issues: 271599372

Code Sample, a copy-pastable example if possible

```python import numpy as np import xarray as xr

a = np.random.randn(4, 3) a[1, 1] = np.NaN da = xr.DataArray(a, dims=('y', 'x'), coords={'y':np.arange(4), 'x':np.arange(3)}) da.to_netcdf('test.nc')

with xr.open_dataarray('test.nc') as da: da.dropna(dim='x', how='any')

---

ValueError Traceback (most recent call last) <ipython-input-37-8d137cf3a813> in <module>() 8 9 with xr.open_dataarray('test.nc') as da: ---> 10 da.dropna(dim='x', how='any')

~/.pyvirtualenvs/py3/lib/python3.5/site-packages/xarray/core/dataarray.py in dropna(self, dim, how, thresh) 1158 DataArray 1159 """ -> 1160 ds = self._to_temp_dataset().dropna(dim, how=how, thresh=thresh) 1161 return self._from_temp_dataset(ds) 1162

~/.pyvirtualenvs/py3/lib/python3.5/site-packages/xarray/core/dataset.py in dropna(self, dim, how, thresh, subset) 2292 raise TypeError('must specify how or thresh') 2293 -> 2294 return self.isel(**{dim: mask}) 2295 2296 def fillna(self, value):

~/.pyvirtualenvs/py3/lib/python3.5/site-packages/xarray/core/dataset.py in isel(self, drop, **indexers) 1291 coord_names = set(variables).intersection(self._coord_names) 1292 selected = self._replace_vars_and_dims(variables, -> 1293 coord_names=coord_names) 1294 1295 # Extract coordinates from indexers

~/.pyvirtualenvs/py3/lib/python3.5/site-packages/xarray/core/dataset.py in _replace_vars_and_dims(self, variables, coord_names, dims, attrs, inplace) 598 """ 599 if dims is None: --> 600 dims = calculate_dimensions(variables) 601 if inplace: 602 self._dims = dims

~/.pyvirtualenvs/py3/lib/python3.5/site-packages/xarray/core/dataset.py in calculate_dimensions(variables) 111 raise ValueError('conflicting sizes for dimension %r: ' 112 'length %s on %r and length %s on %r' % --> 113 (dim, size, k, dims[dim], last_used[dim])) 114 return dims 115

ValueError: conflicting sizes for dimension 'y': length 2 on <this-array> and length 4 on 'y' ```

Problem description

See above. Note that the code runs when: - data is previously read into memory with load() - the DataArray is stored without coordinates (this is strange) - dropna is applied to 'y' instead of 'x'

Expected Output

This used to work in v0.9.6

Output of xr.show_versions()

INSTALLED VERSIONS ------------------ commit: None python: 3.5.2.final.0 python-bits: 64 OS: Linux OS-release: 4.10.0-38-generic machine: x86_64 processor: x86_64 byteorder: little LC_ALL: None LANG: en_US.UTF-8 LOCALE: en_US.UTF-8 xarray: 0.10.0rc1-5-g2a1d392 pandas: 0.21.0 numpy: 1.13.3 scipy: 1.0.0 netCDF4: 1.3.0 h5netcdf: None Nio: None bottleneck: 1.2.1 cyordereddict: None dask: 0.15.4 matplotlib: 2.1.0 cartopy: 0.15.1 seaborn: 0.8.1 setuptools: 36.6.0 pip: 9.0.1 conda: None pytest: 3.2.3 IPython: 6.2.1 sphinx: 1.6.5