BUG: use entire size of DatetimeTZBlock when coercing result (#15855)

doc/source/whatsnew/v0.20.0.txt

@@ -999,6 +999,7 @@ Conversion
 - Bug in ``DataFrame.fillna()`` where the argument ``downcast`` was ignored when fillna value was of type ``dict`` (:issue:`15277`)
 - Bug in ``.asfreq()``, where frequency was not set for empty ``Series`` (:issue:`14320`)
 - Bug in ``DataFrame`` construction with nulls and datetimes in a list-like (:issue:`15869`)
+- Bug in ``DataFrame.fillna()`` with tz-aware datetimes (:issue:`15855`)
 
 Indexing
 ^^^^^^^^

pandas/core/internals.py

@@ -2475,7 +2475,7 @@ def _try_coerce_result(self, result):
         if isinstance(result, np.ndarray):
             # allow passing of > 1dim if its trivial
             if result.ndim > 1:
-                result = result.reshape(len(result))
+                result = result.reshape(np.prod(result.shape))
             result = self.values._shallow_copy(result)
 
         return result

pandas/tests/frame/test_missing.py

@@ -257,6 +257,20 @@ def test_fillna(self):
         result = df.fillna(value={'Date': df['Date2']})
         assert_frame_equal(result, expected)
 
+        # with timezone
+        # GH 15855
+        df = pd.DataFrame({'A': [pd.Timestamp('2012-11-11 00:00:00+01:00'),
+                                 pd.NaT]})
+        exp = pd.DataFrame({'A': [pd.Timestamp('2012-11-11 00:00:00+01:00'),
+                                  pd.Timestamp('2012-11-11 00:00:00+01:00')]})
+        assert_frame_equal(df.fillna(method='pad'), exp)
+
+        df = pd.DataFrame({'A': [pd.NaT,
+                                 pd.Timestamp('2012-11-11 00:00:00+01:00')]})
+        exp = pd.DataFrame({'A': [pd.Timestamp('2012-11-11 00:00:00+01:00'),
+                                  pd.Timestamp('2012-11-11 00:00:00+01:00')]})
+        assert_frame_equal(df.fillna(method='bfill'), exp)
+
     def test_fillna_downcast(self):
         # GH 15277
         # infer int64 from float64

pandas/tests/series/test_missing.py

@@ -258,6 +258,18 @@ def test_datetime64_tz_fillna(self):
             self.assert_series_equal(expected, result)
             self.assert_series_equal(pd.isnull(s), null_loc)
 
+        # with timezone
+        # GH 15855
+        df = pd.Series([pd.Timestamp('2012-11-11 00:00:00+01:00'), pd.NaT])
+        exp = pd.Series([pd.Timestamp('2012-11-11 00:00:00+01:00'),
+                         pd.Timestamp('2012-11-11 00:00:00+01:00')])
+        assert_series_equal(df.fillna(method='pad'), exp)
+
+        df = pd.Series([pd.NaT, pd.Timestamp('2012-11-11 00:00:00+01:00')])
+        exp = pd.Series([pd.Timestamp('2012-11-11 00:00:00+01:00'),
+                         pd.Timestamp('2012-11-11 00:00:00+01:00')])
+        assert_series_equal(df.fillna(method='bfill'), exp)
+
     def test_datetime64tz_fillna_round_issue(self):
         # GH 14872