generic.py

@@ -872,6 +872,47 @@ def test_interp_limit_no_nans(self):
         expected = s
         assert_series_equal(result, expected)
 
+    def test_interp_piecewise_polynomial(self):
+        # GH 10365
+        tm._skip_if_no_scipy()
+        s1 = Series([1, 2, 3, 4, nan, 6, nan])
+        s2 = Series([nan, nan, 3, 4, nan, 6, 7])
+        result1 = s1.interpolate(method='piecewise_polynomial')
+        result2 = s2.interpolate(method='piecewise_polynomial')
+        expected1 = Series([1., 2., 3., 4., 5., 6., 7.])
+        expected2 = Series([nan, nan, 3., 4., 5., 6., 7.])
+
+        assert_series_equal(expected1, result1)
+        assert_series_equal(expected2, result2)
+
+    def test_interp_krogh(self):
+        tm._skip_if_no_scipy()
+        s1 = Series([0, -2, 0, np.nan], index=[0, 0, 1, 5])
+        s2 = Series([nan, 0, -2, 0, np.nan], index=[-1, 0, 0, 1, 5])
+        s3 = Series([nan, 0, 0, 0, np.nan], index=[-1, 0, 0, 0, 5])
+        result1 = s1.interpolate(method='krogh')
+        result2 = s2.interpolate(method='krogh')
+        result3 = s3.interpolate(method='krogh')
+        expected1 = Series([0., -2., 0., 40.], index=[0, 0, 1, 5])
+        expected2 = Series([nan, 0., -2., 0., 40.], index=[-1, 0, 0, 1, 5])
+        expected3 = Series([nan, 0., 0., 0., 0.], index=[-1, 0, 0, 0, 5])
+
+        assert_series_equal(expected1, result1)
+        assert_series_equal(expected2, result2)
+        assert_series_equal(expected3, result3)
+
+    def test_interp_barycentric(self):
+        tm._skip_if_no_scipy()
+        s1 = Series([nan, 0, 0, 0, 0, nan])
+        s2 = Series([nan, 0, 2, 1, nan])
+        result1 = s1.interpolate(method='barycentric')
+        result2 = s2.interpolate(method='barycentric')
+        expected1 = Series([nan, 0., 0., 0., 0., 0.])
+        expected2 = Series([nan, 0., 2., 1., -3.])
+
+        assert_series_equal(expected1, result1)
+        assert_series_equal(expected2, result2)
+
     def test_describe(self):
         _ = self.series.describe()
         _ = self.ts.describe()