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
301888608,MDU6SXNzdWUzMDE4ODg2MDg=,1954,ds.to_zarr error on creation,9453271,closed,0,,,8,2018-03-02T20:05:32Z,2018-03-06T19:00:28Z,2018-03-06T19:00:28Z,CONTRIBUTOR,,,,"Creating just a sample dataset:
```
data    = xr.DataArray(np.random.randn(2, 3), coords={'x': ['a', 'b']}, dims=('x', 'y'))
ds_test = xr.Dataset({'foo': data, 'bar': ('x', [1, 2]), 'baz': np.pi})
print(ds_test)
ds_test.to_zarr(store='foo.zarr', mode='w')
```

Writing this locally. This generates a .zgroup and .zattrs file, but then fails out with, AttributeError: 'Attributes' object has no attribute 'put'. Tried on a couple machines and I get the same error each time.

Run on:

MacOS 10.13.3
CentOS Linux release 7.4.1708 (Core)

```

---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)
<ipython-input-38-d75fe1649815> in <module>()
     11 ).chunk({'y': 50, 'x': 40})
     12 
---> 13 ds.to_zarr(store='/home/kyp/foo.zarr', mode='w')

~/.conda/envs/kai/lib/python3.6/site-packages/xarray/core/dataset.py in to_zarr(self, store, mode, synchronizer, group, encoding)
   1165         from ..backends.api import to_zarr
   1166         return to_zarr(self, store=store, mode=mode, synchronizer=synchronizer,
-> 1167                        group=group, encoding=encoding)
   1168 
   1169     def __unicode__(self):

~/.conda/envs/kai/lib/python3.6/site-packages/xarray/backends/api.py in to_zarr(dataset, store, mode, synchronizer, group, encoding)
    752     # I think zarr stores should always be sync'd immediately
    753     # TODO: figure out how to properly handle unlimited_dims
--> 754     dataset.dump_to_store(store, sync=True, encoding=encoding)
    755     return store

~/.conda/envs/kai/lib/python3.6/site-packages/xarray/core/dataset.py in dump_to_store(self, store, encoder, sync, encoding, unlimited_dims)
   1068 
   1069         store.store(variables, attrs, check_encoding,
-> 1070                     unlimited_dims=unlimited_dims)
   1071         if sync:
   1072             store.sync()

~/.conda/envs/kai/lib/python3.6/site-packages/xarray/backends/zarr.py in store(self, variables, attributes, *args, **kwargs)
    378     def store(self, variables, attributes, *args, **kwargs):
    379         AbstractWritableDataStore.store(self, variables, attributes,
--> 380                                         *args, **kwargs)
    381 
    382 

~/.conda/envs/kai/lib/python3.6/site-packages/xarray/backends/common.py in store(self, variables, attributes, check_encoding_set, unlimited_dims)
    275         variables, attributes = self.encode(variables, attributes)
    276 
--> 277         self.set_attributes(attributes)
    278         self.set_dimensions(variables, unlimited_dims=unlimited_dims)
    279         self.set_variables(variables, check_encoding_set,

~/.conda/envs/kai/lib/python3.6/site-packages/xarray/backends/zarr.py in set_attributes(self, attributes)
    341 
    342     def set_attributes(self, attributes):
--> 343         self.ds.attrs.put(attributes)
    344 
    345     def encode_variable(self, variable):

AttributeError: 'Attributes' object has no attribute 'put'
```
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