BUG: fix writing to groups with h5netcdf (#2181)

* BUG: fix writing to groups with h5netcdf

Fixes GH2177

Our test suite was inadvertently not checking this.

* what's new note

doc/whats-new.rst

@@ -64,7 +64,11 @@ Bug fixes
   longer falsely returns an empty array when the slice includes the value in
   the index) (:issue:`2165`).
   By `Spencer Clark <https://github.com/spencerkclark>`_.
-
+
+- Fix Dataset.to_netcdf() cannot create group with engine="h5netcdf"
+  (:issue:`2177`).
+  By `Stephan Hoyer <https://github.com/shoyer>`_
+
 .. _whats-new.0.10.4:
 
 v0.10.4 (May 16, 2018)

xarray/backends/h5netcdf_.py

@@ -12,7 +12,7 @@
     HDF5_LOCK, DataStorePickleMixin, WritableCFDataStore, find_root)
 from .netCDF4_ import (
     BaseNetCDF4Array, _encode_nc4_variable, _extract_nc4_variable_encoding,
-    _get_datatype, _nc4_group)
+    _get_datatype, _nc4_require_group)
 
 
 class H5NetCDFArrayWrapper(BaseNetCDF4Array):
@@ -57,11 +57,16 @@ def _read_attributes(h5netcdf_var):
     lsd_okay=False, h5py_okay=True, backend='h5netcdf')
 
 
+def _h5netcdf_create_group(dataset, name):
+    return dataset.create_group(name)
+
+
 def _open_h5netcdf_group(filename, mode, group):
     import h5netcdf
     ds = h5netcdf.File(filename, mode=mode)
     with close_on_error(ds):
-        return _nc4_group(ds, group, mode)
+        return _nc4_require_group(
+            ds, group, mode, create_group=_h5netcdf_create_group)
 
 
 class H5NetCDFStore(WritableCFDataStore, DataStorePickleMixin):

xarray/backends/netCDF4_.py

@@ -108,7 +108,11 @@ def _nc4_dtype(var):
     return dtype
 
 
-def _nc4_group(ds, group, mode):
+def _netcdf4_create_group(dataset, name):
+    return dataset.createGroup(name)
+
+
+def _nc4_require_group(ds, group, mode, create_group=_netcdf4_create_group):
     if group in set([None, '', '/']):
         # use the root group
         return ds
@@ -123,7 +127,7 @@ def _nc4_group(ds, group, mode):
                 ds = ds.groups[key]
             except KeyError as e:
                 if mode != 'r':
-                    ds = ds.createGroup(key)
+                    ds = create_group(ds, key)
                 else:
                     # wrap error to provide slightly more helpful message
                     raise IOError('group not found: %s' % key, e)
@@ -210,7 +214,7 @@ def _open_netcdf4_group(filename, mode, group=None, **kwargs):
     ds = nc4.Dataset(filename, mode=mode, **kwargs)
 
     with close_on_error(ds):
-        ds = _nc4_group(ds, group, mode)
+        ds = _nc4_require_group(ds, group, mode)
 
     _disable_auto_decode_group(ds)
 

xarray/tests/test_backends.py

@@ -892,7 +892,7 @@ def test_open_group(self):
 
             # check equivalent ways to specify group
             for group in 'foo', '/foo', 'foo/', '/foo/':
-                with open_dataset(tmp_file, group=group) as actual:
+                with self.open(tmp_file, group=group) as actual:
                     assert_equal(actual['x'], expected['x'])
 
             # check that missing group raises appropriate exception
@@ -920,18 +920,18 @@ def test_open_subgroup(self):
 
             # check equivalent ways to specify group
             for group in 'foo/bar', '/foo/bar', 'foo/bar/', '/foo/bar/':
-                with open_dataset(tmp_file, group=group) as actual:
+                with self.open(tmp_file, group=group) as actual:
                     assert_equal(actual['x'], expected['x'])
 
     def test_write_groups(self):
         data1 = create_test_data()
         data2 = data1 * 2
         with create_tmp_file() as tmp_file:
-            data1.to_netcdf(tmp_file, group='data/1')
-            data2.to_netcdf(tmp_file, group='data/2', mode='a')
-            with open_dataset(tmp_file, group='data/1') as actual1:
+            self.save(data1, tmp_file, group='data/1')
+            self.save(data2, tmp_file, group='data/2', mode='a')
+            with self.open(tmp_file, group='data/1') as actual1:
                 assert_identical(data1, actual1)
-            with open_dataset(tmp_file, group='data/2') as actual2:
+            with self.open(tmp_file, group='data/2') as actual2:
                 assert_identical(data2, actual2)
 
     def test_roundtrip_string_with_fill_value_vlen(self):