id,node_id,number,title,user,state,locked,assignee,milestone,comments,created_at,updated_at,closed_at,author_association,active_lock_reason,draft,pull_request,body,reactions,performed_via_github_app,state_reason,repo,type
217216935,MDU6SXNzdWUyMTcyMTY5MzU=,1329,Cannot open NetCDF file if dimension with time coordinate has length 0 (`ValueError` when decoding CF datetime),500246,closed,0,,,7,2017-03-27T11:33:07Z,2022-08-10T17:25:20Z,2022-08-10T17:25:20Z,CONTRIBUTOR,,,,"If a data set has a zero-sized coordinate that is a time index, reading fails.  A `ValueError` is triggered when xarray tries to decode the array, as shown below:

```
$ cat mwe.py
#!/usr/bin/env python

import numpy
import xarray

ds = xarray.Dataset(
    {""a"": (""x"", [])},
    coords={""x"": numpy.zeros(shape=0, dtype=""M8[ns]"")})

ds.to_netcdf(""/tmp/test.nc"")

xarray.open_dataset(""/tmp/test.nc"")
$ ./mwe.py
Traceback (most recent call last):
  File ""./mwe.py"", line 12, in <module>
    xarray.open_dataset(""/tmp/test.nc"")
  File ""/dev/shm/gerrit/venv/stable-3.5/lib/python3.5/site-packages/xarray/backends/api.py"", line 302, in open_dataset
    return maybe_decode_store(store, lock)
  File ""/dev/shm/gerrit/venv/stable-3.5/lib/python3.5/site-packages/xarray/backends/api.py"", line 223, in maybe_decode_store
    drop_variables=drop_variables)
  File ""/dev/shm/gerrit/venv/stable-3.5/lib/python3.5/site-packages/xarray/conventions.py"", line 952, in decode_cf
    ds = Dataset(vars, attrs=attrs)
  File ""/dev/shm/gerrit/venv/stable-3.5/lib/python3.5/site-packages/xarray/core/dataset.py"", line 358, in __init__
    self._set_init_vars_and_dims(data_vars, coords, compat)
  File ""/dev/shm/gerrit/venv/stable-3.5/lib/python3.5/site-packages/xarray/core/dataset.py"", line 373, in _set_init_vars_and_dims
    data_vars, coords, compat=compat)
  File ""/dev/shm/gerrit/venv/stable-3.5/lib/python3.5/site-packages/xarray/core/merge.py"", line 365, in merge_data_and_coords
    return merge_core(objs, compat, join, explicit_coords=explicit_coords)
  File ""/dev/shm/gerrit/venv/stable-3.5/lib/python3.5/site-packages/xarray/core/merge.py"", line 413, in merge_core
    expanded = expand_variable_dicts(aligned)
  File ""/dev/shm/gerrit/venv/stable-3.5/lib/python3.5/site-packages/xarray/core/merge.py"", line 213, in expand_variable_dicts
    var = as_variable(var, name=name)
  File ""/dev/shm/gerrit/venv/stable-3.5/lib/python3.5/site-packages/xarray/core/variable.py"", line 83, in as_variable
    obj = obj.to_index_variable()
  File ""/dev/shm/gerrit/venv/stable-3.5/lib/python3.5/site-packages/xarray/core/variable.py"", line 322, in to_index_variable
    encoding=self._encoding, fastpath=True)
  File ""/dev/shm/gerrit/venv/stable-3.5/lib/python3.5/site-packages/xarray/core/variable.py"", line 1173, in __init__
    self._data = PandasIndexAdapter(self._data)
  File ""/dev/shm/gerrit/venv/stable-3.5/lib/python3.5/site-packages/xarray/core/indexing.py"", line 497, in __init__
    self.array = utils.safe_cast_to_index(array)
  File ""/dev/shm/gerrit/venv/stable-3.5/lib/python3.5/site-packages/xarray/core/utils.py"", line 57, in safe_cast_to_index
    index = pd.Index(np.asarray(array), **kwargs)
  File ""/dev/shm/gerrit/venv/stable-3.5/lib/python3.5/site-packages/numpy/core/numeric.py"", line 531, in asarray
    return array(a, dtype, copy=False, order=order)
  File ""/dev/shm/gerrit/venv/stable-3.5/lib/python3.5/site-packages/xarray/core/indexing.py"", line 373, in __array__
    return np.asarray(array[self.key], dtype=None)
  File ""/dev/shm/gerrit/venv/stable-3.5/lib/python3.5/site-packages/xarray/conventions.py"", line 408, in __getitem__
    calendar=self.calendar)
  File ""/dev/shm/gerrit/venv/stable-3.5/lib/python3.5/site-packages/xarray/conventions.py"", line 151, in decode_cf_datetime
    pd.to_timedelta(flat_num_dates.min(), delta) + ref_date
  File ""/dev/shm/gerrit/venv/stable-3.5/lib/python3.5/site-packages/numpy/core/_methods.py"", line 29, in _amin
    return umr_minimum(a, axis, None, out, keepdims)
ValueError: zero-size array to reduction operation minimum which has no identity
$ ncdump /tmp/test.nc 
netcdf test {
dimensions:
        x = UNLIMITED ; // (0 currently)
variables:
        double a(x) ;
                a:_FillValue = NaN ;
        int64 x(x) ;
                x:units = ""days since 1970-01-01 00:00:00"" ;
                x:calendar = ""proleptic_gregorian"" ;

// global attributes:
                :_NCProperties = ""version=1|netcdflibversion=4.4.1|hdf5libversion=1.8.18"" ;
data:
}
```
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