DEPR: stricter downcast values in fillna

doc/source/whatsnew/v2.1.0.rst

@@ -259,6 +259,7 @@ Deprecations
 - Deprecated unused "closed" and "normalize" keywords in the :class:`DatetimeIndex` constructor (:issue:`52628`)
 - Deprecated unused "closed" keyword in the :class:`TimedeltaIndex` constructor (:issue:`52628`)
 - Deprecated logical operation between two non boolean :class:`Series` with different indexes always coercing the result to bool dtype. In a future version, this will maintain the return type of the inputs. (:issue:`52500`, :issue:`52538`)
+- Deprecated allowing ``downcast`` keyword other than ``None``, ``False``, "infer", or a dict with these as values in :meth:`Series.fillna`, :meth:`DataFrame.fillna` (:issue:`40988`)
 - Deprecated constructing :class:`SparseArray` from scalar data, pass a sequence instead (:issue:`53039`)
 -
 
@@ -360,7 +361,7 @@ Indexing
 
 Missing
 ^^^^^^^
--
+- Bug in :meth:`Series.interpolate` and :meth:`DataFrame.interpolate` failing to raise on invalid ``downcast`` keyword, which can be only ``None`` or "infer" (:issue:`53103`)
 -
 
 MultiIndex

pandas/core/generic.py

@@ -6972,6 +6972,25 @@ def fillna(
         inplace = validate_bool_kwarg(inplace, "inplace")
         value, method = validate_fillna_kwargs(value, method)
 
+        if isinstance(downcast, dict):
+            # GH#40988
+            for dc in downcast.values():
+                if dc is not None and dc is not False and dc != "infer":
+                    warnings.warn(
+                        "downcast entries other than None, False, and 'infer' "
+                        "are deprecated and will raise in a future version",
+                        FutureWarning,
+                        stacklevel=find_stack_level(),
+                    )
+        elif downcast is not None and downcast is not False and downcast != "infer":
+            # GH#40988
+            warnings.warn(
+                "downcast other than None, False, and 'infer' are deprecated "
+                "and will raise in a future version",
+                FutureWarning,
+                stacklevel=find_stack_level(),
+            )
+
         # set the default here, so functions examining the signaure
         # can detect if something was set (e.g. in groupby) (GH9221)
         if axis is None:
@@ -7548,7 +7567,7 @@ def interpolate(
         inplace: bool_t = False,
         limit_direction: Literal["forward", "backward", "both"] | None = None,
         limit_area: Literal["inside", "outside"] | None = None,
-        downcast: str | None = None,
+        downcast: Literal["infer"] | None = None,
         **kwargs,
     ) -> Self | None:
         """
@@ -7746,6 +7765,9 @@ def interpolate(
         3    16.0
         Name: d, dtype: float64
         """
+        if downcast is not None and downcast != "infer":
+            raise ValueError("downcast must be either None or 'infer'")
+
         inplace = validate_bool_kwarg(inplace, "inplace")
 
         axis = self._get_axis_number(axis)

pandas/core/internals/blocks.py

@@ -7,6 +7,7 @@
     Any,
     Callable,
     Iterable,
+    Literal,
     Sequence,
     cast,
     final,
@@ -1327,7 +1328,7 @@ def interpolate(
         limit_direction: str = "forward",
         limit_area: str | None = None,
         fill_value: Any | None = None,
-        downcast: str | None = None,
+        downcast: Literal["infer"] | None = None,
         using_cow: bool = False,
         **kwargs,
     ) -> list[Block]:

pandas/tests/frame/methods/test_fillna.py

@@ -297,7 +297,11 @@ def test_fillna_downcast_noop(self, frame_or_series):
         #  2) _can_hold_na + noop + not can_hold_element
 
         obj = frame_or_series([1, 2, 3], dtype=np.int64)
-        res = obj.fillna("foo", downcast=np.dtype(np.int32))
+
+        msg = "downcast other than None, False, and 'infer' are deprecated"
+        with tm.assert_produces_warning(FutureWarning, match=msg):
+            # GH#40988
+            res = obj.fillna("foo", downcast=np.dtype(np.int32))
         expected = obj.astype(np.int32)
         tm.assert_equal(res, expected)
 
@@ -306,7 +310,9 @@ def test_fillna_downcast_noop(self, frame_or_series):
         expected2 = obj  # get back int64
         tm.assert_equal(res2, expected2)
 
-        res3 = obj2.fillna("foo", downcast=np.dtype(np.int32))
+        with tm.assert_produces_warning(FutureWarning, match=msg):
+            # GH#40988
+            res3 = obj2.fillna("foo", downcast=np.dtype(np.int32))
         tm.assert_equal(res3, expected)
 
     @pytest.mark.parametrize("columns", [["A", "A", "B"], ["A", "A"]])
@@ -605,7 +611,10 @@ def test_fill_corner(self, float_frame, float_string_frame):
     def test_fillna_downcast_dict(self):
         # GH#40809
         df = DataFrame({"col1": [1, np.nan]})
-        result = df.fillna({"col1": 2}, downcast={"col1": "int64"})
+
+        msg = "downcast entries other than None, False, and 'infer' are deprecated"
+        with tm.assert_produces_warning(FutureWarning, match=msg):
+            result = df.fillna({"col1": 2}, downcast={"col1": "int64"})
         expected = DataFrame({"col1": [1, 2]})
         tm.assert_frame_equal(result, expected)
 

pandas/tests/frame/methods/test_interpolate.py

@@ -151,6 +151,23 @@ def test_interp_combo(self):
         expected = Series([1, 2, 3, 4], name="A")
         tm.assert_series_equal(result, expected)
 
+    def test_inerpolate_invalid_downcast(self):
+        # GH#53103
+        df = DataFrame(
+            {
+                "A": [1.0, 2.0, np.nan, 4.0],
+                "B": [1, 4, 9, np.nan],
+                "C": [1, 2, 3, 5],
+                "D": list("abcd"),
+            }
+        )
+
+        msg = "downcast must be either None or 'infer'"
+        with pytest.raises(ValueError, match=msg):
+            df.interpolate(downcast="int64")
+        with pytest.raises(ValueError, match=msg):
+            df["A"].interpolate(downcast="int64")
+
     def test_interp_nan_idx(self):
         df = DataFrame({"A": [1, 2, np.nan, 4], "B": [np.nan, 2, 3, 4]})
         df = df.set_index("A")