Backport PR pandas-dev#54818: DEP: Deprecate passing fill_value and freq to shift

Author: phofl

doc/source/whatsnew/v2.1.0.rst

@@ -591,6 +591,7 @@ Other Deprecations
 - Deprecated values ``"pad"``, ``"ffill"``, ``"bfill"``, ``"backfill"`` for :meth:`Series.interpolate` and :meth:`DataFrame.interpolate`, use ``obj.ffill()`` or ``obj.bfill()`` instead (:issue:`53581`)
 - Deprecated the behavior of :meth:`Index.argmax`, :meth:`Index.argmin`, :meth:`Series.argmax`, :meth:`Series.argmin` with either all-NAs and ``skipna=True`` or any-NAs and ``skipna=False`` returning -1; in a future version this will raise ``ValueError`` (:issue:`33941`, :issue:`33942`)
 - Deprecated allowing non-keyword arguments in :meth:`DataFrame.to_sql` except ``name`` and ``con`` (:issue:`54229`)
+- Deprecated silently ignoring ``fill_value`` when passing both ``freq`` and ``fill_value`` to :meth:`DataFrame.shift`, :meth:`Series.shift` and :meth:`.DataFrameGroupBy.shift`; in a future version this will raise ``ValueError`` (:issue:`53832`)
 
 .. ---------------------------------------------------------------------------
 .. _whatsnew_210.performance:
@@ -875,7 +876,6 @@ Other
 - Bug in :meth:`.DataFrameGroupBy.first`, :meth:`.DataFrameGroupBy.last`, :meth:`.SeriesGroupBy.first`, and :meth:`.SeriesGroupBy.last` where an empty group would return ``np.nan`` instead of the corresponding :class:`.ExtensionArray` NA value (:issue:`39098`)
 - Bug in :meth:`DataFrame.pivot_table` with casting the mean of ints back to an int (:issue:`16676`)
 - Bug in :meth:`DataFrame.reindex` with a ``fill_value`` that should be inferred with a :class:`ExtensionDtype` incorrectly inferring ``object`` dtype (:issue:`52586`)
-- Bug in :meth:`DataFrame.shift` and :meth:`Series.shift` and :meth:`.DataFrameGroupBy.shift` when passing both ``freq`` and ``fill_value`` silently ignoring ``fill_value`` instead of raising ``ValueError`` (:issue:`53832`)
 - Bug in :meth:`DataFrame.shift` with ``axis=1`` on a :class:`DataFrame` with a single :class:`ExtensionDtype` column giving incorrect results (:issue:`53832`)
 - Bug in :meth:`Index.sort_values` when a ``key`` is passed (:issue:`52764`)
 - Bug in :meth:`Series.align`, :meth:`DataFrame.align`, :meth:`Series.reindex`, :meth:`DataFrame.reindex`, :meth:`Series.interpolate`, :meth:`DataFrame.interpolate`, incorrectly failing to raise with method="asfreq" (:issue:`53620`)

pandas/core/frame.py

@@ -5617,10 +5617,14 @@ def shift(
     ) -> DataFrame:
         if freq is not None and fill_value is not lib.no_default:
             # GH#53832
-            raise ValueError(
-                "Cannot pass both 'freq' and 'fill_value' to "
-                f"{type(self).__name__}.shift"
+            warnings.warn(
+                "Passing a 'freq' together with a 'fill_value' silently ignores "
+                "the fill_value and is deprecated. This will raise in a future "
+                "version.",
+                FutureWarning,
+                stacklevel=find_stack_level(),
             )
+            fill_value = lib.no_default
 
         axis = self._get_axis_number(axis)
 

pandas/core/generic.py

@@ -10802,10 +10802,14 @@ def shift(
 
         if freq is not None and fill_value is not lib.no_default:
             # GH#53832
-            raise ValueError(
-                "Cannot pass both 'freq' and 'fill_value' to "
-                f"{type(self).__name__}.shift"
+            warnings.warn(
+                "Passing a 'freq' together with a 'fill_value' silently ignores "
+                "the fill_value and is deprecated. This will raise in a future "
+                "version.",
+                FutureWarning,
+                stacklevel=find_stack_level(),
             )
+            fill_value = lib.no_default
 
         if periods == 0:
             return self.copy(deep=None)

pandas/tests/frame/methods/test_shift.py

@@ -32,19 +32,23 @@ def test_shift_axis1_with_valid_fill_value_one_array(self):
         expected2 = DataFrame([12345] * 5, dtype="Float64")
         tm.assert_frame_equal(res2, expected2)
 
-    def test_shift_disallow_freq_and_fill_value(self, frame_or_series):
+    def test_shift_deprecate_freq_and_fill_value(self, frame_or_series):
         # Can't pass both!
         obj = frame_or_series(
             np.random.default_rng(2).standard_normal(5),
             index=date_range("1/1/2000", periods=5, freq="H"),
         )
 
-        msg = "Cannot pass both 'freq' and 'fill_value' to (Series|DataFrame).shift"
-        with pytest.raises(ValueError, match=msg):
+        msg = (
+            "Passing a 'freq' together with a 'fill_value' silently ignores the "
+            "fill_value"
+        )
+        with tm.assert_produces_warning(FutureWarning, match=msg):
             obj.shift(1, fill_value=1, freq="H")
 
         if frame_or_series is DataFrame:
-            with pytest.raises(ValueError, match=msg):
+            obj.columns = date_range("1/1/2000", periods=1, freq="H")
+            with tm.assert_produces_warning(FutureWarning, match=msg):
                 obj.shift(1, axis=1, fill_value=1, freq="H")
 
     @pytest.mark.parametrize(
@@ -716,8 +720,11 @@ def test_shift_with_iterable_freq_and_fill_value(self):
             df.shift(1, freq="H"),
         )
 
-        msg = r"Cannot pass both 'freq' and 'fill_value' to.*"
-        with pytest.raises(ValueError, match=msg):
+        msg = (
+            "Passing a 'freq' together with a 'fill_value' silently ignores the "
+            "fill_value"
+        )
+        with tm.assert_produces_warning(FutureWarning, match=msg):
             df.shift([1, 2], fill_value=1, freq="H")
 
     def test_shift_with_iterable_check_other_arguments(self):

pandas/tests/groupby/test_groupby_shift_diff.py

@@ -166,12 +166,14 @@ def test_shift_periods_freq():
     tm.assert_frame_equal(result, expected)
 
 
-def test_shift_disallow_freq_and_fill_value():
+def test_shift_deprecate_freq_and_fill_value():
     # GH 53832
     data = {"a": [1, 2, 3, 4, 5, 6], "b": [0, 0, 0, 1, 1, 1]}
     df = DataFrame(data, index=date_range(start="20100101", periods=6))
-    msg = "Cannot pass both 'freq' and 'fill_value' to (Series|DataFrame).shift"
-    with pytest.raises(ValueError, match=msg):
+    msg = (
+        "Passing a 'freq' together with a 'fill_value' silently ignores the fill_value"
+    )
+    with tm.assert_produces_warning(FutureWarning, match=msg):
         df.groupby(df.index).shift(periods=-2, freq="D", fill_value="1")
 
 
@@ -238,12 +240,15 @@ def test_group_shift_with_multiple_periods_and_fill_value():
     tm.assert_frame_equal(shifted_df, expected_df)
 
 
-def test_group_shift_with_multiple_periods_and_both_fill_and_freq_fails():
+def test_group_shift_with_multiple_periods_and_both_fill_and_freq_deprecated():
     # GH#44424
     df = DataFrame(
         {"a": [1, 2, 3, 4, 5], "b": [True, True, False, False, True]},
         index=date_range("1/1/2000", periods=5, freq="H"),
     )
-    msg = r"Cannot pass both 'freq' and 'fill_value' to.*"
-    with pytest.raises(ValueError, match=msg):
+    msg = (
+        "Passing a 'freq' together with a 'fill_value' silently ignores the "
+        "fill_value"
+    )
+    with tm.assert_produces_warning(FutureWarning, match=msg):
         df.groupby("b")[["a"]].shift([1, 2], fill_value=1, freq="H")