Add dayofyear and dayofweek accessors (#2599)

doc/time-series.rst

@@ -263,14 +263,16 @@ For data indexed by a :py:class:`~xarray.CFTimeIndex` xarray currently supports:
    da.sel(time=slice('0001-05', '0002-02'))
 
 - Access of basic datetime components via the ``dt`` accessor (in this case
-  just "year", "month", "day", "hour", "minute", "second", "microsecond", and
-  "season"): 
+  just "year", "month", "day", "hour", "minute", "second", "microsecond",
+  "season", "dayofyear", and "dayofweek"): 
 
 .. ipython:: python
 
    da.time.dt.year
    da.time.dt.month
    da.time.dt.season
+   da.time.dt.dayofyear
+   da.time.dt.dayofweek
 
 - Group-by operations based on datetime accessor attributes (e.g. by month of
   the year):

doc/whats-new.rst

@@ -41,6 +41,9 @@ Enhancements
 - :py:class:`CFTimeIndex` uses slicing for string indexing when possible (like
   :py:class:`pandas.DatetimeIndex`), which avoids unnecessary copies.
   By `Stephan Hoyer <https://github.com/shoyer>`_
+- Like :py:class:`pandas.DatetimeIndex`, :py:class:`CFTimeIndex` now supports
+  "dayofyear" and "dayofweek" accessors (:issue:`2597`).  By `Spencer Clark
+  <https://github.com/spencerkclark>`_. 
 
 Bug fixes
 ~~~~~~~~~

xarray/coding/cftimeindex.py

@@ -207,6 +207,9 @@ class CFTimeIndex(pd.Index):
     second = _field_accessor('second', 'The seconds of the datetime')
     microsecond = _field_accessor('microsecond',
                                   'The microseconds of the datetime')
+    dayofyear = _field_accessor('dayofyr',
+                                'The ordinal day of year of the datetime')
+    dayofweek = _field_accessor('dayofwk', 'The day of week of the datetime')
     date_type = property(get_date_type)
 
     def __new__(cls, data, name=None):

xarray/tests/test_accessors.py

@@ -158,7 +158,8 @@ def times_3d(times):
 
 
 @pytest.mark.skipif(not has_cftime, reason='cftime not installed')
-@pytest.mark.parametrize('field', ['year', 'month', 'day', 'hour'])
+@pytest.mark.parametrize('field', ['year', 'month', 'day', 'hour',
+                                   'dayofyear', 'dayofweek'])
 def test_field_access(data, field):
     result = getattr(data.time.dt, field)
     expected = xr.DataArray(
@@ -170,7 +171,8 @@ def test_field_access(data, field):
 
 @pytest.mark.skipif(not has_dask, reason='dask not installed')
 @pytest.mark.skipif(not has_cftime, reason='cftime not installed')
-@pytest.mark.parametrize('field', ['year', 'month', 'day', 'hour'])
+@pytest.mark.parametrize('field', ['year', 'month', 'day', 'hour',
+                                   'dayofyear', 'dayofweek'])
 def test_dask_field_access_1d(data, field):
     import dask.array as da
 
@@ -186,7 +188,8 @@ def test_dask_field_access_1d(data, field):
 
 @pytest.mark.skipif(not has_dask, reason='dask not installed')
 @pytest.mark.skipif(not has_cftime, reason='cftime not installed')
-@pytest.mark.parametrize('field', ['year', 'month', 'day', 'hour'])
+@pytest.mark.parametrize('field', ['year', 'month', 'day', 'hour', 'dayofyear',
+                                   'dayofweek'])
 def test_dask_field_access(times_3d, data, field):
     import dask.array as da
 

xarray/tests/test_cftimeindex.py

@@ -175,6 +175,20 @@ def test_cftimeindex_field_accessors(index, field, expected):
     assert_array_equal(result, expected)
 
 
+@pytest.mark.skipif(not has_cftime, reason='cftime not installed')
+def test_cftimeindex_dayofyear_accessor(index):
+    result = index.dayofyear
+    expected = [date.dayofyr for date in index]
+    assert_array_equal(result, expected)
+
+
+@pytest.mark.skipif(not has_cftime, reason='cftime not installed')
+def test_cftimeindex_dayofweek_accessor(index):
+    result = index.dayofweek
+    expected = [date.dayofwk for date in index]
+    assert_array_equal(result, expected)
+
+
 @pytest.mark.skipif(not has_cftime, reason='cftime not installed')
 @pytest.mark.parametrize(('string', 'date_args', 'reso'), [
     ('1999', (1999, 1, 1), 'year'),