issue_comments

Has anyone found a workaround for this issue? I am able to recreate the ValueError via @Thomas-Z's example using xarray 2022.6.0.

Hello,

Using the same code sample:

``` 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") ```

It works on xarray 0.17 but does not work anymore with xarray 0.18 & 0.18.2.

This addition seems to be responsible (coming from this commit).

I don't know if this issue is still relevant for xarray But I encountered the same error with https://github.com/euroargodev/argopy and may be surprisingly, only with xarray version 0.16.1

I ran into this issue with a file from the GOES-17 lightning mapper.

A simple script to reproduce is: d=xr.open_dataset('OR_GLM-L2-LCFA_G17_s20191110644200_e20191110644370_c20191110645086.nc') d.load()

giving the error

``` ----> 1 d.load()

~/anaconda/envs/isatss/lib/python3.7/site-packages/xarray/core/dataset.py in load(self, **kwargs) 516 for k, v in self.variables.items(): 517 if k not in lazy_data: --> 518 v.load() 519 520 return self

~/anaconda/envs/isatss/lib/python3.7/site-packages/xarray/core/variable.py in load(self, kwargs) 325 self._data = as_compatible_data(self._data.compute(kwargs)) 326 elif not isinstance(self._data, np.ndarray): --> 327 self._data = np.asarray(self._data) 328 return self 329

~/anaconda/envs/isatss/lib/python3.7/site-packages/numpy/core/numeric.py in asarray(a, dtype, order) 499 500 """ --> 501 return array(a, dtype, copy=False, order=order) 502 503

~/anaconda/envs/isatss/lib/python3.7/site-packages/xarray/core/indexing.py in array(self, dtype) 624 625 def array(self, dtype=None): --> 626 self._ensure_cached() 627 return np.asarray(self.array, dtype=dtype) 628

~/anaconda/envs/isatss/lib/python3.7/site-packages/xarray/core/indexing.py in _ensure_cached(self) 621 def _ensure_cached(self): 622 if not isinstance(self.array, NumpyIndexingAdapter): --> 623 self.array = NumpyIndexingAdapter(np.asarray(self.array)) 624 625 def array(self, dtype=None):

~/anaconda/envs/isatss/lib/python3.7/site-packages/numpy/core/numeric.py in asarray(a, dtype, order) 499 500 """ --> 501 return array(a, dtype, copy=False, order=order) 502 503

~/anaconda/envs/isatss/lib/python3.7/site-packages/xarray/core/indexing.py in array(self, dtype) 602 603 def array(self, dtype=None): --> 604 return np.asarray(self.array, dtype=dtype) 605 606 def getitem(self, key):

~/anaconda/envs/isatss/lib/python3.7/site-packages/numpy/core/numeric.py in asarray(a, dtype, order) 499 500 """ --> 501 return array(a, dtype, copy=False, order=order) 502 503

~/anaconda/envs/isatss/lib/python3.7/site-packages/xarray/core/indexing.py in array(self, dtype) 508 def array(self, dtype=None): 509 array = as_indexable(self.array) --> 510 return np.asarray(array[self.key], dtype=None) 511 512 def transpose(self, order):

~/anaconda/envs/isatss/lib/python3.7/site-packages/numpy/core/numeric.py in asarray(a, dtype, order) 499 500 """ --> 501 return array(a, dtype, copy=False, order=order) 502 503

~/anaconda/envs/isatss/lib/python3.7/site-packages/xarray/coding/variables.py in array(self, dtype) 66 67 def array(self, dtype=None): ---> 68 return self.func(self.array) 69 70 def repr(self):

~/anaconda/envs/isatss/lib/python3.7/site-packages/xarray/coding/times.py in decode_cf_datetime(num_dates, units, calendar, use_cftime) 174 try: 175 dates = _decode_datetime_with_pandas(flat_num_dates, units, --> 176 calendar) 177 except (OutOfBoundsDatetime, OverflowError): 178 dates = _decode_datetime_with_cftime(

~/anaconda/envs/isatss/lib/python3.7/site-packages/xarray/coding/times.py in _decode_datetime_with_pandas(flat_num_dates, units, calendar) 139 warnings.filterwarnings('ignore', 'invalid value encountered', 140 RuntimeWarning) --> 141 pd.to_timedelta(flat_num_dates.min(), delta) + ref_date 142 pd.to_timedelta(flat_num_dates.max(), delta) + ref_date 143

~/anaconda/envs/isatss/lib/python3.7/site-packages/numpy/core/_methods.py in _amin(a, axis, out, keepdims, initial) 30 def _amin(a, axis=None, out=None, keepdims=False, 31 initial=_NoValue): ---> 32 return umr_minimum(a, axis, None, out, keepdims, initial) 33 34 def _sum(a, axis=None, dtype=None, out=None, keepdims=False,

ValueError: zero-size array to reduction operation minimum which has no identity ```

Versions: xarray = 0.12.1, pandas = 0.24.1

I'd still like to fix this but I have too much workload at the moment. However, I've noticed it's also triggered if the time axis is not empty, but we subselect data such that it becomes empty, then run ds.load().

I might try it out but most likely not before the end of the week.

Thanks for the report -- marking this as a bug.

If you are able to put together a PR to fix this that would be appreciated.