DEPR: NDFrame.interpolate with ffill/bfill methods

doc/source/whatsnew/v2.1.0.rst

@@ -284,6 +284,7 @@ Deprecations
 - Deprecated option "mode.use_inf_as_na", convert inf entries to ``NaN`` before instead (:issue:`51684`)
 - Deprecated positional indexing on :class:`Series` with :meth:`Series.__getitem__` and :meth:`Series.__setitem__`, in a future version ``ser[item]`` will *always* interpret ``item`` as a label, not a position (:issue:`50617`)
 - Deprecated the "method" and "limit" keywords on :meth:`Series.fillna`, :meth:`DataFrame.fillna`, :meth:`SeriesGroupBy.fillna`, :meth:`DataFrameGroupBy.fillna`, and :meth:`Resampler.fillna`, use ``obj.bfill()`` or ``obj.ffill()`` instead (:issue:`53394`)
+- Deprecated values "pad", "ffill", "bfill", "backfill" for :meth:`Series.interpolate` and :meth:`DataFrame.interpolate`, use ``obj.ffill()`` or ``obj.bfill()`` instead (:issue:`53581`)
 -
 
 .. ---------------------------------------------------------------------------

pandas/core/generic.py

@@ -7806,35 +7806,6 @@ def interpolate(
         3    3.0
         dtype: float64
 
-        Filling in ``NaN`` in a Series by padding, but filling at most two
-        consecutive ``NaN`` at a time.
-
-        >>> s = pd.Series([np.nan, "single_one", np.nan,
-        ...                "fill_two_more", np.nan, np.nan, np.nan,
-        ...                4.71, np.nan])
-        >>> s
-        0              NaN
-        1       single_one
-        2              NaN
-        3    fill_two_more
-        4              NaN
-        5              NaN
-        6              NaN
-        7             4.71
-        8              NaN
-        dtype: object
-        >>> s.interpolate(method='pad', limit=2)
-        0              NaN
-        1       single_one
-        2       single_one
-        3    fill_two_more
-        4    fill_two_more
-        5    fill_two_more
-        6              NaN
-        7             4.71
-        8             4.71
-        dtype: object
-
         Filling in ``NaN`` in a Series via polynomial interpolation or splines:
         Both 'polynomial' and 'spline' methods require that you also specify
         an ``order`` (int).
@@ -7899,6 +7870,17 @@ def interpolate(
                 return None
             return self.copy()
 
+        if not isinstance(method, str):
+            raise ValueError("'method' should be a string, not None.")
+        elif method.lower() in fillna_methods:
+            # GH#53581
+            warnings.warn(
+                f"{type(self).__name__}.interpolate with method={method} is "
+                "deprecated and will raise in a future version. "
+                "Use obj.ffill() or obj.bfill() instead.",
+                FutureWarning,
+                stacklevel=find_stack_level(),
+            )
         if method not in fillna_methods:
             axis = self._info_axis_number
 

pandas/tests/copy_view/test_interp_fillna.py

@@ -20,7 +20,12 @@ def test_interpolate_no_op(using_copy_on_write, method):
     df = DataFrame({"a": [1, 2]})
     df_orig = df.copy()
 
-    result = df.interpolate(method=method)
+    warn = None
+    if method == "pad":
+        warn = FutureWarning
+    msg = "DataFrame.interpolate with method=pad is deprecated"
+    with tm.assert_produces_warning(warn, match=msg):
+        result = df.interpolate(method=method)
 
     if using_copy_on_write:
         assert np.shares_memory(get_array(result, "a"), get_array(df, "a"))
@@ -125,7 +130,9 @@ def test_interpolate_cleaned_fill_method(using_copy_on_write):
 def test_interpolate_object_convert_no_op(using_copy_on_write):
     df = DataFrame({"a": ["a", "b", "c"], "b": 1})
     arr_a = get_array(df, "a")
-    df.interpolate(method="pad", inplace=True)
+    msg = "DataFrame.interpolate with method=pad is deprecated"
+    with tm.assert_produces_warning(FutureWarning, match=msg):
+        df.interpolate(method="pad", inplace=True)
 
     # Now CoW makes a copy, it should not!
     if using_copy_on_write:
@@ -136,7 +143,9 @@ def test_interpolate_object_convert_no_op(using_copy_on_write):
 def test_interpolate_object_convert_copies(using_copy_on_write):
     df = DataFrame({"a": Series([1, 2], dtype=object), "b": 1})
     arr_a = get_array(df, "a")
-    df.interpolate(method="pad", inplace=True)
+    msg = "DataFrame.interpolate with method=pad is deprecated"
+    with tm.assert_produces_warning(FutureWarning, match=msg):
+        df.interpolate(method="pad", inplace=True)
 
     if using_copy_on_write:
         assert df._mgr._has_no_reference(0)
@@ -146,7 +155,9 @@ def test_interpolate_object_convert_copies(using_copy_on_write):
 def test_interpolate_downcast(using_copy_on_write):
     df = DataFrame({"a": [1, np.nan, 2.5], "b": 1})
     arr_a = get_array(df, "a")
-    df.interpolate(method="pad", inplace=True, downcast="infer")
+    msg = "DataFrame.interpolate with method=pad is deprecated"
+    with tm.assert_produces_warning(FutureWarning, match=msg):
+        df.interpolate(method="pad", inplace=True, downcast="infer")
 
     if using_copy_on_write:
         assert df._mgr._has_no_reference(0)
@@ -158,7 +169,9 @@ def test_interpolate_downcast_reference_triggers_copy(using_copy_on_write):
     df_orig = df.copy()
     arr_a = get_array(df, "a")
     view = df[:]
-    df.interpolate(method="pad", inplace=True, downcast="infer")
+    msg = "DataFrame.interpolate with method=pad is deprecated"
+    with tm.assert_produces_warning(FutureWarning, match=msg):
+        df.interpolate(method="pad", inplace=True, downcast="infer")
 
     if using_copy_on_write:
         assert df._mgr._has_no_reference(0)

pandas/tests/frame/methods/test_interpolate.py

@@ -436,7 +436,9 @@ def test_interp_fillna_methods(self, request, axis, method, using_array_manager)
         )
         method2 = method if method != "pad" else "ffill"
         expected = getattr(df, method2)(axis=axis)
-        result = df.interpolate(method=method, axis=axis)
+        msg = f"DataFrame.interpolate with method={method} is deprecated"
+        with tm.assert_produces_warning(FutureWarning, match=msg):
+            result = df.interpolate(method=method, axis=axis)
         tm.assert_frame_equal(result, expected)
 
     def test_interpolate_empty_df(self):

pandas/tests/series/methods/test_interpolate.py

@@ -347,6 +347,8 @@ def test_interp_invalid_method(self, invalid_method):
         s = Series([1, 3, np.nan, 12, np.nan, 25])
 
         msg = f"method must be one of.* Got '{invalid_method}' instead"
+        if invalid_method is None:
+            msg = "'method' should be a string, not None"
         with pytest.raises(ValueError, match=msg):
             s.interpolate(method=invalid_method)
 
@@ -360,8 +362,10 @@ def test_interp_invalid_method_and_value(self):
         ser = Series([1, 3, np.nan, 12, np.nan, 25])
 
         msg = "Cannot pass both fill_value and method"
+        msg2 = "Series.interpolate with method=pad"
         with pytest.raises(ValueError, match=msg):
-            ser.interpolate(fill_value=3, method="pad")
+            with tm.assert_produces_warning(FutureWarning, match=msg2):
+                ser.interpolate(fill_value=3, method="pad")
 
     def test_interp_limit_forward(self):
         s = Series([1, 3, np.nan, np.nan, np.nan, 11])
@@ -470,8 +474,10 @@ def test_interp_limit_direction_raises(self, method, limit_direction, expected):
         s = Series([1, 2, 3])
 
         msg = f"`limit_direction` must be '{expected}' for method `{method}`"
+        msg2 = "Series.interpolate with method="
         with pytest.raises(ValueError, match=msg):
-            s.interpolate(method=method, limit_direction=limit_direction)
+            with tm.assert_produces_warning(FutureWarning, match=msg2):
+                s.interpolate(method=method, limit_direction=limit_direction)
 
     @pytest.mark.parametrize(
         "data, expected_data, kwargs",
@@ -513,7 +519,9 @@ def test_interp_limit_area_with_pad(self, data, expected_data, kwargs):
 
         s = Series(data)
         expected = Series(expected_data)
-        result = s.interpolate(**kwargs)
+        msg = "Series.interpolate with method=pad"
+        with tm.assert_produces_warning(FutureWarning, match=msg):
+            result = s.interpolate(**kwargs)
         tm.assert_series_equal(result, expected)
 
     @pytest.mark.parametrize(
@@ -546,7 +554,9 @@ def test_interp_limit_area_with_backfill(self, data, expected_data, kwargs):
 
         s = Series(data)
         expected = Series(expected_data)
-        result = s.interpolate(**kwargs)
+        msg = "Series.interpolate with method=bfill"
+        with tm.assert_produces_warning(FutureWarning, match=msg):
+            result = s.interpolate(**kwargs)
         tm.assert_series_equal(result, expected)
 
     def test_interp_limit_direction(self):
@@ -642,7 +652,15 @@ def test_interp_datetime64(self, method, tz_naive_fixture):
         df = Series(
             [1, np.nan, 3], index=date_range("1/1/2000", periods=3, tz=tz_naive_fixture)
         )
-        result = df.interpolate(method=method)
+        warn = None if method == "nearest" else FutureWarning
+        msg = "Series.interpolate with method=pad is deprecated"
+        with tm.assert_produces_warning(warn, match=msg):
+            result = df.interpolate(method=method)
+        if warn is not None:
+            # check the "use ffill instead" is equivalent
+            alt = df.ffill()
+            tm.assert_series_equal(result, alt)
+
         expected = Series(
             [1.0, 1.0, 3.0],
             index=date_range("1/1/2000", periods=3, tz=tz_naive_fixture),
@@ -654,7 +672,13 @@ def test_interp_pad_datetime64tz_values(self):
         dti = date_range("2015-04-05", periods=3, tz="US/Central")
         ser = Series(dti)
         ser[1] = pd.NaT
-        result = ser.interpolate(method="pad")
+
+        msg = "Series.interpolate with method=pad is deprecated"
+        with tm.assert_produces_warning(FutureWarning, match=msg):
+            result = ser.interpolate(method="pad")
+        # check the "use ffill instead" is equivalent
+        alt = ser.ffill()
+        tm.assert_series_equal(result, alt)
 
         expected = Series(dti)
         expected[1] = expected[0]