Backport PR #49720 on branch 1.5.x (Suppress spurious warning) (#49726)

Backport PR #49720: Suppress spurious warning

Co-authored-by: jbrockmendel <jbrockmendel@gmail.com>

Author: meeseeksmachine

pandas/core/frame.py

@@ -8255,7 +8255,9 @@ def update(
             if mask.all():
                 continue
 
-            self.loc[:, col] = expressions.where(mask, this, that)
+            with warnings.catch_warnings():
+                warnings.filterwarnings("ignore", "In a future version, `df.iloc")
+                self.loc[:, col] = expressions.where(mask, this, that)
 
     # ----------------------------------------------------------------------
     # Data reshaping

pandas/tests/frame/methods/test_update.py

@@ -136,7 +136,8 @@ def test_update_from_non_df(self):
     def test_update_datetime_tz(self):
         # GH 25807
         result = DataFrame([pd.Timestamp("2019", tz="UTC")])
-        result.update(result)
+        with tm.assert_produces_warning(None):
+            result.update(result)
         expected = DataFrame([pd.Timestamp("2019", tz="UTC")])
         tm.assert_frame_equal(result, expected)