issues: 279161550

I'm doing a reduction like mean on a dask-backed DataArray, and passing it to dask.compute

```python In [3]: from dask import compute ...: import numpy as np ...: import xarray as xr ...:

In [4]: data = xr.DataArray(np.random.random(size=(10, 2)), ...: dims=['samples', 'features']).chunk((5, 2)) ...:

In [5]: compute(data.mean(axis=0)) ```

```pytb

ValueError Traceback (most recent call last) <ipython-input-5-47605102585c> in <module>() ----> 1 compute(data.mean(axis=0))

~/Envs/dask-dev/lib/python3.6/site-packages/dask/dask/base.py in compute(args, kwargs) 334 results_iter = iter(results) 335 return tuple(a if f is None else f(next(results_iter), a) --> 336 for f, a in postcomputes) 337 338

~/Envs/dask-dev/lib/python3.6/site-packages/dask/dask/base.py in <genexpr>(.0) 334 results_iter = iter(results) 335 return tuple(a if f is None else f(next(results_iter), *a) --> 336 for f, a in postcomputes) 337 338

~/Envs/dask-dev/lib/python3.6/site-packages/xarray/xarray/core/dataarray.py in _dask_finalize(results, func, args, name) 607 @staticmethod 608 def _dask_finalize(results, func, args, name): --> 609 ds = func(results, *args) 610 variable = ds._variables.pop(_THIS_ARRAY) 611 coords = ds._variables

~/Envs/dask-dev/lib/python3.6/site-packages/xarray/xarray/core/dataset.py in _dask_postcompute(results, info, args) 551 func, args2 = v 552 r = results2.pop() --> 553 result = func(r, args2) 554 else: 555 result = v

~/Envs/dask-dev/lib/python3.6/site-packages/xarray/xarray/core/variable.py in _dask_finalize(results, array_func, array_args, dims, attrs, encoding) 389 results = {k: v for k, v in results.items() if k[0] == name} # cull 390 data = array_func(results, *array_args) --> 391 return Variable(dims, data, attrs=attrs, encoding=encoding) 392 393 @property

~/Envs/dask-dev/lib/python3.6/site-packages/xarray/xarray/core/variable.py in init(self, dims, data, attrs, encoding, fastpath) 267 """ 268 self._data = as_compatible_data(data, fastpath=fastpath) --> 269 self._dims = self._parse_dimensions(dims) 270 self._attrs = None 271 self._encoding = None

~/Envs/dask-dev/lib/python3.6/site-packages/xarray/xarray/core/variable.py in _parse_dimensions(self, dims) 431 raise ValueError('dimensions %s must have the same length as the ' 432 'number of data dimensions, ndim=%s' --> 433 % (dims, self.ndim)) 434 return dims 435

ValueError: dimensions ('features',) must have the same length as the number of data dimensions, ndim=0 ```

The expected output is the .compute version, which works correctly:

python In [7]: data.mean(axis=0).compute() Out[7]: <xarray.DataArray (features: 2)> array([0.535643, 0.459406]) Dimensions without coordinates: features

``` In [6]: xr.show_versions() INSTALLED VERSIONS ------------------ commit: c2b205f29467a4431baa80b5c07fe31bda67fbef python: 3.6.1.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: 0.10.0-5-gc2b205f pandas: 0.22.0.dev0+118.g4c6387520 numpy: 1.14.0.dev0+2995e6a scipy: 1.1.0.dev0+b6fd544 netCDF4: 1.3.1 h5netcdf: None Nio: None bottleneck: None cyordereddict: None dask: 0.16.0+15.gcbc62fbef matplotlib: 2.1.0 cartopy: None seaborn: 0.8.1 setuptools: 36.7.2 pip: 10.0.0.dev0 conda: None pytest: 3.2.3 IPython: 6.2.1 sphinx: 1.6.5 ```

Apologies if I'm doing something silly here, I don't know xarray :)