API: improve dtype in df.where with EA other (#62038)

Co-authored-by: Matthew Roeschke <10647082+mroeschke@users.noreply.github.com>
Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

Author: 3 people

doc/source/whatsnew/v3.0.0.rst

@@ -90,12 +90,14 @@ Other enhancements
 - Added support to read and write from and to Apache Iceberg tables with the new :func:`read_iceberg` and :meth:`DataFrame.to_iceberg` functions (:issue:`61383`)
 - Errors occurring during SQL I/O will now throw a generic :class:`.DatabaseError` instead of the raw Exception type from the underlying driver manager library (:issue:`60748`)
 - Implemented :meth:`Series.str.isascii` and :meth:`Series.str.isascii` (:issue:`59091`)
+- Improve the resulting dtypes in :meth:`DataFrame.where` and :meth:`DataFrame.mask` with :class:`ExtensionDtype` ``other`` (:issue:`62038`)
 - Improved deprecation message for offset aliases (:issue:`60820`)
 - Multiplying two :class:`DateOffset` objects will now raise a ``TypeError`` instead of a ``RecursionError`` (:issue:`59442`)
 - Restore support for reading Stata 104-format and enable reading 103-format dta files (:issue:`58554`)
 - Support passing a :class:`Iterable[Hashable]` input to :meth:`DataFrame.drop_duplicates` (:issue:`59237`)
 - Support reading Stata 102-format (Stata 1) dta files (:issue:`58978`)
 - Support reading Stata 110-format (Stata 7) dta files (:issue:`47176`)
+-
 
 .. ---------------------------------------------------------------------------
 .. _whatsnew_300.notable_bug_fixes:

pandas/core/generic.py

@@ -9788,14 +9788,40 @@ def _where(
                     raise InvalidIndexError
 
                 if other.ndim < self.ndim:
-                    # TODO(EA2D): avoid object-dtype cast in EA case GH#38729
                     other = other._values
-                    if axis == 0:
-                        other = np.reshape(other, (-1, 1))
-                    elif axis == 1:
-                        other = np.reshape(other, (1, -1))
-
-                    other = np.broadcast_to(other, self.shape)
+                    if isinstance(other, np.ndarray):
+                        # TODO(EA2D): could also do this for NDArrayBackedEA cases?
+                        if axis == 0:
+                            other = np.reshape(other, (-1, 1))
+                        elif axis == 1:
+                            other = np.reshape(other, (1, -1))
+
+                        other = np.broadcast_to(other, self.shape)
+                    else:
+                        # GH#38729, GH#62038 avoid lossy casting or object-casting
+                        if axis == 0:
+                            res_cols = [
+                                self.iloc[:, i]._where(
+                                    cond.iloc[:, i],
+                                    other,
+                                )
+                                for i in range(self.shape[1])
+                            ]
+                        elif axis == 1:
+                            # TODO: can we use a zero-copy alternative to "repeat"?
+                            res_cols = [
+                                self.iloc[:, i]._where(
+                                    cond.iloc[:, i],
+                                    other[i : i + 1].repeat(len(self)),
+                                )
+                                for i in range(self.shape[1])
+                            ]
+                        res = self._constructor(dict(enumerate(res_cols)))
+                        res.index = self.index
+                        res.columns = self.columns
+                        if inplace:
+                            return self._update_inplace(res)
+                        return res.__finalize__(self)
 
             # slice me out of the other
             else:

pandas/tests/frame/indexing/test_where.py

@@ -698,22 +698,30 @@ def test_where_categorical_filtering(self):
         tm.assert_equal(result, expected)
 
     def test_where_ea_other(self):
-        # GH#38729/GH#38742
+        # GH#38729/GH#38742, GH#62038
         df = DataFrame({"A": [1, 2, 3], "B": [4, 5, 6]})
         arr = pd.array([7, pd.NA, 9])
         ser = Series(arr)
         mask = np.ones(df.shape, dtype=bool)
         mask[1, :] = False
 
-        # TODO: ideally we would get Int64 instead of object
-        result = df.where(mask, ser, axis=0)
-        expected = DataFrame({"A": [1, np.nan, 3], "B": [4, np.nan, 6]})
-        tm.assert_frame_equal(result, expected)
+        result1 = df.where(mask, ser, axis=0)
+        expected1 = DataFrame({"A": [1, pd.NA, 3], "B": [4, pd.NA, 6]}, dtype="Int64")
+        tm.assert_frame_equal(result1, expected1)
 
         ser2 = Series(arr[:2], index=["A", "B"])
-        expected = DataFrame({"A": [1, 7, 3], "B": [4, np.nan, 6]})
-        result = df.where(mask, ser2, axis=1)
-        tm.assert_frame_equal(result, expected)
+        expected2 = DataFrame({"A": [1, 7, 3], "B": [4, pd.NA, 6]})
+        expected2["B"] = expected2["B"].astype("Int64")
+        result2 = df.where(mask, ser2, axis=1)
+        tm.assert_frame_equal(result2, expected2)
+
+        result3 = df.copy()
+        result3.mask(mask, ser, axis=0, inplace=True)
+        tm.assert_frame_equal(result3, expected1)
+
+        result4 = df.copy()
+        result4.mask(mask, ser2, axis=1, inplace=True)
+        tm.assert_frame_equal(result4, expected2)
 
     def test_where_interval_noop(self):
         # GH#44181