fix: scalar timestamp assignment (pandas-dev#19843) (pandas-dev#19973)

Author: DylanDmitri

doc/source/whatsnew/v0.24.0.txt

@@ -562,6 +562,7 @@ Timezones
 - Bug in :class:`DatetimeIndex` where constructing with an integer and tz would not localize correctly (:issue:`12619`)
 - Fixed bug where :meth:`DataFrame.describe` and :meth:`Series.describe` on tz-aware datetimes did not show `first` and `last` result (:issue:`21328`)
 - Bug in :class:`DatetimeIndex` comparisons failing to raise ``TypeError`` when comparing timezone-aware ``DatetimeIndex`` against ``np.datetime64`` (:issue:`22074`)
+- Bug in ``DataFrame`` assignment with a timezone-aware scalar (:issue:`19843`)
 
 Offsets
 ^^^^^^^

pandas/core/frame.py

@@ -27,6 +27,7 @@
     maybe_upcast,
     cast_scalar_to_array,
     construct_1d_arraylike_from_scalar,
+    infer_dtype_from_scalar,
     maybe_cast_to_datetime,
     maybe_infer_to_datetimelike,
     maybe_convert_platform,
@@ -3507,9 +3508,13 @@ def reindexer(value):
                 value = maybe_infer_to_datetimelike(value)
 
         else:
-            # upcast the scalar
+            # cast ignores pandas dtypes. so save the dtype first
+            infer_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)
+            value = maybe_cast_to_datetime(value, infer_dtype)
 
         # return internal types directly
         if is_extension_type(value) or is_extension_array_dtype(value):

pandas/tests/frame/test_indexing.py

@@ -3135,6 +3135,14 @@ 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')
+        expected = pd.DataFrame(
+            {'now': pd.Timestamp('20130101', tz='UTC')}, index=[0, 1, 2])
+        tm.assert_frame_equal(df, expected)
+
 
 class TestDataFrameIndexingUInt64(TestData):