MCVE Code Sample

```python import gcsfs from gcsfs import mapping import xarray as xr

gcs_bucket_name = '<bucket_name_here>' gcsmap = mapping.GCSMap(gcs_bucket_name) ds.to_zarr(store=gcsmap, mode='a', consolidated=True, compute=False, append_dim='time')

```

Expected Output

Zarr store in bucket appended on time dimension.

Problem Description

```python ValueError Traceback (most recent call last) <ipython-input-103-cb22683dde66> in <module> ----> 1 ds.to_zarr(store=gcsmap, mode='a', consolidated=True, compute=False, append_dim='time') ~/anaconda3/envs/intake/lib/python3.7/site-packages/xarray/core/dataset.py in to_zarr(self, store, mode, synchronizer, group, encoding, compute, consolidated, append_dim) 1431 return to_zarr(self, store=store, mode=mode, synchronizer=synchronizer, 1432 group=group, encoding=encoding, compute=compute, -> 1433 consolidated=consolidated, append_dim=append_dim) 1434 1435 def __repr__(self): ~/anaconda3/envs/intake/lib/python3.7/site-packages/xarray/backends/api.py in to_zarr(dataset, store, mode, synchronizer, group, encoding, compute, consolidated, append_dim) 1090 group=group, 1091 consolidated=consolidated, -> 1092 encoding=encoding) 1093 1094 zstore = backends.ZarrStore.open_group(store=store, mode=mode, ~/anaconda3/envs/intake/lib/python3.7/site-packages/xarray/backends/api.py in _validate_append_dim_and_encoding(ds_to_append, store, append_dim, encoding, **open_kwargs) 1053 if append_dim not in ds.dims: 1054 raise ValueError( -> 1055 "{} not a valid dimension in the Dataset".format(append_dim) 1056 ) 1057 for data_var in ds_to_append: ValueError: time not a valid dimension in the Dataset ```

Line 1296 in xarray.backends.api

```python def to_zarr( dataset, store=None, mode=None, synchronizer=None, group=None, encoding=None, compute=True, consolidated=False, append_dim=None, ): """This function creates an appropriate datastore for writing a dataset to a zarr ztore See `Dataset.to_zarr` for full API docs. """ if isinstance(store, Path): store = str(store) ```

The mutable mapping created by gcsfs gets turned into a string (IE '<fsspec.mapping.FSMap object at 0x10fae1278>') which fails to read in the zarr store correctly. This fails silently and then the ValueError for line 1055 gets tripped because there are no dims to compare the append_dim too.

This was tested by running

```python import gcsfs from gcsfs import mapping import xarray as xr from xarray import backends

gcs_bucket_name = '<bucket_name_here>' gcsmap = mapping.GCSMap(gcs_bucket_name)

same way it is called in _validate_append_dim_and_encoding minus str gcsmap

ds = backends.zarr.open_zarr(gcsmap) ```

and

```python import gcsfs from gcsfs import mapping import xarray as xr from xarray import backends

gcs_bucket_name = '<bucket_name_here>' gcsmap = mapping.GCSMap(gcs_bucket_name)

same way it is called in _validate_append_dim_and_encoding

ds = backends.zarr.open_zarr(str(gcsmap)) ```

The top example reads the data in correctly while the second one fails with:

```python ValueError Traceback (most recent call last) <ipython-input-102-52e941f0a8f2> in <module> ----> 1 ds = backends.zarr.open_zarr(str(gcsmap)) ~/anaconda3/envs/intake/lib/python3.7/site-packages/xarray/backends/zarr.py in open_zarr(store, group, synchronizer, chunks, decode_cf, mask_and_scale, decode_times, concat_characters, decode_coords, drop_variables, consolidated, overwrite_encoded_chunks, **kwargs) 554 zarr_store = ZarrStore.open_group(store, mode=mode, 555 synchronizer=synchronizer, --> 556 group=group, consolidated=consolidated) 557 ds = maybe_decode_store(zarr_store) 558 ~/anaconda3/envs/intake/lib/python3.7/site-packages/xarray/backends/zarr.py in open_group(cls, store, mode, synchronizer, group, consolidated, consolidate_on_close) 250 zarr_group = zarr.open_consolidated(store, **open_kwargs) 251 else: --> 252 zarr_group = zarr.open_group(store, **open_kwargs) 253 return cls(zarr_group, consolidate_on_close) 254 ~/anaconda3/envs/intake/lib/python3.7/site-packages/zarr/hierarchy.py in open_group(store, mode, cache_attrs, synchronizer, path, chunk_store) 1114 err_contains_array(path) 1115 elif not contains_group(store, path=path): -> 1116 err_group_not_found(path) 1117 1118 elif mode == 'w': ~/anaconda3/envs/intake/lib/python3.7/site-packages/zarr/errors.py in err_group_not_found(path) 27 28 def err_group_not_found(path): ---> 29 raise ValueError('group not found at path %r' % path) 30 31 ValueError: group not found at path '' ```

Output of xr.show_versions()

# Paste the output here xr.show_versions() here INSTALLED VERSIONS ------------------ commit: None python: 3.7.3 | packaged by conda-forge | (default, Jul 1 2019, 14:38:56) [Clang 4.0.1 (tags/RELEASE_401/final)] python-bits: 64 OS: Darwin OS-release: 18.7.0 machine: x86_64 processor: i386 byteorder: little LC_ALL: None LANG: en_US.UTF-8 LOCALE: en_US.UTF-8 libhdf5: 1.10.5 libnetcdf: 4.6.2 xarray: 0.12.3 pandas: 0.25.0 numpy: 1.17.0 scipy: None netCDF4: 1.5.1.2 pydap: None h5netcdf: 0.7.4 h5py: 2.9.0 Nio: None zarr: 2.3.2 cftime: 1.0.3.4 nc_time_axis: None PseudoNetCDF: None rasterio: None cfgrib: None iris: None bottleneck: None dask: 2.2.0 distributed: 2.2.0 matplotlib: 3.1.1 cartopy: None seaborn: None numbagg: None setuptools: 41.0.1 pip: 19.2.2 conda: None pytest: None IPython: 7.7.0 sphinx: None