BUG in Series.interpolate: limit_area/limit_direction kwargs with method="pad"/"bfill" have no effect

doc/source/whatsnew/v1.2.0.rst

@@ -670,6 +670,7 @@ Missing
 
 - Bug in :meth:`.SeriesGroupBy.transform` now correctly handles missing values for ``dropna=False`` (:issue:`35014`)
 - Bug in :meth:`Series.nunique` with ``dropna=True`` was returning incorrect results when both ``NA`` and ``None`` missing values were present (:issue:`37566`)
+- Bug in :meth:`Series.interpolate` where kwarg ``limit_area`` and ``limit_direction`` had no effect when using methods ``pad`` and ``backfill`` (:issue:`31048`)
 -
 
 MultiIndex

pandas/core/internals/blocks.py

@@ -1261,6 +1261,7 @@ def interpolate(
                 axis=axis,
                 inplace=inplace,
                 limit=limit,
+                limit_area=limit_area,
                 downcast=downcast,
             )
         # validate the interp method
@@ -1287,6 +1288,7 @@ def _interpolate_with_fill(
         axis: int = 0,
         inplace: bool = False,
         limit: Optional[int] = None,
+        limit_area: Optional[str] = None,
         downcast: Optional[str] = None,
     ) -> List["Block"]:
         """ fillna but using the interpolate machinery """
@@ -1301,6 +1303,7 @@ def _interpolate_with_fill(
             method=method,
             axis=axis,
             limit=limit,
+            limit_area=limit_area,
         )
 
         blocks = [self.make_block_same_class(values, ndim=self.ndim)]

pandas/core/missing.py

@@ -1,13 +1,13 @@
 """
 Routines for filling missing data.
 """
-
+from functools import partial
 from typing import Any, List, Optional, Set, Union
 
 import numpy as np
 
 from pandas._libs import algos, lib
-from pandas._typing import ArrayLike, DtypeObj
+from pandas._typing import ArrayLike, Axis, DtypeObj
 from pandas.compat._optional import import_optional_dependency
 
 from pandas.core.dtypes.cast import infer_dtype_from_array
@@ -528,16 +528,92 @@ def _cubicspline_interpolate(xi, yi, x, axis=0, bc_type="not-a-knot", extrapolat
     return P(x)
 
 
+def _interpolate_with_limit_area(
+    values: ArrayLike, method: str, limit: Optional[int], limit_area: Optional[str]
+) -> ArrayLike:
+    """
+    Apply interpolation and limit_area logic to values along a to-be-specified axis.
+
+    Parameters
+    ----------
+    values: array-like
+        Input array.
+    method: str
+        Interpolation method. Could be "bfill" or "pad"
+    limit: int, optional
+        Index limit on interpolation.
+    limit_area: str
+        Limit area for interpolation. Can be "inside" or "outside"
+
+    Returns
+    -------
+    values: array-like
+        Interpolated array.
+    """
+
+    invalid = isna(values)
+
+    if not invalid.all():
+        first = find_valid_index(values, "first")
+        last = find_valid_index(values, "last")
+
+        values = interpolate_2d(
+            values,
+            method=method,
+            limit=limit,
+        )
+
+        if limit_area == "inside":
+            invalid[first : last + 1] = False
+        elif limit_area == "outside":
+            invalid[:first] = invalid[last + 1 :] = False
+
+        values[invalid] = np.nan
+
+    return values
+
+
 def interpolate_2d(
     values,
-    method="pad",
-    axis=0,
-    limit=None,
+    method: str = "pad",
+    axis: Axis = 0,
+    limit: Optional[int] = None,
+    limit_area: Optional[str] = None,
 ):
     """
     Perform an actual interpolation of values, values will be make 2-d if
     needed fills inplace, returns the result.
+
+       Parameters
+    ----------
+    values: array-like
+        Input array.
+    method: str, default "pad"
+        Interpolation method. Could be "bfill" or "pad"
+    axis: 0 or 1
+        Interpolation axis
+    limit: int, optional
+        Index limit on interpolation.
+    limit_area: str, optional
+        Limit area for interpolation. Can be "inside" or "outside"
+
+    Returns
+    -------
+    values: array-like
+        Interpolated array.
     """
+    if limit_area is not None:
+        return np.apply_along_axis(
+            partial(
+                _interpolate_with_limit_area,
+                method=method,
+                limit=limit,
+                limit_area=limit_area,
+            ),
+            axis,
+            values,
+        )
+
     orig_values = values
 
     transf = (lambda x: x) if axis == 0 else (lambda x: x.T)

pandas/tests/series/methods/test_interpolate.py

@@ -458,6 +458,82 @@ def test_interp_limit_direction_raises(self, method, limit_direction, expected):
         with pytest.raises(ValueError, match=msg):
             s.interpolate(method=method, limit_direction=limit_direction)
 
+    @pytest.mark.parametrize(
+        "data, expected_data, kwargs",
+        (
+            (
+                [np.nan, np.nan, 3, np.nan, np.nan, np.nan, 7, np.nan, np.nan],
+                [np.nan, np.nan, 3.0, 3.0, 3.0, 3.0, 7.0, np.nan, np.nan],
+                {"method": "pad", "limit_area": "inside"},
+            ),
+            (
+                [np.nan, np.nan, 3, np.nan, np.nan, np.nan, 7, np.nan, np.nan],
+                [np.nan, np.nan, 3.0, 3.0, np.nan, np.nan, 7.0, np.nan, np.nan],
+                {"method": "pad", "limit_area": "inside", "limit": 1},
+            ),
+            (
+                [np.nan, np.nan, 3, np.nan, np.nan, np.nan, 7, np.nan, np.nan],
+                [np.nan, np.nan, 3.0, np.nan, np.nan, np.nan, 7.0, 7.0, 7.0],
+                {"method": "pad", "limit_area": "outside"},
+            ),
+            (
+                [np.nan, np.nan, 3, np.nan, np.nan, np.nan, 7, np.nan, np.nan],
+                [np.nan, np.nan, 3.0, np.nan, np.nan, np.nan, 7.0, 7.0, np.nan],
+                {"method": "pad", "limit_area": "outside", "limit": 1},
+            ),
+            (
+                [np.nan, np.nan, np.nan, np.nan, np.nan, np.nan, np.nan],
+                [np.nan, np.nan, np.nan, np.nan, np.nan, np.nan, np.nan],
+                {"method": "pad", "limit_area": "outside", "limit": 1},
+            ),
+            (
+                range(5),
+                range(5),
+                {"method": "pad", "limit_area": "outside", "limit": 1},
+            ),
+        ),
+    )
+    def test_interp_limit_area_with_pad(self, data, expected_data, kwargs):
+        # GH26796
+
+        s = Series(data)
+        expected = Series(expected_data)
+        result = s.interpolate(**kwargs)
+        tm.assert_series_equal(result, expected)
+
+    @pytest.mark.parametrize(
+        "data, expected_data, kwargs",
+        (
+            (
+                [np.nan, np.nan, 3, np.nan, np.nan, np.nan, 7, np.nan, np.nan],
+                [np.nan, np.nan, 3.0, 7.0, 7.0, 7.0, 7.0, np.nan, np.nan],
+                {"method": "bfill", "limit_area": "inside"},
+            ),
+            (
+                [np.nan, np.nan, 3, np.nan, np.nan, np.nan, 7, np.nan, np.nan],
+                [np.nan, np.nan, 3.0, np.nan, np.nan, 7.0, 7.0, np.nan, np.nan],
+                {"method": "bfill", "limit_area": "inside", "limit": 1},
+            ),
+            (
+                [np.nan, np.nan, 3, np.nan, np.nan, np.nan, 7, np.nan, np.nan],
+                [3.0, 3.0, 3.0, np.nan, np.nan, np.nan, 7.0, np.nan, np.nan],
+                {"method": "bfill", "limit_area": "outside"},
+            ),
+            (
+                [np.nan, np.nan, 3, np.nan, np.nan, np.nan, 7, np.nan, np.nan],
+                [np.nan, 3.0, 3.0, np.nan, np.nan, np.nan, 7.0, np.nan, np.nan],
+                {"method": "bfill", "limit_area": "outside", "limit": 1},
+            ),
+        ),
+    )
+    def test_interp_limit_area_with_backfill(self, data, expected_data, kwargs):
+        # GH26796
+
+        s = Series(data)
+        expected = Series(expected_data)
+        result = s.interpolate(**kwargs)
+        tm.assert_series_equal(result, expected)
+
     def test_interp_limit_direction(self):
         # These tests are for issue #9218 -- fill NaNs in both directions.
         s = Series([1, 3, np.nan, np.nan, np.nan, 11])