REGR: DataFrame.update emits spurious warning about downcasting (#57485)

Author: rhshadrach

doc/source/whatsnew/v2.2.1.rst

@@ -32,6 +32,7 @@ Fixed regressions
 - Fixed regression in :meth:`DataFrame.to_dict` with ``orient='list'`` and datetime or timedelta types returning integers (:issue:`54824`)
 - Fixed regression in :meth:`DataFrame.to_json` converting nullable integers to floats (:issue:`57224`)
 - Fixed regression in :meth:`DataFrame.to_sql` when ``method="multi"`` is passed and the dialect type is not Oracle (:issue:`57310`)
+- Fixed regression in :meth:`DataFrame.update` emitting incorrect warnings about downcasting (:issue:`57124`)
 - Fixed regression in :meth:`DataFrameGroupBy.idxmin`, :meth:`DataFrameGroupBy.idxmax`, :meth:`SeriesGroupBy.idxmin`, :meth:`SeriesGroupBy.idxmax` ignoring the ``skipna`` argument (:issue:`57040`)
 - Fixed regression in :meth:`DataFrameGroupBy.idxmin`, :meth:`DataFrameGroupBy.idxmax`, :meth:`SeriesGroupBy.idxmin`, :meth:`SeriesGroupBy.idxmax` where values containing the minimum or maximum value for the dtype could produce incorrect results (:issue:`57040`)
 - Fixed regression in :meth:`ExtensionArray.to_numpy` raising for non-numeric masked dtypes (:issue:`56991`)

pandas/core/frame.py

@@ -8962,6 +8962,7 @@ def update(
         1  2  500.0
         2  3    6.0
         """
+
         if not PYPY and using_copy_on_write():
             if sys.getrefcount(self) <= REF_COUNT:
                 warnings.warn(
@@ -9010,7 +9011,17 @@ def update(
             if mask.all():
                 continue
 
-            self.loc[:, col] = self[col].where(mask, that)
+            with warnings.catch_warnings():
+                warnings.filterwarnings(
+                    "ignore",
+                    message="Downcasting behavior",
+                    category=FutureWarning,
+                )
+                # GH#57124 - `that` might get upcasted because of NA values, and then
+                # downcasted in where because of the mask. Ignoring the warning
+                # is a stopgap, will replace with a new implementation of update
+                # in 3.0.
+                self.loc[:, col] = self[col].where(mask, that)
 
     # ----------------------------------------------------------------------
     # Data reshaping

pandas/tests/frame/methods/test_update.py

@@ -48,16 +48,18 @@ def test_update(self):
     def test_update_dtypes(self):
         # gh 3016
         df = DataFrame(
-            [[1.0, 2.0, False, True], [4.0, 5.0, True, False]],
-            columns=["A", "B", "bool1", "bool2"],
+            [[1.0, 2.0, 1, False, True], [4.0, 5.0, 2, True, False]],
+            columns=["A", "B", "int", "bool1", "bool2"],
         )
 
-        other = DataFrame([[45, 45]], index=[0], columns=["A", "B"])
+        other = DataFrame(
+            [[45, 45, 3, True]], index=[0], columns=["A", "B", "int", "bool1"]
+        )
         df.update(other)
 
         expected = DataFrame(
-            [[45.0, 45.0, False, True], [4.0, 5.0, True, False]],
-            columns=["A", "B", "bool1", "bool2"],
+            [[45.0, 45.0, 3, True, True], [4.0, 5.0, 2, True, False]],
+            columns=["A", "B", "int", "bool1", "bool2"],
         )
         tm.assert_frame_equal(df, expected)