API/BUG: Enforce "normalized" pytz timezones for DatetimeIndex (#20510)

doc/source/whatsnew/v0.23.0.txt

@@ -770,6 +770,8 @@ Datetimelike API Changes
 - :func:`pandas.merge` provides a more informative error message when trying to merge on timezone-aware and timezone-naive columns (:issue:`15800`)
 - For :class:`DatetimeIndex` and :class:`TimedeltaIndex` with ``freq=None``, addition or subtraction of integer-dtyped array or ``Index`` will raise ``NullFrequencyError`` instead of ``TypeError`` (:issue:`19895`)
 - :class:`Timestamp` constructor now accepts a `nanosecond` keyword or positional argument (:issue:`18898`)
+- :class:`DatetimeIndex` will now raise an ``AttributeError`` when the ``tz`` attribute is set after instantiation (:issue:`3746`)
+- :class:`DatetimeIndex` with a ``pytz`` timezone will now return a consistent ``pytz`` timezone (:issue:`18595`)
 
 .. _whatsnew_0230.api.other:
 
@@ -1127,6 +1129,7 @@ Groupby/Resample/Rolling
 - Bug in :func:`DataFrame.resample().aggregate` not raising a ``KeyError`` when aggregating a non-existent column (:issue:`16766`, :issue:`19566`)
 - Fixed a performance regression for ``GroupBy.nth`` and ``GroupBy.last`` with some object columns (:issue:`19283`)
 - Bug in :func:`DataFrameGroupBy.cumsum` and :func:`DataFrameGroupBy.cumprod` when ``skipna`` was passed (:issue:`19806`)
+- Bug in :func:`Dataframe.resample` that dropped timezone information (:issue:`13238`)
 
 Sparse
 ^^^^^^

pandas/_libs/tslibs/timestamps.pyx

@@ -700,6 +700,12 @@ class Timestamp(_Timestamp):
         """
         return self.tzinfo
 
+    @tz.setter
+    def tz(self, value):
+        # GH 3746: Prevent localizing or converting the index by setting tz
+        raise AttributeError("Cannot directly set timezone. Use tz_localize() "
+                             "or tz_convert() as appropriate")
+
     def __setstate__(self, state):
         self.value = state[0]
         self.freq = state[1]

pandas/_libs/tslibs/timezones.pyx

@@ -314,3 +314,41 @@ cpdef bint tz_compare(object start, object end):
     """
     # GH 18523
     return get_timezone(start) == get_timezone(end)
+
+
+cpdef tz_standardize(object tz):
+    """
+    If the passed tz is a pytz timezone object, "normalize" it to the a
+    consistent version
+
+    Parameters
+    ----------
+    tz : tz object
+
+    Returns:
+    -------
+    tz object
+
+    Examples:
+    --------
+    >>> tz
+    <DstTzInfo 'US/Pacific' PST-1 day, 16:00:00 STD>
+
+    >>> tz_standardize(tz)
+    <DstTzInfo 'US/Pacific' LMT-1 day, 16:07:00 STD>
+
+    >>> tz
+    <DstTzInfo 'US/Pacific' LMT-1 day, 16:07:00 STD>
+
+    >>> tz_standardize(tz)
+    <DstTzInfo 'US/Pacific' LMT-1 day, 16:07:00 STD>
+
+    >>> tz
+    dateutil.tz.tz.tzutc
+
+    >>> tz_standardize(tz)
+    dateutil.tz.tz.tzutc
+    """
+    if treat_tz_as_pytz(tz):
+        return pytz.timezone(str(tz))
+    return tz

pandas/core/indexes/datetimelike.py

@@ -1005,7 +1005,7 @@ def shift(self, n, freq=None):
             result = self + offset
 
             if hasattr(self, 'tz'):
-                result.tz = self.tz
+                result._tz = self.tz
 
             return result
 

pandas/core/indexes/datetimes.py

@@ -511,13 +511,7 @@ def _generate(cls, start, end, periods, name, offset,
                             'different timezones')
 
         inferred_tz = timezones.maybe_get_tz(inferred_tz)
-
-        # these may need to be localized
         tz = timezones.maybe_get_tz(tz)
-        if tz is not None:
-            date = start or end
-            if date.tzinfo is not None and hasattr(tz, 'localize'):
-                tz = tz.localize(date.replace(tzinfo=None)).tzinfo
 
         if tz is not None and inferred_tz is not None:
             if not timezones.tz_compare(inferred_tz, tz):
@@ -654,7 +648,8 @@ def _simple_new(cls, values, name=None, freq=None, tz=None,
         result._data = values
         result.name = name
         result.offset = freq
-        result.tz = timezones.maybe_get_tz(tz)
+        result._tz = timezones.maybe_get_tz(tz)
+        result._tz = timezones.tz_standardize(result._tz)
         result._reset_identity()
         return result
 
@@ -684,6 +679,17 @@ def _values(self):
         else:
             return self.values
 
+    @property
+    def tz(self):
+        # GH 18595
+        return self._tz
+
+    @tz.setter
+    def tz(self, value):
+        # GH 3746: Prevent localizing or converting the index by setting tz
+        raise AttributeError("Cannot directly set timezone. Use tz_localize() "
+                             "or tz_convert() as appropriate")
+
     @property
     def tzinfo(self):
         """
@@ -754,7 +760,7 @@ def _cached_range(cls, start=None, end=None, periods=None, offset=None,
 
             cachedRange = DatetimeIndex._simple_new(arr)
             cachedRange.offset = offset
-            cachedRange.tz = None
+            cachedRange = cachedRange.tz_localize(None)
             cachedRange.name = None
             drc[offset] = cachedRange
         else:
@@ -831,7 +837,7 @@ def __setstate__(self, state):
 
                 self.name = own_state[0]
                 self.offset = own_state[1]
-                self.tz = own_state[2]
+                self._tz = timezones.tz_standardize(own_state[2])
 
                 # provide numpy < 1.7 compat
                 if nd_state[2] == 'M8[us]':
@@ -1175,7 +1181,7 @@ def union(self, other):
         else:
             result = Index.union(this, other)
             if isinstance(result, DatetimeIndex):
-                result.tz = this.tz
+                result._tz = timezones.tz_standardize(this.tz)
                 if (result.freq is None and
                         (this.freq is not None or other.freq is not None)):
                     result.offset = to_offset(result.inferred_freq)
@@ -1223,7 +1229,7 @@ def union_many(self, others):
                 tz = this.tz
                 this = Index.union(this, other)
                 if isinstance(this, DatetimeIndex):
-                    this.tz = tz
+                    this._tz = timezones.tz_standardize(tz)
 
         if this.freq is None:
             this.offset = to_offset(this.inferred_freq)

pandas/tests/frame/test_alter_axes.py

@@ -249,8 +249,8 @@ def test_set_index_cast_datetimeindex(self):
         # convert to utc
         df['C'] = i.to_series().reset_index(drop=True)
         result = df['C']
-        comp = pd.DatetimeIndex(expected.values).copy()
-        comp.tz = None
+        comp = pd.DatetimeIndex(expected.values)
+        comp = comp.tz_localize(None)
         tm.assert_numpy_array_equal(result.values, comp.values)
 
         # list of datetimes with a tz

pandas/tests/indexes/datetimes/test_construction.py

@@ -441,6 +441,34 @@ def test_000constructor_resolution(self):
 
         assert idx.nanosecond[0] == t1.nanosecond
 
+    def test_disallow_setting_tz(self):
+        # GH 3746
+        dti = DatetimeIndex(['2010'], tz='UTC')
+        with pytest.raises(AttributeError):
+            dti.tz = pytz.timezone('US/Pacific')
+
+    @pytest.mark.parametrize('tz', [
+        None, 'America/Los_Angeles', pytz.timezone('America/Los_Angeles'),
+        Timestamp('2000', tz='America/Los_Angeles').tz])
+    def test_constructor_start_end_with_tz(self, tz):
+        # GH 18595
+        start = Timestamp('2013-01-01 06:00:00', tz='America/Los_Angeles')
+        end = Timestamp('2013-01-02 06:00:00', tz='America/Los_Angeles')
+        result = DatetimeIndex(freq='D', start=start, end=end, tz=tz)
+        expected = DatetimeIndex(['2013-01-01 06:00:00',
+                                  '2013-01-02 06:00:00'],
+                                 tz='America/Los_Angeles')
+        tm.assert_index_equal(result, expected)
+        # Especially assert that the timezone is consistent for pytz
+        assert pytz.timezone('America/Los_Angeles') is result.tz
+
+    @pytest.mark.parametrize('tz', ['US/Pacific', 'US/Eastern', 'Asia/Tokyo'])
+    def test_constructor_with_non_normalized_pytz(self, tz):
+        # GH 18595
+        non_norm_tz = Timestamp('2010', tz=tz).tz
+        result = DatetimeIndex(['2010'], tz=non_norm_tz)
+        assert pytz.timezone(tz) is result.tz
+
 
 class TestTimeSeries(object):
 

pandas/tests/scalar/timestamp/test_timestamp.py

@@ -521,6 +521,13 @@ def test_today(self):
         assert (abs(ts_from_string_tz.tz_localize(None) -
                     ts_from_method_tz.tz_localize(None)) < delta)
 
+    @pytest.mark.parametrize('tz', [None, pytz.timezone('US/Pacific')])
+    def test_disallow_setting_tz(self, tz):
+        # GH 3746
+        ts = Timestamp('2010')
+        with pytest.raises(AttributeError):
+            ts.tz = tz
+
 
 class TestTimestamp(object):
 

pandas/tests/test_resample.py

@@ -2532,6 +2532,18 @@ def test_with_local_timezone_pytz(self):
         expected = Series(1, index=expected_index)
         assert_series_equal(result, expected)
 
+    def test_resample_with_pytz(self):
+        # GH 13238
+        s = Series(2, index=pd.date_range('2017-01-01', periods=48, freq="H",
+                                          tz="US/Eastern"))
+        result = s.resample("D").mean()
+        expected = Series(2, index=pd.DatetimeIndex(['2017-01-01',
+                                                     '2017-01-02'],
+                                                    tz="US/Eastern"))
+        assert_series_equal(result, expected)
+        # Especially assert that the timezone is LMT for pytz
+        assert result.index.tz == pytz.timezone('US/Eastern')
+
     def test_with_local_timezone_dateutil(self):
         # see gh-5430
         local_timezone = 'dateutil/America/Los_Angeles'