`.sel()` failures when using latest cftime release (v1.0.4)

[andersy005]

I just updated to the latest cftime release, and all of a sudden sel() appears to be broken:

In [1]: import xarray as xr                                                                                                                        

In [2]: import cftime                                                                                                                              

In [3]: ds = xr.tutorial.load_dataset('rasm')                                                                                                      

In [4]: ds                                                                                                                                         
Out[4]: 
<xarray.Dataset>
Dimensions:  (time: 36, x: 275, y: 205)
Coordinates:
  * time     (time) object 1980-09-16 12:00:00 ... 1983-08-17 00:00:00
    xc       (y, x) float64 189.2 189.4 189.6 189.7 ... 17.65 17.4 17.15 16.91
    yc       (y, x) float64 16.53 16.78 17.02 17.27 ... 28.26 28.01 27.76 27.51
Dimensions without coordinates: x, y
Data variables:
    Tair     (time, y, x) float64 nan nan nan nan nan ... 29.8 28.66 28.19 28.21
Attributes:
    title:                     /workspace/jhamman/processed/R1002RBRxaaa01a/l...
    institution:               U.W.
    source:                    RACM R1002RBRxaaa01a
    output_frequency:          daily
    output_mode:               averaged
    convention:                CF-1.4
    references:                Based on the initial model of Liang et al., 19...
    comment:                   Output from the Variable Infiltration Capacity...
    nco_openmp_thread_number:  1
    NCO:                       "4.6.0"
    history:                   Tue Dec 27 14:15:22 2016: ncatted -a dimension...

In [5]: ds.sel(time=slice("1980", "1982"))                                                                                                         
---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
<ipython-input-5-2c26e36a673a> in <module>
----> 1 ds.sel(time=slice("1980", "1982"))

~/opt/miniconda3/envs/intake-esm-dev/lib/python3.7/site-packages/xarray/core/dataset.py in sel(self, indexers, method, tolerance, drop, **indexers_kwargs)
   1998         indexers = either_dict_or_kwargs(indexers, indexers_kwargs, "sel")
   1999         pos_indexers, new_indexes = remap_label_indexers(
-> 2000             self, indexers=indexers, method=method, tolerance=tolerance
   2001         )
   2002         result = self.isel(indexers=pos_indexers, drop=drop)

~/opt/miniconda3/envs/intake-esm-dev/lib/python3.7/site-packages/xarray/core/coordinates.py in remap_label_indexers(obj, indexers, method, tolerance, **indexers_kwargs)
    390 
    391     pos_indexers, new_indexes = indexing.remap_label_indexers(
--> 392         obj, v_indexers, method=method, tolerance=tolerance
    393     )
    394     # attach indexer's coordinate to pos_indexers

~/opt/miniconda3/envs/intake-esm-dev/lib/python3.7/site-packages/xarray/core/indexing.py in remap_label_indexers(data_obj, indexers, method, tolerance)
    259             coords_dtype = data_obj.coords[dim].dtype
    260             label = maybe_cast_to_coords_dtype(label, coords_dtype)
--> 261             idxr, new_idx = convert_label_indexer(index, label, dim, method, tolerance)
    262             pos_indexers[dim] = idxr
    263             if new_idx is not None:

~/opt/miniconda3/envs/intake-esm-dev/lib/python3.7/site-packages/xarray/core/indexing.py in convert_label_indexer(index, label, index_name, method, tolerance)
    123             _sanitize_slice_element(label.start),
    124             _sanitize_slice_element(label.stop),
--> 125             _sanitize_slice_element(label.step),
    126         )
    127         if not isinstance(indexer, slice):

~/opt/miniconda3/envs/intake-esm-dev/lib/python3.7/site-packages/pandas/core/indexes/base.py in slice_indexer(self, start, end, step, kind)
   5032         slice(1, 3)
   5033         """
-> 5034         start_slice, end_slice = self.slice_locs(start, end, step=step, kind=kind)
   5035 
   5036         # return a slice

~/opt/miniconda3/envs/intake-esm-dev/lib/python3.7/site-packages/pandas/core/indexes/base.py in slice_locs(self, start, end, step, kind)
   5246         start_slice = None
   5247         if start is not None:
-> 5248             start_slice = self.get_slice_bound(start, "left", kind)
   5249         if start_slice is None:
   5250             start_slice = 0

~/opt/miniconda3/envs/intake-esm-dev/lib/python3.7/site-packages/pandas/core/indexes/base.py in get_slice_bound(self, label, side, kind)
   5158         # For datetime indices label may be a string that has to be converted
   5159         # to datetime boundary according to its resolution.
-> 5160         label = self._maybe_cast_slice_bound(label, side, kind)
   5161 
   5162         # we need to look up the label

~/opt/miniconda3/envs/intake-esm-dev/lib/python3.7/site-packages/xarray/coding/cftimeindex.py in _maybe_cast_slice_bound(self, label, side, kind)
    336         pandas.tseries.index.DatetimeIndex._maybe_cast_slice_bound"""
    337         if isinstance(label, str):
--> 338             parsed, resolution = _parse_iso8601_with_reso(self.date_type, label)
    339             start, end = _parsed_string_to_bounds(self.date_type, resolution, parsed)
    340             if self.is_monotonic_decreasing and len(self) > 1:

~/opt/miniconda3/envs/intake-esm-dev/lib/python3.7/site-packages/xarray/coding/cftimeindex.py in _parse_iso8601_with_reso(date_type, timestr)
    114     # 1.0.3.4.
    115     replace["dayofwk"] = -1
--> 116     return default.replace(**replace), resolution
    117 
    118 

cftime/_cftime.pyx in cftime._cftime.datetime.replace()

ValueError: Replacing the dayofyr or dayofwk of a datetime is not supported.

Output of xr.show_versions()

# Paste the output here xr.show_versions() here

In [6]: xr.show_versions()                                                                                                                         

INSTALLED VERSIONS
------------------
commit: None
python: 3.7.3 | packaged by conda-forge | (default, Jul  1 2019, 14:38:56) 
[Clang 4.0.1 (tags/RELEASE_401/final)]
python-bits: 64
OS: Darwin
OS-release: 18.7.0
machine: x86_64
processor: i386
byteorder: little
LC_ALL: None
LANG: en_US.UTF-8
LOCALE: en_US.UTF-8
libhdf5: 1.10.5
libnetcdf: 4.7.1

xarray: 0.14.0
pandas: 0.25.2
numpy: 1.17.2
scipy: None
netCDF4: 1.5.1.2
pydap: None
h5netcdf: None
h5py: None
Nio: None
zarr: 2.3.2
cftime: 1.0.4
nc_time_axis: None
PseudoNetCDF: None
rasterio: None
cfgrib: None
iris: None
bottleneck: None
dask: 2.2.0
distributed: 2.5.1
matplotlib: None
cartopy: None
seaborn: None
numbagg: None
setuptools: 41.4.0
pip: 19.2.1
conda: None
pytest: 5.0.1
IPython: 7.8.0
sphinx: 2.1.2

Expected Output

I can confirm that everything works just fine with an older version of cftime:

In [4]: ds.sel(time=slice("1980", "1982"))                                                                                                         
Out[4]: 
<xarray.Dataset>
Dimensions:  (time: 28, x: 275, y: 205)
Coordinates:
  * time     (time) object 1980-09-16 12:00:00 ... 1982-12-17 00:00:00
    xc       (y, x) float64 ...
    yc       (y, x) float64 ...
Dimensions without coordinates: x, y
Data variables:
    Tair     (time, y, x) float64 ...
Attributes:
    title:                     /workspace/jhamman/processed/R1002RBRxaaa01a/l...
    institution:               U.W.
    source:                    RACM R1002RBRxaaa01a
    output_frequency:          daily
    output_mode:               averaged
    convention:                CF-1.4
    references:                Based on the initial model of Liang et al., 19...
    comment:                   Output from the Variable Infiltration Capacity...
    nco_openmp_thread_number:  1
    NCO:                       "4.6.0"
    history:                   Tue Dec 27 14:15:22 2016: ncatted -a dimension...
In [5]: import cftime                                                                                                                              

In [6]: cftime.__version__                                                                                                                         
Out[6]: '1.0.3.4'

In [7]: xr.__version__                                                                                                                             
Out[7]: '0.14.0'

[spencerkclark]

Thanks for noting this @andersy005; indeed I knew this time would come at some point :). See #2593 (comment). Feel free to have a go at fixing this yourself if you feel inclined. If not I'll try and submit a fix in a few days.

[dcherian]

we should add cftime git tip to the upstream-dev config.

[spencerkclark]

we should add cftime git tip to the upstream-dev config.

Yes, +1 for testing against cftime master in a CI build.