CFTimeIndex Resampling

doc/time-series.rst

@@ -309,31 +309,34 @@ For data indexed by a :py:class:`~xarray.CFTimeIndex` xarray currently supports:
 
    da.differentiate('time')
 
-- And serialization:
+- Serialization:
 
 .. ipython:: python
 
    da.to_netcdf('example-no-leap.nc')
    xr.open_dataset('example-no-leap.nc')
 
+- And resampling along the time dimension for data indexed by a :py:class:`~xarray.CFTimeIndex`:
+
+.. ipython:: python
+
+    da.resample(time='81T', closed='right', label='right', base=3).mean()
+
 .. note::
 
    While much of the time series functionality that is possible for standard
    dates has been implemented for dates from non-standard calendars, there are
    still some remaining important features that have yet to be implemented,
    for example:
 
-   - Resampling along the time dimension for data indexed by a
-     :py:class:`~xarray.CFTimeIndex` (:issue:`2191`, :issue:`2458`)
    - Built-in plotting of data with :py:class:`cftime.datetime` coordinate axes
      (:issue:`2164`).   
 
    For some use-cases it may still be useful to convert from
    a :py:class:`~xarray.CFTimeIndex` to a :py:class:`pandas.DatetimeIndex`,
-   despite the difference in calendar types (e.g. to allow the use of some
-   forms of resample with non-standard calendars).  The recommended way of
-   doing this is to use the built-in
-   :py:meth:`~xarray.CFTimeIndex.to_datetimeindex` method:
+   despite the difference in calendar types. The recommended way of doing this
+   is to use the built-in :py:meth:`~xarray.CFTimeIndex.to_datetimeindex`
+   method:
 
    .. ipython:: python
       :okwarning:
@@ -343,8 +346,7 @@ For data indexed by a :py:class:`~xarray.CFTimeIndex` xarray currently supports:
        da
        datetimeindex = da.indexes['time'].to_datetimeindex()
        da['time'] = datetimeindex
-       da.resample(time='Y').mean('time')
-
+
    However in this case one should use caution to only perform operations which
    do not depend on differences between dates (e.g. differentiation,
    interpolation, or upsampling with resample), as these could introduce subtle

doc/whats-new.rst

@@ -47,6 +47,10 @@ Enhancements
   report showing what exactly differs between the two objects (dimensions /
   coordinates / variables / attributes)  (:issue:`1507`).
   By `Benoit Bovy <https://github.com/benbovy>`_.
+- Resampling of standard and non-standard calendars indexed by
+  :py:class:`~xarray.CFTimeIndex` is now possible. (:issue:`2191`).
+  By `Jwen Fai Low <https://github.com/jwenfai>`_ and
+  `Spencer Clark <https://github.com/spencerkclark>`_.
 - Add ``tolerance`` option to ``resample()`` methods ``bfill``, ``pad``,
   ``nearest``. (:issue:`2695`)
   By `Hauke Schulz <https://github.com/observingClouds>`_.

xarray/coding/cftime_offsets.py

@@ -358,29 +358,41 @@ def rollback(self, date):
 class Day(BaseCFTimeOffset):
     _freq = 'D'
 
+    def as_timedelta(self):
+        return timedelta(days=self.n)
+
     def __apply__(self, other):
-        return other + timedelta(days=self.n)
+        return other + self.as_timedelta()
 
 
 class Hour(BaseCFTimeOffset):
     _freq = 'H'
 
+    def as_timedelta(self):
+        return timedelta(hours=self.n)
+
     def __apply__(self, other):
-        return other + timedelta(hours=self.n)
+        return other + self.as_timedelta()
 
 
 class Minute(BaseCFTimeOffset):
     _freq = 'T'
 
+    def as_timedelta(self):
+        return timedelta(minutes=self.n)
+
     def __apply__(self, other):
-        return other + timedelta(minutes=self.n)
+        return other + self.as_timedelta()
 
 
 class Second(BaseCFTimeOffset):
     _freq = 'S'
 
+    def as_timedelta(self):
+        return timedelta(seconds=self.n)
+
     def __apply__(self, other):
-        return other + timedelta(seconds=self.n)
+        return other + self.as_timedelta()
 
 
 _FREQUENCIES = {
@@ -427,6 +439,11 @@ def __apply__(self, other):
     _FREQUENCY_CONDITION)
 
 
+# pandas defines these offsets as "Tick" objects, which for instance have
+# distinct behavior from monthly or longer frequencies in resample.
+CFTIME_TICKS = (Day, Hour, Minute, Second)
+
+
 def to_offset(freq):
     """Convert a frequency string to the appropriate subclass of
     BaseCFTimeOffset."""

xarray/core/common.py

@@ -756,23 +756,16 @@ def resample(self, indexer=None, skipna=None, closed=None, label=None,
         dim_coord = self[dim]
 
         if isinstance(self.indexes[dim_name], CFTimeIndex):
-            raise NotImplementedError(
-                'Resample is currently not supported along a dimension '
-                'indexed by a CFTimeIndex.  For certain kinds of downsampling '
-                'it may be possible to work around this by converting your '
-                'time index to a DatetimeIndex using '
-                'CFTimeIndex.to_datetimeindex.  Use caution when doing this '
-                'however, because switching to a DatetimeIndex from a '
-                'CFTimeIndex with a non-standard calendar entails a change '
-                'in the calendar type, which could lead to subtle and silent '
-                'errors.'
-            )
-
+            from .resample_cftime import CFTimeGrouper
+            grouper = CFTimeGrouper(freq, closed, label, base, loffset)
+        else:
+            # TODO: to_offset() call required for pandas==0.19.2
+            grouper = pd.Grouper(freq=freq, closed=closed, label=label,
+                                 base=base,
+                                 loffset=pd.tseries.frequencies.to_offset(
+                                     loffset))
         group = DataArray(dim_coord, coords=dim_coord.coords,
                           dims=dim_coord.dims, name=RESAMPLE_DIM)
-        # TODO: to_offset() call required for pandas==0.19.2
-        grouper = pd.Grouper(freq=freq, closed=closed, label=label, base=base,
-                             loffset=pd.tseries.frequencies.to_offset(loffset))
         resampler = self._resample_cls(self, group=group, dim=dim_name,
                                        grouper=grouper,
                                        resample_dim=RESAMPLE_DIM)

xarray/core/groupby.py

@@ -258,12 +258,8 @@ def __init__(self, obj, group, squeeze=False, grouper=None, bins=None,
             if not index.is_monotonic:
                 # TODO: sort instead of raising an error
                 raise ValueError('index must be monotonic for resampling')
-            s = pd.Series(np.arange(index.size), index)
-            first_items = s.groupby(grouper).first()
-            _apply_loffset(grouper, first_items)
-            full_index = first_items.index
-            if first_items.isnull().any():
-                first_items = first_items.dropna()
+            full_index, first_items = self._get_index_and_items(
+                index, grouper)
             sbins = first_items.values.astype(np.int64)
             group_indices = ([slice(i, j)
                               for i, j in zip(sbins[:-1], sbins[1:])] +
@@ -310,6 +306,19 @@ def __len__(self):
     def __iter__(self):
         return zip(self._unique_coord.values, self._iter_grouped())
 
+    def _get_index_and_items(self, index, grouper):
+        from .resample_cftime import CFTimeGrouper
+        s = pd.Series(np.arange(index.size), index)
+        if isinstance(grouper, CFTimeGrouper):
+            first_items = grouper.first_items(index)
+        else:
+            first_items = s.groupby(grouper).first()
+            _apply_loffset(grouper, first_items)
+        full_index = first_items.index
+        if first_items.isnull().any():
+            first_items = first_items.dropna()
+        return full_index, first_items
+
     def _iter_grouped(self):
         """Iterate over each element in this group"""
         for indices in self._group_indices: