CF: also decode time bounds when available

doc/whats-new.rst

@@ -39,6 +39,9 @@ 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>`_
+- Time bounds variables are now also decoded according to CF conventions
+  (:issue:`2565`).
+  By `Fabien Maussion <https://github.com/fmaussion>`_.
 
 Bug fixes
 ~~~~~~~~~

xarray/conventions.py

@@ -320,11 +320,39 @@ def decode_cf_variable(name, var, concat_characters=True, mask_and_scale=True,
     return Variable(dimensions, data, attributes, encoding=encoding)
 
 
+def _update_bounds_attributes(variables):
+    """Adds time attributes to time bounds variables.
+
+    Variables handling time bounds ("Cell boundaries" in the CF
+    conventions) do not necessarily carry the necessary attributes to be
+    decoded. This copies the attributes from the time variable to the
+    associated boundaries.
+
+    See Also:
+
+    http://cfconventions.org/Data/cf-conventions/cf-conventions-1.7/
+         cf-conventions.html#cell-boundaries
+
+    https://github.com/pydata/xarray/issues/2565
+    """
+
+    # For all time variables with bounds
+    for v in variables.values():
+        attrs = v.attrs
+        has_date_units = 'units' in attrs and 'since' in attrs['units']
+        if has_date_units and 'bounds' in attrs:
+            if attrs['bounds'] in variables:
+                to_update = variables[attrs['bounds']].attrs
+                to_update.setdefault('units', attrs['units'])
+                if 'calendar' in attrs:
+                    to_update.setdefault('calendar', attrs['calendar'])
+
+
 def decode_cf_variables(variables, attributes, concat_characters=True,
                         mask_and_scale=True, decode_times=True,
                         decode_coords=True, drop_variables=None):
     """
-    Decode a several CF encoded variables.
+    Decode several CF encoded variables.
 
     See: decode_cf_variable
     """
@@ -350,6 +378,10 @@ def stackable(dim):
         drop_variables = []
     drop_variables = set(drop_variables)
 
+    # Time bounds coordinates might miss the decoding attributes
+    if decode_times:
+        _update_bounds_attributes(variables)
+
     new_vars = OrderedDict()
     for k, v in iteritems(variables):
         if k in drop_variables:

xarray/tests/test_coding_times.py

@@ -10,6 +10,7 @@
 from xarray import DataArray, Variable, coding, decode_cf
 from xarray.coding.times import (_import_cftime, cftime_to_nptime,
                                  decode_cf_datetime, encode_cf_datetime)
+from xarray.conventions import _update_bounds_attributes
 from xarray.core.common import contains_cftime_datetimes
 
 from . import (
@@ -624,6 +625,41 @@ def test_decode_cf(calendar):
             assert ds.test.dtype == np.dtype('M8[ns]')
 
 
+def test_decode_cf_time_bounds():
+
+    da = DataArray(np.arange(6, dtype='int64').reshape((3, 2)),
+                   coords={'time': [1, 2, 3]},
+                   dims=('time', 'nbnd'), name='time_bnds')
+
+    attrs = {'units': 'days since 2001-01',
+             'calendar': 'standard',
+             'bounds': 'time_bnds'}
+
+    ds = da.to_dataset()
+    ds['time'].attrs.update(attrs)
+    _update_bounds_attributes(ds.variables)
+    assert ds.variables['time_bnds'].attrs == {'units': 'days since 2001-01',
+                                               'calendar': 'standard'}
+    dsc = decode_cf(ds)
+    assert dsc.time_bnds.dtype == np.dtype('M8[ns]')
+    dsc = decode_cf(ds, decode_times=False)
+    assert dsc.time_bnds.dtype == np.dtype('int64')
+
+    # Do not overwrite existing attrs
+    ds = da.to_dataset()
+    ds['time'].attrs.update(attrs)
+    bnd_attr = {'units': 'hours since 2001-01', 'calendar': 'noleap'}
+    ds['time_bnds'].attrs.update(bnd_attr)
+    _update_bounds_attributes(ds.variables)
+    assert ds.variables['time_bnds'].attrs == bnd_attr
+
+    # If bounds variable not available do not complain
+    ds = da.to_dataset()
+    ds['time'].attrs.update(attrs)
+    ds['time'].attrs['bounds'] = 'fake_var'
+    _update_bounds_attributes(ds.variables)
+
+
 @pytest.fixture(params=_ALL_CALENDARS)
 def calendar(request):
     return request.param