TENTATIVE: Fix for combine_first; tests for combine_first and where.

No solution yet for where, and combine_first might require cleanup.

Author: Liam3851

pandas/core/series.py

@@ -2305,7 +2305,11 @@ def combine_first(self, other):
         # TODO: do we need name?
         name = ops.get_op_result_name(self, other)  # noqa
         rs_vals = com._where_compat(isna(this), other._values, this._values)
-        return self._constructor(rs_vals, index=new_index).__finalize__(self)
+        result = self._constructor(rs_vals, index=new_index).__finalize__(self)
+        # TODO DK can we simplify this using internals rather than public astype
+        if is_datetime64tz_dtype(self.dtype):
+            result = result.astype(self.dtype)
+        return result
 
     def update(self, other):
         """

pandas/tests/series/indexing/test_boolean.py

@@ -551,6 +551,19 @@ def test_where_datetime_conversion():
     assert_series_equal(rs, expected)
 
 
+def test_where_dt_tz_values(self):
+    dts1 = pd.date_range('20150101', '20150105', tz='America/New_York')
+    df1 = pd.DataFrame({'date':dts1})
+    dts2 = pd.date_range('20150103', '20150107', tz='America/New_York')
+    df2 = pd.DataFrame({'date':dts2})
+    result = df1.date.where(df1.date < df1.date[3], df2.date)
+    exp_vals = pd.DatetimeIndex(['20150101', '20150102', '20150103',
+                                 '20150106', '20150107'],
+                                tz='America/New_York')
+    exp = Series(exp_vals)
+    assert_series_equal(exp, result)
+
+
 def test_mask():
     # compare with tested results in test_where
     s = Series(np.random.randn(5))

pandas/tests/series/test_combine_concat.py

@@ -170,6 +170,19 @@ def get_result_type(dtype, dtype2):
                                     ]).dtype
                 assert result.kind == expected
 
+    def test_combine_first_dt_tz_values(self):
+        dts1 = pd.date_range('20150101', '20150105', tz='America/New_York')
+        df1 = pd.DataFrame({'date':dts1})
+        dts2 = pd.date_range('20160514', '20160518', tz='America/New_York')
+        df2 = pd.DataFrame({'date':dts2}, index=range(3, 8))
+        result = df1.date.combine_first(df2.date)
+        exp_vals = pd.DatetimeIndex(['20150101', '20150102', '20150103',
+                                     '20150104', '20150105', '20160516',
+                                     '20160517', '20160518'],
+                                    tz='America/New_York')
+        exp = pd.Series(exp_vals, name='date')
+        assert_series_equal(exp, result)
+
     def test_concat_empty_series_dtypes(self):
 
         # booleans