Dask arrays and DataArray coords that share name with dimensions

[jhamman]

First reported by @mrocklin in here.

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

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

[shoyer]

Case 1 looks like a bug.

Case 2 is intentional: we always evaluate coordinates corresponding to dimensions and store them as pandas.Index objects.

[jhamman]

@shoyer - I had forgotten about case 2. Thanks. I'll look into case 1.

[jhamman]

fix is up in #1685