BUG: DataFrame.update not operating in-place for datetime64[ns, UTC] dtype (#56228)

* inplace update

* copy-on-write fixups

* Update doc/source/whatsnew/v2.2.0.rst

Author: MarcoGorelli

doc/source/whatsnew/v2.2.0.rst

@@ -521,7 +521,7 @@ Indexing
 
 Missing
 ^^^^^^^
--
+- Bug in :meth:`DataFrame.update` wasn't updating in-place for tz-aware datetime64 dtypes (:issue:`56227`)
 -
 
 MultiIndex

pandas/core/frame.py

@@ -8852,14 +8852,14 @@ def update(
         in the original dataframe.
 
         >>> df = pd.DataFrame({'A': [1, 2, 3],
-        ...                    'B': [400, 500, 600]})
+        ...                    'B': [400., 500., 600.]})
         >>> new_df = pd.DataFrame({'B': [4, np.nan, 6]})
         >>> df.update(new_df)
         >>> df
-           A    B
-        0  1    4
-        1  2  500
-        2  3    6
+           A      B
+        0  1    4.0
+        1  2  500.0
+        2  3    6.0
         """
         if not PYPY and using_copy_on_write():
             if sys.getrefcount(self) <= REF_COUNT:
@@ -8876,8 +8876,6 @@ def update(
                     stacklevel=2,
                 )
 
-        from pandas.core.computation import expressions
-
         # TODO: Support other joins
         if join != "left":  # pragma: no cover
             raise NotImplementedError("Only left join is supported")
@@ -8911,7 +8909,7 @@ def update(
             if mask.all():
                 continue
 
-            self.loc[:, col] = expressions.where(mask, this, that)
+            self.loc[:, col] = self[col].where(mask, that)
 
     # ----------------------------------------------------------------------
     # Data reshaping

pandas/tests/frame/methods/test_update.py

@@ -140,6 +140,22 @@ def test_update_datetime_tz(self):
         expected = DataFrame([pd.Timestamp("2019", tz="UTC")])
         tm.assert_frame_equal(result, expected)
 
+    def test_update_datetime_tz_in_place(self, using_copy_on_write, warn_copy_on_write):
+        # https://github.com/pandas-dev/pandas/issues/56227
+        result = DataFrame([pd.Timestamp("2019", tz="UTC")])
+        orig = result.copy()
+        view = result[:]
+        with tm.assert_produces_warning(
+            FutureWarning if warn_copy_on_write else None, match="Setting a value"
+        ):
+            result.update(result + pd.Timedelta(days=1))
+        expected = DataFrame([pd.Timestamp("2019-01-02", tz="UTC")])
+        tm.assert_frame_equal(result, expected)
+        if not using_copy_on_write:
+            tm.assert_frame_equal(view, expected)
+        else:
+            tm.assert_frame_equal(view, orig)
+
     def test_update_with_different_dtype(self, using_copy_on_write):
         # GH#3217
         df = DataFrame({"a": [1, 3], "b": [np.nan, 2]})