BUG: read_json converted date strings with Z to UTC

doc/source/whatsnew/v0.25.0.rst

@@ -290,6 +290,7 @@ Timezones
 - Bug in :func:`Series.at` where setting :class:`Timestamp` with timezone raises ``TypeError`` (:issue:`25506`)
 - Bug in :func:`DataFrame.update` when updating with timezone aware data would return timezone naive data (:issue:`25807`)
 - Bug in :func:`to_datetime` where an uninformative ``RuntimeError`` was raised when passing a naive :class:`Timestamp` with datetime strings with mixed UTC offsets (:issue:`25978`)
+- Bug in :func:`to_datetime` with ``unit='ns'`` would drop timezone information from the parsed argument (:issue:`26168`)
 
 Numeric
 ^^^^^^^
@@ -371,6 +372,7 @@ I/O
 - Fixed bug in loading objects from S3 that contain ``#`` characters in the URL (:issue:`25945`)
 - Adds ``use_bqstorage_api`` parameter to :func:`read_gbq` to speed up downloads of large data frames. This feature requires version 0.10.0 of the ``pandas-gbq`` library as well as the ``google-cloud-bigquery-storage`` and ``fastavro`` libraries. (:issue:`26104`)
 - Fixed memory leak in :meth:`DataFrame.to_json` when dealing with numeric data (:issue:`24889`)
+- Bug in :func:`read_json` where date strings with ``Z`` were not converted to a UTC timezone (:issue:`26168`)
 
 Plotting
 ^^^^^^^^

pandas/_libs/tslib.pyx

@@ -311,6 +311,10 @@ def array_with_unit_to_datetime(ndarray values, object unit,
       - ignore: return non-convertible values as the same unit
       - coerce: NaT for non-convertibles
 
+    Returns
+    -------
+    result : ndarray of m8 values
+    tz : parsed timezone offset or None
     """
     cdef:
         Py_ssize_t i, j, n=len(values)
@@ -323,13 +327,15 @@ def array_with_unit_to_datetime(ndarray values, object unit,
         bint need_to_iterate = True
         ndarray[int64_t] iresult
         ndarray[object] oresult
+        object tz = None
 
     assert is_ignore or is_coerce or is_raise
 
     if unit == 'ns':
         if issubclass(values.dtype.type, np.integer):
-            return values.astype('M8[ns]')
-        return array_to_datetime(values.astype(object), errors=errors)[0]
+            return values.astype('M8[ns]'), tz
+        # This will return a tz
+        return array_to_datetime(values.astype(object), errors=errors)
 
     m = cast_from_unit(None, unit)
 
@@ -357,7 +363,7 @@ def array_with_unit_to_datetime(ndarray values, object unit,
             result = (iresult * m).astype('M8[ns]')
             iresult = result.view('i8')
             iresult[mask] = NPY_NAT
-            return result
+            return result, tz
 
     result = np.empty(n, dtype='M8[ns]')
     iresult = result.view('i8')
@@ -419,7 +425,7 @@ def array_with_unit_to_datetime(ndarray values, object unit,
 
                 iresult[i] = NPY_NAT
 
-        return result
+        return result, tz
 
     except AssertionError:
         pass
@@ -451,7 +457,7 @@ def array_with_unit_to_datetime(ndarray values, object unit,
             else:
                 oresult[i] = val
 
-    return oresult
+    return oresult, tz
 
 
 @cython.wraparound(False)

pandas/core/tools/datetimes.py

@@ -200,19 +200,27 @@ def _convert_listlike_datetimes(arg, box, format, name=None, tz=None,
         if format is not None:
             raise ValueError("cannot specify both format and unit")
         arg = getattr(arg, 'values', arg)
-        result = tslib.array_with_unit_to_datetime(arg, unit,
-                                                   errors=errors)
+        result, tz_parsed = tslib.array_with_unit_to_datetime(arg, unit,
+                                                              errors=errors)
         if box:
             if errors == 'ignore':
                 from pandas import Index
                 result = Index(result, name=name)
-                # GH 23758: We may still need to localize the result with tz
-                try:
-                    return result.tz_localize(tz)
-                except AttributeError:
-                    return result
-
-            return DatetimeIndex(result, tz=tz, name=name)
+            else:
+                result = DatetimeIndex(result, name=name)
+            # GH 23758: We may still need to localize the result with tz
+            # GH 25546: Apply tz_parsed first (from arg), then tz (from caller)
+            # result will be naive but in UTC
+            try:
+                result = result.tz_localize('UTC').tz_convert(tz_parsed)
+            except AttributeError:
+                # Regular Index from 'ignore' path
+                return result
+            if tz is not None:
+                if result.tz is None:
+                    result = result.tz_localize(tz)
+                else:
+                    result = result.tz_convert(tz)
         return result
     elif getattr(arg, 'ndim', 1) > 1:
         raise TypeError('arg must be a string, datetime, list, tuple, '

pandas/tests/indexes/datetimes/test_tools.py

@@ -1967,3 +1967,16 @@ def test_processing_order(self):
         result = pd.to_datetime(300 * 365, unit='D', origin='1870-01-01')
         expected = Timestamp('2169-10-20 00:00:00')
         assert result == expected
+
+    @pytest.mark.parametrize('offset,utc,exp', [
+        ["Z", True, "2019-01-01T00:00:00.000Z"],
+        ["Z", None, "2019-01-01T00:00:00.000Z"],
+        ["-01:00", True, "2019-01-01T01:00:00.000Z"],
+        ["-01:00", None, "2019-01-01T00:00:00.000-01:00"],
+    ])
+    def test_arg_tz_ns_unit(self, offset, utc, exp):
+        # GH 25546
+        arg = "2019-01-01T00:00:00.000" + offset
+        result = to_datetime([arg], unit='ns', utc=utc)
+        expected = to_datetime([exp])
+        tm.assert_index_equal(result, expected)

pandas/tests/io/json/test_pandas.py

@@ -762,7 +762,10 @@ def test_w_date(date, date_unit=None):
             else:
                 json = df.to_json(date_format='iso')
             result = read_json(json)
-            assert_frame_equal(result, df)
+            expected = df.copy()
+            expected.index = expected.index.tz_localize('UTC')
+            expected['date'] = expected['date'].dt.tz_localize('UTC')
+            assert_frame_equal(result, expected)
 
         test_w_date('20130101 20:43:42.123')
         test_w_date('20130101 20:43:42', date_unit='s')
@@ -784,7 +787,10 @@ def test_w_date(date, date_unit=None):
             else:
                 json = ts.to_json(date_format='iso')
             result = read_json(json, typ='series')
-            assert_series_equal(result, ts)
+            expected = ts.copy()
+            expected.index = expected.index.tz_localize('UTC')
+            expected = expected.dt.tz_localize('UTC')
+            assert_series_equal(result, expected)
 
         test_w_date('20130101 20:43:42.123')
         test_w_date('20130101 20:43:42', date_unit='s')
@@ -880,11 +886,15 @@ def test_round_trip_exception_(self):
 
     @network
     @pytest.mark.single
-    def test_url(self):
+    @pytest.mark.parametrize('field,dtype', [
+        ['created_at', pd.DatetimeTZDtype(tz='UTC')],
+        ['closed_at', 'datetime64[ns]'],
+        ['updated_at', pd.DatetimeTZDtype(tz='UTC')]
+    ])
+    def test_url(self, field, dtype):
         url = 'https://api.github.com/repos/pandas-dev/pandas/issues?per_page=5'  # noqa
         result = read_json(url, convert_dates=True)
-        for c in ['created_at', 'closed_at', 'updated_at']:
-            assert result[c].dtype == 'datetime64[ns]'
+        assert result[field].dtype == dtype
 
     def test_timedelta(self):
         converter = lambda x: pd.to_timedelta(x, unit='ms')
@@ -1298,3 +1308,12 @@ def test_index_false_from_json_to_json(self, orient, index):
         dfjson = expected.to_json(orient=orient, index=index)
         result = read_json(dfjson, orient=orient)
         assert_frame_equal(result, expected)
+
+    def test_read_timezone_information(self):
+        # GH 25546
+        result = read_json('{"2019-01-01T11:00:00.000Z":88}',
+                           typ='series', orient='index')
+        expected = Series([88],
+                          index=DatetimeIndex(['2019-01-01 11:00:00'],
+                                              tz='UTC'))
+        assert_series_equal(result, expected)