issues: 270808895

First reported by @mrocklin in here.

```python In [1]: import xarray

In [2]: import dask.array as da

In [3]: coord = da.arange(8, chunks=(4,)) ...: data = da.random.random((8, 8), chunks=(4, 4)) + 1 ...: array = xarray.DataArray(data, ...: coords={'x': coord, 'y': coord}, ...: dims=['x', 'y']) ...:

---

ValueError Traceback (most recent call last) <ipython-input-3-b90a33ebf436> in <module>() 3 array = xarray.DataArray(data, 4 coords={'x': coord, 'y': coord}, ----> 5 dims=['x', 'y'])

/home/mrocklin/workspace/xarray/xarray/core/dataarray.py in init(self, data, coords, dims, name, attrs, encoding, fastpath) 227 228 data = as_compatible_data(data) --> 229 coords, dims = _infer_coords_and_dims(data.shape, coords, dims) 230 variable = Variable(dims, data, attrs, encoding, fastpath=True) 231

/home/mrocklin/workspace/xarray/xarray/core/dataarray.py in _infer_coords_and_dims(shape, coords, dims) 68 if utils.is_dict_like(coords): 69 for k, v in coords.items(): ---> 70 new_coords[k] = as_variable(v, name=k) 71 elif coords is not None: 72 for dim, coord in zip(dims, coords):

/home/mrocklin/workspace/xarray/xarray/core/variable.py in as_variable(obj, name) 94 '{}'.format(obj)) 95 elif utils.is_scalar(obj): ---> 96 obj = Variable([], obj) 97 elif getattr(obj, 'name', None) is not None: 98 obj = Variable(obj.name, obj)

/home/mrocklin/workspace/xarray/xarray/core/variable.py in init(self, dims, data, attrs, encoding, fastpath) 275 """ 276 self._data = as_compatible_data(data, fastpath=fastpath) --> 277 self._dims = self._parse_dimensions(dims) 278 self._attrs = None 279 self._encoding = None

/home/mrocklin/workspace/xarray/xarray/core/variable.py in _parse_dimensions(self, dims) 439 raise ValueError('dimensions %s must have the same length as the ' 440 'number of data dimensions, ndim=%s' --> 441 % (dims, self.ndim)) 442 return dims 443

ValueError: dimensions () must have the same length as the number of data dimensions, ndim=1 ```

or a similiar setup that computes the coordinates imediately

```Python In [18]: x = xr.Variable('x', da.arange(8, chunks=(4,))) ...: y = xr.Variable('y', da.arange(8, chunks=(4,)) * 2) ...: data = da.random.random((8, 8), chunks=(4, 4)) + 1 ...: array = xr.DataArray(data, ...: dims=['x', 'y']) ...: array.coords['x'] = x ...: array.coords['y'] = y ...:

In [19]: array Out[19]: <xarray.DataArray 'add-7d8ed340e5dd8fe107ea681573c72e87' (x: 8, y: 8)> dask.array<shape=(8, 8), dtype=float64, chunksize=(4, 4)> Coordinates: * x (x) int64 0 1 2 3 4 5 6 7 * y (y) int64 0 2 4 6 8 10 12 14 ```

Problem description

I think we have two, possiblely related problems with using dask arrays as DataArray coordinates.

1. As the first snippet shows, the constructor fails when coordinates are specified as raw dask arrays. This does not occur when coord is a numpy array.
2. When coordinates are specified as dask arrays via the coords attribute, they are computed immediately.

Expected Output

Output of xr.show_versions()

In [23]: xr.show_versions() INSTALLED VERSIONS ------------------ commit: None python: 3.6.0.final.0 python-bits: 64 OS: Darwin OS-release: 16.7.0 machine: x86_64 processor: i386 byteorder: little LC_ALL: None LANG: en_US.UTF-8 LOCALE: en_US.UTF-8 xarray: 0.10.0rc1 pandas: 0.20.3 numpy: 1.13.1 scipy: 0.19.1 netCDF4: None h5netcdf: 0.3.1 Nio: None bottleneck: 1.2.0 cyordereddict: None dask: 0.15.4 matplotlib: 2.0.2 cartopy: 0.15.1 seaborn: 0.8.1 setuptools: 36.6.0 pip: 9.0.1 conda: 4.3.29 pytest: 3.0.5 IPython: 5.1.0 sphinx: 1.5.1