id,node_id,number,title,user,state,locked,assignee,milestone,comments,created_at,updated_at,closed_at,author_association,active_lock_reason,draft,pull_request,body,reactions,performed_via_github_app,state_reason,repo,type
269143043,MDU6SXNzdWUyNjkxNDMwNDM=,1663,ds.notnull() fails with AttributeError on pandas 0.21.0rc1,500246,closed,0,,,8,2017-10-27T15:19:33Z,2017-11-01T05:27:18Z,2017-11-01T05:27:18Z,CONTRIBUTOR,,,,"`xarray.Dataset({""A"": (""x"", arange(5))}).notnull()` fails with an 'AttributeError' when using `numpy` 1.13.3, `xarray` 0.9.6, and `pandas` 0.21.0rc1.  The `AttributeError` is raised by `pandas`; see below.

```
$ cat mwe.py
#!/usr/bin/env python3.6

import numpy
print(numpy.__version__)
import xarray
print(xarray.__version__)
import pandas
print(pandas.__version__)

xarray.Dataset({""A"": (""x"", numpy.arange(5))}).notnull()
$ ./mwe.py
1.13.3
0.9.6
0.21.0rc1
Traceback (most recent call last):
  File ""/dev/shm/gerrit/venv/stable-3.6/lib/python3.6/site-packages/xarray/core/ops.py"", line 193, in func
    return getattr(self, name)(*args, **kwargs)
AttributeError: 'Variable' object has no attribute 'notna'

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File ""./mwe.py"", line 10, in <module>
    xarray.Dataset({""A"": (""x"", numpy.arange(5))}).notnull()
  File ""/dev/shm/gerrit/venv/stable-3.6/lib/python3.6/site-packages/xarray/core/dataset.py"", line 2485, in func
    ds._variables[k] = f(self._variables[k], *args, **kwargs)
  File ""/dev/shm/gerrit/venv/stable-3.6/lib/python3.6/site-packages/xarray/core/ops.py"", line 195, in func
    return f(self, *args, **kwargs)
  File ""/dev/shm/gerrit/venv/stable-3.6/lib/python3.6/site-packages/pandas/core/dtypes/missing.py"", line 212, in notna
    res = isna(obj)
  File ""/dev/shm/gerrit/venv/stable-3.6/lib/python3.6/site-packages/pandas/core/dtypes/missing.py"", line 45, in isna
    return _isna(obj)
  File ""/dev/shm/gerrit/venv/stable-3.6/lib/python3.6/site-packages/pandas/core/dtypes/missing.py"", line 60, in _isna_new
    return obj._constructor(obj._data.isna(func=isna))
AttributeError: 'Variable' object has no attribute '_constructor'
```","{""url"": ""https://api.github.com/repos/pydata/xarray/issues/1663/reactions"", ""total_count"": 1, ""+1"": 1, ""-1"": 0, ""laugh"": 0, ""hooray"": 0, ""confused"": 0, ""heart"": 0, ""rocket"": 0, ""eyes"": 0}",,completed,13221727,issue