Backport PR #36115 on branch 1.1.x (REGR: append tz-aware DataFrame with tz-naive values)

doc/source/whatsnew/v1.1.2.rst

@@ -16,6 +16,7 @@ Fixed regressions
 ~~~~~~~~~~~~~~~~~
 - Regression in :meth:`DatetimeIndex.intersection` incorrectly raising ``AssertionError`` when intersecting against a list (:issue:`35876`)
 - Fix regression in updating a column inplace (e.g. using ``df['col'].fillna(.., inplace=True)``) (:issue:`35731`)
+- Fix regression in :meth:`DataFrame.append` mixing tz-aware and tz-naive datetime columns (:issue:`35460`)
 - Performance regression for :meth:`RangeIndex.format` (:issue:`35712`)
 - Regression where :meth:`MultiIndex.get_loc` would return a slice spanning the full index when passed an empty list (:issue:`35878`)
 - Fix regression in invalid cache after an indexing operation; this can manifest when setting which does not update the data (:issue:`35521`)

pandas/core/dtypes/concat.py

@@ -148,15 +148,17 @@ def is_nonempty(x) -> bool:
     any_ea = any(is_extension_array_dtype(x.dtype) for x in to_concat)
 
     if any_ea:
+        # we ignore axis here, as internally concatting with EAs is always
+        # for axis=0
         if not single_dtype:
             target_dtype = find_common_type([x.dtype for x in to_concat])
             to_concat = [_cast_to_common_type(arr, target_dtype) for arr in to_concat]
 
-        if isinstance(to_concat[0], ExtensionArray) and axis == 0:
+        if isinstance(to_concat[0], ExtensionArray):
             cls = type(to_concat[0])
             return cls._concat_same_type(to_concat)
         else:
-            return np.concatenate(to_concat, axis=axis)
+            return np.concatenate(to_concat)
 
     elif _contains_datetime or "timedelta" in typs:
         return concat_datetime(to_concat, axis=axis, typs=typs)

pandas/core/internals/concat.py

@@ -23,7 +23,7 @@
 from pandas.core.dtypes.missing import isna
 
 import pandas.core.algorithms as algos
-from pandas.core.arrays import ExtensionArray
+from pandas.core.arrays import DatetimeArray, ExtensionArray
 from pandas.core.internals.blocks import make_block
 from pandas.core.internals.managers import BlockManager
 
@@ -334,9 +334,13 @@ def _concatenate_join_units(join_units, concat_axis, copy):
         # the non-EA values are 2D arrays with shape (1, n)
         to_concat = [t if isinstance(t, ExtensionArray) else t[0, :] for t in to_concat]
         concat_values = concat_compat(to_concat, axis=0)
-        if not isinstance(concat_values, ExtensionArray):
+        if not isinstance(concat_values, ExtensionArray) or (
+            isinstance(concat_values, DatetimeArray) and concat_values.tz is None
+        ):
             # if the result of concat is not an EA but an ndarray, reshape to
             # 2D to put it a non-EA Block
+            # special case DatetimeArray, which *is* an EA, but is put in a
+            # consolidated 2D block
             concat_values = np.atleast_2d(concat_values)
     else:
         concat_values = concat_compat(to_concat, axis=concat_axis,)

pandas/tests/reshape/test_concat.py

@@ -1110,6 +1110,23 @@ def test_append_empty_frame_to_series_with_dateutil_tz(self):
         result = df.append([s, s], ignore_index=True)
         tm.assert_frame_equal(result, expected)
 
+    def test_append_empty_tz_frame_with_datetime64ns(self):
+        # https://github.com/pandas-dev/pandas/issues/35460
+        df = pd.DataFrame(columns=["a"]).astype("datetime64[ns, UTC]")
+
+        # pd.NaT gets inferred as tz-naive, so append result is tz-naive
+        result = df.append({"a": pd.NaT}, ignore_index=True)
+        expected = pd.DataFrame({"a": [pd.NaT]}).astype("datetime64[ns]")
+        tm.assert_frame_equal(result, expected)
+
+        # also test with typed value to append
+        df = pd.DataFrame(columns=["a"]).astype("datetime64[ns, UTC]")
+        result = df.append(
+            pd.Series({"a": pd.NaT}, dtype="datetime64[ns]"), ignore_index=True
+        )
+        expected = pd.DataFrame({"a": [pd.NaT]}).astype("datetime64[ns]")
+        tm.assert_frame_equal(result, expected)
+
 
 class TestConcatenate:
     def test_concat_copy(self):