ENH: Ability to tz localize when index is implicility in tz

doc/source/api.rst

@@ -868,3 +868,149 @@ Serialization / IO / Conversion
    Panel.to_frame
    Panel.to_clipboard
 
+.. currentmodule:: pandas.core.index
+
+.. _api.index
+
+Index
+-----
+
+**Many of these methods or variants thereof are available on the objects that contain an index (Series/Dataframe)
+and those should most likely be used before calling these methods directly.**
+
+   * **values**
+Modifying and Computations
+~~~~~~~~~~~~~~~~~~~~~~~~~~
+.. autosummary::
+   :toctree: generated/
+
+   Index.copy
+   Index.delete
+   Index.diff
+   Index.drop
+   Index.equals
+   Index.identical
+   Index.insert
+   Index.order
+   Index.reindex
+   Index.repeat
+   Index.set_names
+   Index.unique
+
+Conversion
+~~~~~~~~~~
+.. autosummary::
+   :toctree: generated/
+
+   Index.astype
+   Index.tolist
+   Index.to_datetime
+   Index.to_series
+
+Sorting
+~~~~~~~
+.. autosummary::
+   :toctree: generated/
+
+   Index.argsort
+   Index.order
+   Index.sort
+
+Time-specific operations
+~~~~~~~~~~~~~~~~~~~~~~~~
+.. autosummary::
+   :toctree: generated/
+
+   Index.shift
+
+Combining / joining / merging
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+.. autosummary::
+   :toctree: generated/
+
+   Index.append
+   Index.intersection
+   Index.join
+   Index.union
+
+Selecting
+~~~~~~~~~
+.. autosummary::
+   :toctree: generated/
+
+   Index.get_indexer
+   Index.get_indexer_non_unique
+   Index.get_level_values
+   Index.get_loc
+   Index.get_value
+   Index.isin
+   Index.slice_indexer
+   Index.slice_locs
+
+Properties
+~~~~~~~~~~
+.. autosummary::
+   :toctree: generated/
+
+   Index.is_monotonic
+   Index.is_numeric
+
+.. currentmodule:: pandas.tseries.index
+
+.. _api.datetimeindex:
+
+DatetimeIndex
+-------------
+
+Time/Date Components
+~~~~~~~~~~~~~~~~~~~~
+  * **year**
+  * **month**
+  * **day**
+  * **hour**
+  * **minute**
+  * **second**
+  * **microsecond**
+  * **nanosecond**
+
+  * **weekofyear**
+  * **week**: Same as weekofyear
+  * **dayofweek**: (0=Monday, 6=Sunday)
+  * **weekday**: (0=Monday, 6=Sunday)
+  * **dayofyear** 
+  * **quarter**
+
+  * **date**: Returns date component of Timestamps
+  * **time**: Returns time component of Timestamps
+
+
+Selecting
+~~~~~~~~~
+.. autosummary::
+   :toctree: generated/
+
+   DatetimeIndex.indexer_at_time
+   DatetimeIndex.indexer_between_time
+
+
+Time-specific operations
+~~~~~~~~~~~~~~~~~~~~~~~~
+.. autosummary::
+   :toctree: generated/
+
+   DatetimeIndex.normalize
+   DatetimeIndex.snap
+   DatetimeIndex.tz_convert
+   DatetimeIndex.tz_localize
+
+
+Conversion
+~~~~~~~~~~
+.. autosummary::
+   :toctree: generated/
+
+   DatetimeIndex.to_datetime
+   DatetimeIndex.to_period
+   DatetimeIndex.to_pydatetime
+
+

doc/source/release.rst

@@ -160,6 +160,9 @@ Improvements to existing features
     :issue:`4998`)
   - ``to_dict`` now takes ``records`` as a possible outtype.  Returns an array
     of column-keyed dictionaries. (:issue:`4936`)
+  - ``tz_localize`` can infer a fall daylight savings transition based on the 
+    structure of unlocalized data (:issue:`4230`)
+  - DatetimeIndex is now in the API documentation
 
 API Changes
 ~~~~~~~~~~~

doc/source/timeseries.rst

@@ -1108,6 +1108,20 @@ TimeSeries, aligning the data on the UTC timestamps:
 
 .. _timeseries.timedeltas:
 
+In some cases, localize cannot determine the DST and non-DST hours when there are
+duplicates.  This often happens when reading files that simply duplicate the hours.
+The infer_dst argument in tz_localize will attempt
+to determine the right offset.
+
+.. ipython:: python
+
+   rng_hourly = DatetimeIndex(['11/06/2011 00:00', '11/06/2011 01:00', 
+                               '11/06/2011 01:00', '11/06/2011 02:00', 
+                               '11/06/2011 03:00'])
+   rng_hourly.tz_localize('US/Eastern')
+   rng_hourly_eastern = rng_hourly.tz_localize('US/Eastern', infer_dst=True)
+   rng_hourly_eastern.values
+
 Time Deltas
 -----------
 

doc/source/v0.13.0.txt

@@ -8,7 +8,7 @@ enhancements along with a large number of bug fixes.
 
 .. warning::
 
-   In 0.13.0 ``Series`` has internaly been refactored to no longer sub-class ``ndarray``
+   In 0.13.0 ``Series`` has internally been refactored to no longer sub-class ``ndarray``
    but instead subclass ``NDFrame``, similarly to the rest of the pandas containers. This should be
    a transparent change with only very limited API implications. See :ref:`Internal Refactoring<whatsnew_0130.refactoring>`
 
@@ -481,6 +481,10 @@ Enhancements
 
       :ref:`See the docs<indexing.basics.indexing_isin>` for more.
 
+  - ``tz_localize`` can infer a fall daylight savings transition based on the structure
+    of the unlocalized data (:issue:`4230`), see :ref:`here<timeseries.timezone>`
+  - DatetimeIndex is now in the API documentation, see :ref:`here<api.datetimeindex>`
+
 .. _whatsnew_0130.experimental:
 
 Experimental

pandas/core/generic.py

@@ -2752,7 +2752,7 @@ def tz_convert(self, tz, axis=0, copy=True):
 
         return new_obj
 
-    def tz_localize(self, tz, axis=0, copy=True):
+    def tz_localize(self, tz, axis=0, copy=True, infer_dst=False):
         """
         Localize tz-naive TimeSeries to target time zone
 
@@ -2761,6 +2761,8 @@ def tz_localize(self, tz, axis=0, copy=True):
         tz : string or pytz.timezone object
         copy : boolean, default True
             Also make a copy of the underlying data
+        infer_dst : boolean, default False
+            Attempt to infer fall dst-transition times based on order
 
         Returns
         -------
@@ -2778,7 +2780,7 @@ def tz_localize(self, tz, axis=0, copy=True):
             new_data = new_data.copy()
 
         new_obj = self._constructor(new_data)
-        new_ax = ax.tz_localize(tz)
+        new_ax = ax.tz_localize(tz, infer_dst=infer_dst)
 
         if axis == 0:
             new_obj._set_axis(1, new_ax)

pandas/core/series.py

@@ -2331,7 +2331,7 @@ def tz_convert(self, tz, copy=True):
 
         return self._constructor(new_values, index=new_index, name=self.name)
 
-    def tz_localize(self, tz, copy=True):
+    def tz_localize(self, tz, copy=True, infer_dst=False):
         """
         Localize tz-naive TimeSeries to target time zone
         Entries will retain their "naive" value but will be annotated as
@@ -2345,6 +2345,8 @@ def tz_localize(self, tz, copy=True):
         tz : string or pytz.timezone object
         copy : boolean, default True
             Also make a copy of the underlying data
+        infer_dst : boolean, default False
+            Attempt to infer fall dst-transition hours based on order
 
         Returns
         -------
@@ -2358,7 +2360,7 @@ def tz_localize(self, tz, copy=True):
 
             new_index = DatetimeIndex([], tz=tz)
         else:
-            new_index = self.index.tz_localize(tz)
+            new_index = self.index.tz_localize(tz, infer_dst=infer_dst)
 
         new_values = self.values
         if copy:

pandas/tseries/index.py

@@ -147,6 +147,7 @@ def __new__(cls, data=None,
 
         dayfirst = kwds.pop('dayfirst', None)
         yearfirst = kwds.pop('yearfirst', None)
+        infer_dst = kwds.pop('infer_dst', False)
         warn = False
         if 'offset' in kwds and kwds['offset']:
             freq = kwds['offset']
@@ -183,7 +184,8 @@ def __new__(cls, data=None,
 
         if data is None:
             return cls._generate(start, end, periods, name, offset,
-                                 tz=tz, normalize=normalize)
+                                 tz=tz, normalize=normalize, 
+                                 infer_dst=infer_dst)
 
         if not isinstance(data, np.ndarray):
             if np.isscalar(data):
@@ -209,7 +211,7 @@ def __new__(cls, data=None,
                         data.name = name
 
                     if tz is not None:
-                        return data.tz_localize(tz)
+                        return data.tz_localize(tz, infer_dst=infer_dst)
 
                     return data
 
@@ -261,7 +263,8 @@ def __new__(cls, data=None,
                         getattr(data, 'tz', None) is None):
                     # Convert tz-naive to UTC
                     ints = subarr.view('i8')
-                    subarr = tslib.tz_localize_to_utc(ints, tz)
+                    subarr = tslib.tz_localize_to_utc(ints, tz,
+                                                      infer_dst=infer_dst)
 
                 subarr = subarr.view(_NS_DTYPE)
 
@@ -286,7 +289,7 @@ def __new__(cls, data=None,
 
     @classmethod
     def _generate(cls, start, end, periods, name, offset,
-                  tz=None, normalize=False):
+                  tz=None, normalize=False, infer_dst=False):
         if com._count_not_none(start, end, periods) != 2:
             raise ValueError('Must specify two of start, end, or periods')
 
@@ -375,7 +378,8 @@ def _generate(cls, start, end, periods, name, offset,
                 index = _generate_regular_range(start, end, periods, offset)
 
             if tz is not None and getattr(index, 'tz', None) is None:
-                index = tslib.tz_localize_to_utc(com._ensure_int64(index), tz)
+                index = tslib.tz_localize_to_utc(com._ensure_int64(index), tz,
+                                                 infer_dst=infer_dst)
                 index = index.view(_NS_DTYPE)
 
         index = index.view(cls)
@@ -1537,9 +1541,17 @@ def tz_convert(self, tz):
         # No conversion since timestamps are all UTC to begin with
         return self._simple_new(self.values, self.name, self.offset, tz)
 
-    def tz_localize(self, tz):
+    def tz_localize(self, tz, infer_dst=False):
         """
         Localize tz-naive DatetimeIndex to given time zone (using pytz)
+
+        Parameters
+        ----------
+        tz : string or pytz.timezone
+            Time zone for time. Corresponding timestamps would be converted to
+            time zone of the TimeSeries
+        infer_dst : boolean, default False
+            Attempt to infer fall dst-transition hours based on order
 
         Returns
         -------
@@ -1550,7 +1562,7 @@ def tz_localize(self, tz):
         tz = tools._maybe_get_tz(tz)
 
         # Convert to UTC
-        new_dates = tslib.tz_localize_to_utc(self.asi8, tz)
+        new_dates = tslib.tz_localize_to_utc(self.asi8, tz, infer_dst=infer_dst)
         new_dates = new_dates.view(_NS_DTYPE)
 
         return self._simple_new(new_dates, self.name, self.offset, tz)

pandas/tseries/tests/test_timezones.py

@@ -360,6 +360,32 @@ def test_with_tz_ambiguous_times(self):
         dr = date_range(datetime(2011, 3, 13), periods=48,
                         freq=datetools.Minute(30), tz=pytz.utc)
 
+    def test_infer_dst(self):
+        # November 6, 2011, fall back, repeat 2 AM hour
+        # With no repeated hours, we cannot infer the transition
+        tz = pytz.timezone('US/Eastern')
+        dr = date_range(datetime(2011, 11, 6, 0), periods=5,
+                        freq=datetools.Hour())
+        self.assertRaises(pytz.AmbiguousTimeError, dr.tz_localize, 
+                          tz, infer_dst=True)
+
+        # With repeated hours, we can infer the transition
+        dr = date_range(datetime(2011, 11, 6, 0), periods=5, 
+                        freq=datetools.Hour(), tz=tz)
+        di = DatetimeIndex(['11/06/2011 00:00', '11/06/2011 01:00', 
+                            '11/06/2011 01:00', '11/06/2011 02:00', 
+                            '11/06/2011 03:00'])
+        localized = di.tz_localize(tz, infer_dst=True)
+        self.assert_(np.array_equal(dr, localized))
+
+        # When there is no dst transition, nothing special happens
+        dr = date_range(datetime(2011, 6, 1, 0), periods=10,
+                        freq=datetools.Hour())
+        localized = dr.tz_localize(tz)
+        localized_infer = dr.tz_localize(tz, infer_dst=True)
+        self.assert_(np.array_equal(localized, localized_infer))
+
+
     # test utility methods
     def test_infer_tz(self):
         eastern = pytz.timezone('US/Eastern')