resample fails with datetime

[matt-long]

I am having trouble with esmlab resample. I am working with monthly data with standard calendar.

Here are a two failure modes.

1. Read dataset with decode_times=True

In [1]: ds = xr.open_dataset(file_in, drop_variables='date', decode_times=True)

In [2]: ds_ann = esmlab.resample(ds, freq='ann')

Error:

---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-15-0640091b3d29> in <module>
----> 1 ds_ann = esmlab.resample(ds, freq='ann')

/gpfs/u/home/mclong/codes/esmlab/esmlab/core.py in resample(dset, freq, weights, time_coord_name)
    723
    724     else:
--> 725         ds = dset.esmlab.set_time(time_coord_name=time_coord_name).compute_ann_mean(weights=weights)
    726
    727     new_history = f'\n{datetime.now()} esmlab.resample(<DATASET>, freq="{freq}")'

/gpfs/u/home/mclong/codes/esmlab/esmlab/core.py in set_time(self, time_coord_name, year_offset)
    319                     self._ds[self.tb_name],
    320                     units=self.time_attrs['units'],--> 321                     calendar=self.time_attrs['calendar'],
    322                 )
    323                 self.time_bound.data = tb_data

cftime/_cftime.pyx in cftime._cftime.date2num()
TypeError: ufunc subtract cannot use operands with types dtype('<M8[ns]') and dtype('O')

2. Read dataset with decode_times=False

In [1]: file_in = '/glade/work/mclong/pco2-landschutzer/downloaded/MPI-SOM_FFN_GCB2018.nc'

In [2]: ds = xr.open_dataset(file_in, drop_variables='date', decode_times=False)

In [3]: ds_ann = esmlab.resample(ds, freq='ann')

Error:

---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-11-0640091b3d29> in <module>
----> 1 ds_ann = esmlab.resample(ds, freq='ann')

/gpfs/u/home/mclong/codes/esmlab/esmlab/core.py in resample(dset, freq, weights, time_coord_name)
    723
    724     else:
--> 725         ds = dset.esmlab.set_time(time_coord_name=time_coord_name).compute_ann_mean(weights=weights)
    726
    727     new_history = f'\n{datetime.now()} esmlab.resample(<DATASET>, freq="{freq}")'

/gpfs/u/home/mclong/codes/esmlab/esmlab/core.py in set_time(self, time_coord_name, year_offset)
    322                 )
    323                 self.time_bound.data = tb_data
--> 324         self.setup()
    325         return self
    326

/gpfs/u/home/mclong/codes/esmlab/esmlab/core.py in setup(self)
    327     def setup(self):
    328         self.get_variables()
--> 329         self.compute_time()
    330         self.get_original_metadata()
    331

/gpfs/u/home/mclong/codes/esmlab/esmlab/core.py in compute_time(self)
     80
     81         if self.time_bound is not None:
---> 82             groupby_coord = self.get_time_decoded(midpoint=True)
     83
     84         else:

/gpfs/u/home/mclong/codes/esmlab/esmlab/core.py in get_time_decoded(self, midpoint)
    181         time_out.data = xr.CFTimeIndex(
    182             cftime.num2date(
--> 183                 time_data, units=self.time_attrs['units'], calendar=self.time_attrs['calendar']
    184             )
    185         )

/glade/work/mclong/miniconda3/envs/dev/lib/python3.7/site-packages/xarray/coding/cftimeindex.py in __new__(cls, data, name)
    235         result = object.__new__(cls)
    236         result._data = np.array(data, dtype='O')
--> 237         assert_all_valid_date_type(result._data)
    238         result.name = name
    239         return result

/glade/work/mclong/miniconda3/envs/dev/lib/python3.7/site-packages/xarray/coding/cftimeindex.py in assert_all_valid_date_type(data)
    191             raise TypeError(
    192                 'CFTimeIndex requires cftime.datetime '
--> 193                 'objects. Got object of {}.'.format(date_type))
    194         if not all(isinstance(value, date_type) for value in data):
    195             raise TypeError(

TypeError: CFTimeIndex requires cftime.datetime objects. Got object of <class 'cftime._cftime.real_datetime'>.

Workaround:

In [1]: ds = xr.open_dataset(file_in, drop_variables='date', decode_times=False)

In [2]: ds['time'] = cftime.num2date(ds.time, units=ds.time.units, 
    ...:                              only_use_cftime_datetimes=True)

From cftime documentation:
only_use_cftime_datetimes: if False (default), datetime.datetime objects are returned from num2date where possible; if True dates which subclass cftime.datetime are returned for all calendars.

Conclusion: esmlab is not datetime friendly.

[andersy005]

Partially fixed by #122