fix: scalar timestamp assignment (#19843)

doc/source/whatsnew/v0.23.0.txt

@@ -854,6 +854,7 @@ Datetimelike
 - Bug in :func:`to_datetime` where passing an out-of-bounds datetime with ``errors='coerce'`` and ``utc=True`` would raise ``OutOfBoundsDatetime`` instead of parsing to ``NaT`` (:issue:`19612`)
 - Bug in :class:`DatetimeIndex` and :class:`TimedeltaIndex` addition and subtraction where name of the returned object was not always set consistently. (:issue:`19744`)
 - Bug in :class:`DatetimeIndex` and :class:`TimedeltaIndex` addition and subtraction where operations with numpy arrays raised ``TypeError`` (:issue:`19847`)
+- Bug in `DataFrame` assignment with a timezone-aware object (:issue:`19843`)
 
 Timedelta
 ^^^^^^^^^

pandas/core/frame.py

@@ -2868,9 +2868,15 @@ def reindexer(value):
                 value = maybe_infer_to_datetimelike(value)
 
         else:
-            # upcast the scalar
+            # cast ignores pandas dtypes. so save the dtype first
+            from pandas.core.dtypes.cast import infer_dtype_from_scalar
+            pd_dtype, _ = infer_dtype_from_scalar(value, pandas_dtype=True)
+
+            # upcast
             value = cast_scalar_to_array(len(self.index), value)
-            value = maybe_cast_to_datetime(value, value.dtype)
+
+            # then add dtype back in
+            value = maybe_cast_to_datetime(value, pd_dtype)
 
         # return internal types directly
         if is_extension_type(value) or is_extension_array_dtype(value):

pandas/tests/frame/test_indexing.py

@@ -37,6 +37,8 @@
 
 from pandas.tests.frame.common import TestData
 
+from pandas.core.dtypes.dtypes import DatetimeTZDtype
+
 
 class TestDataFrameIndexing(TestData):
 
@@ -3042,6 +3044,20 @@ def test_transpose(self):
         expected.index = ['A', 'B']
         assert_frame_equal(result, expected)
 
+    def test_scalar_assignment(self):
+        # issue #19843
+        df = pd.DataFrame(index=(0, 1, 2))
+        df['now'] = pd.Timestamp('20130101', tz='UTC')
+        assert isinstance(df.dtypes[0], DatetimeTZDtype)
+
+    def test_datetime_index_assignment(self):
+        # issue #19843
+        df = pd.DataFrame(index=(0, 1, 2))
+        di = pd.DatetimeIndex(
+            [pd.Timestamp('20130101', tz='UTC')]).repeat(len(df))
+        df['now'] = di
+        assert isinstance(df.dtypes[0], DatetimeTZDtype)
+
 
 class TestDataFrameIndexingUInt64(TestData):