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

Fix to issue pandas-dev#4230 which allows to localize an index which is
implicitly in a tz (e.g., reading from a file) by passing infer_dst to
tz_localize.

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')

pandas/tslib.pyx

@@ -1630,7 +1630,7 @@ cpdef ndarray _unbox_utcoffsets(object transinfo):
 
 @cython.boundscheck(False)
 @cython.wraparound(False)
-def tz_localize_to_utc(ndarray[int64_t] vals, object tz):
+def tz_localize_to_utc(ndarray[int64_t] vals, object tz, bint infer_dst=False):
     """
     Localize tzinfo-naive DateRange to given time zone (using pytz). If
     there are ambiguities in the values, raise AmbiguousTimeError.
@@ -1644,7 +1644,7 @@ def tz_localize_to_utc(ndarray[int64_t] vals, object tz):
         Py_ssize_t i, idx, pos, ntrans, n = len(vals)
         int64_t *tdata
         int64_t v, left, right
-        ndarray[int64_t] result, result_a, result_b
+        ndarray[int64_t] result, result_a, result_b, dst_hours
         pandas_datetimestruct dts
 
     # Vectorized version of DstTzInfo.localize
@@ -1701,6 +1701,48 @@ def tz_localize_to_utc(ndarray[int64_t] vals, object tz):
         # timestamp falls to the right side of the DST transition
         if v + deltas[pos] == vals[i]:
             result_b[i] = v
+
+
+    if infer_dst:
+        dst_hours = np.empty(n, dtype=np.int64)
+        dst_hours.fill(NPY_NAT)
+
+        # Get the ambiguous hours (given the above, these are the hours
+        # where result_a != result_b and neither of them are NAT) 
+        both_nat = np.logical_and(result_a != NPY_NAT, result_b != NPY_NAT)
+        both_eq  = result_a == result_b
+        trans_idx = np.squeeze(np.nonzero(np.logical_and(both_nat, ~both_eq)))
+        if trans_idx.size == 1:
+            stamp = Timestamp(vals[trans_idx])
+            raise pytz.AmbiguousTimeError("Cannot infer dst time from %s as" 
+                                          "there are no repeated times" % stamp)
+        # Split the array into contiguous chunks (where the difference between
+        # indices is 1).  These are effectively dst transitions in different years
+        # which is useful for checking that there is not an ambiguous transition
+        # in an individual year.
+        if trans_idx.size > 0:
+            one_diff = np.where(np.diff(trans_idx)!=1)[0]+1
+            trans_grp = np.array_split(trans_idx, one_diff)
+
+            # Iterate through each day, if there are no hours where the delta is negative
+            # (indicates a repeat of hour) the switch cannot be inferred
+            for grp in trans_grp:
+
+                delta = np.diff(result_a[grp])
+                if grp.size == 1 or np.all(delta>0):
+                    stamp = Timestamp(vals[grp[0]])
+                    raise pytz.AmbiguousTimeError(stamp)
+
+                # Find the index for the switch and pull from a for dst and b for standard
+                switch_idx = (delta<=0).nonzero()[0]
+                if switch_idx.size > 1:
+                    raise pytz.AmbiguousTimeError("There are %i dst switches "
+                                                  "when there should only be 1." 
+                                                  % switch_idx.size)
+                switch_idx = switch_idx[0]+1 # Pull the only index and adjust
+                a_idx = grp[:switch_idx]
+                b_idx = grp[switch_idx:]
+                dst_hours[grp] = np.hstack((result_a[a_idx], result_b[b_idx]))
 
     for i in range(n):
         left = result_a[i]
@@ -1709,8 +1751,13 @@ def tz_localize_to_utc(ndarray[int64_t] vals, object tz):
             if left == right:
                 result[i] = left
             else:
-                stamp = Timestamp(vals[i])
-                raise pytz.AmbiguousTimeError(stamp)
+                if infer_dst and dst_hours[i] != NPY_NAT:
+                    result[i] = dst_hours[i]
+                else:
+                    stamp = Timestamp(vals[i])
+                    raise pytz.AmbiguousTimeError("Cannot infer dst time from %r, "\
+                                                  "try using the 'infer_dst' argument" 
+                                                  % stamp)
         elif left != NPY_NAT:
             result[i] = left
         elif right != NPY_NAT:

vb_suite/timeseries.py

@@ -225,3 +225,21 @@ def date_range(start=None, end=None, periods=None, freq=None):
 
 datetimeindex_unique = Benchmark('index.unique()', setup,
                                  start_date=datetime(2012, 7, 1))
+
+# tz_localize with infer argument.  This is an attempt to emulate the results
+# of read_csv with duplicated data.  Not passing infer_dst will fail
+setup = common_setup + """
+dst_rng = date_range('10/29/2000 1:00:00', 
+                     '10/29/2000 1:59:59', freq='S')
+index = date_range('10/29/2000', '10/29/2000 00:59:59', freq='S')
+index = index.append(dst_rng)
+index = index.append(dst_rng)
+index = index.append(date_range('10/29/2000 2:00:00', 
+                                '10/29/2000 3:00:00', freq='S'))
+"""
+
+datetimeindex_infer_dst = \
+Benchmark('index.tz_localize("US/Eastern", infer_dst=True)',
+          setup, start_date=datetime(2013, 9, 30))
+
+