Fix group kwarg

conftest.py

@@ -24,6 +24,17 @@ def pytest_runtest_setup(item):
         )
 
 
+@pytest.fixture
+def empty_netcdf4_file(tmpdir):
+    # Set up example xarray dataset
+    ds = xr.Dataset()  # Save it to disk as netCDF (in temporary directory)
+    filepath = f"{tmpdir}/empty.nc"
+    ds.to_netcdf(filepath, format="NETCDF4")
+    ds.close()
+
+    return filepath
+
+
 @pytest.fixture
 def netcdf4_file(tmpdir):
     # Set up example xarray dataset
@@ -37,6 +48,18 @@ def netcdf4_file(tmpdir):
     return filepath
 
 
+@pytest.fixture
+def netcdf4_file_with_data_in_multiple_groups(tmpdir):
+    filepath = str(tmpdir / "test.nc")
+
+    ds1 = xr.DataArray([1, 2, 3], name="foo").to_dataset()
+    ds1.to_netcdf(filepath)
+    ds2 = xr.DataArray([4, 5], name="bar").to_dataset()
+    ds2.to_netcdf(filepath, group="subgroup", mode="a")
+
+    return filepath
+
+
 @pytest.fixture
 def netcdf4_files_factory(tmpdir) -> callable:
     def create_netcdf4_files(

virtualizarr/readers/hdf5.py

@@ -43,7 +43,9 @@ def open_virtual_dataset(
             filepath, inline_threshold=0, **reader_options
         ).translate()
 
-        refs = extract_group(refs, group)
+        # both group=None and group='' mean to read root group
+        if group:
+            refs = extract_group(refs, group)
 
         virtual_vars, attrs, coord_names = virtual_vars_and_metadata_from_kerchunk_refs(
             refs,

virtualizarr/tests/test_backend.py

@@ -5,7 +5,7 @@
 import pytest
 import xarray as xr
 import xarray.testing as xrt
-from xarray import open_dataset
+from xarray import Dataset, open_dataset
 from xarray.core.indexes import Index
 
 from virtualizarr import open_virtual_dataset
@@ -309,6 +309,43 @@ def test_virtualizarr_vs_local_nisar(self, hdf_backend):
         xrt.assert_equal(dsXR, dsV)
 
 
+@requires_kerchunk
+def test_open_empty_group(empty_netcdf4_file):
+    vds = open_virtual_dataset(empty_netcdf4_file, indexes={})
+    assert isinstance(vds, xr.Dataset)
+    expected = Dataset()
+    xrt.assert_identical(vds, expected)
+
+
+@requires_kerchunk
+class TestOpenVirtualDatasetHDFGroup:
+    def test_open_subgroup(self, netcdf4_file_with_data_in_multiple_groups):
+        vds = open_virtual_dataset(
+            netcdf4_file_with_data_in_multiple_groups, group="subgroup", indexes={}
+        )
+        assert list(vds.variables) == ["bar"]
+        assert isinstance(vds["bar"].data, ManifestArray)
+        assert vds["bar"].shape == (2,)
+
+    def test_open_root_group_manually(self, netcdf4_file_with_data_in_multiple_groups):
+        vds = open_virtual_dataset(
+            netcdf4_file_with_data_in_multiple_groups, group="", indexes={}
+        )
+        assert list(vds.variables) == ["foo"]
+        assert isinstance(vds["foo"].data, ManifestArray)
+        assert vds["foo"].shape == (3,)
+
+    def test_open_root_group_by_default(
+        self, netcdf4_file_with_data_in_multiple_groups
+    ):
+        vds = open_virtual_dataset(
+            netcdf4_file_with_data_in_multiple_groups, indexes={}
+        )
+        assert list(vds.variables) == ["foo"]
+        assert isinstance(vds["foo"].data, ManifestArray)
+        assert vds["foo"].shape == (3,)
+
+
 @requires_kerchunk
 class TestLoadVirtualDataset:
     @pytest.mark.parametrize("hdf_backend", [HDF5VirtualBackend, HDFVirtualBackend])
@@ -356,8 +393,6 @@ def test_group_kwarg(self, hdf5_groups_file, hdf_backend):
                     hdf5_groups_file, group="doesnt_exist", backend=hdf_backend
                 )
         if hdf_backend == HDF5VirtualBackend:
-            with pytest.raises(ValueError, match="Multiple HDF Groups found"):
-                open_virtual_dataset(hdf5_groups_file)
             with pytest.raises(ValueError, match="not found in"):
                 open_virtual_dataset(hdf5_groups_file, group="doesnt_exist")
 

virtualizarr/tests/test_integration.py

@@ -13,7 +13,6 @@
 from virtualizarr.tests import requires_kerchunk
 from virtualizarr.translators.kerchunk import (
     dataset_from_kerchunk_refs,
-    find_var_names,
 )
 from virtualizarr.zarr import ZArray
 
@@ -49,12 +48,6 @@ def test_kerchunk_roundtrip_in_memory_no_concat():
     xrt.assert_equal(roundtrip, ds)
 
 
-def test_no_duplicates_find_var_names():
-    """Verify that we get a deduplicated list of var names"""
-    ref_dict = {"refs": {"x/something": {}, "x/otherthing": {}}}
-    assert len(find_var_names(ref_dict)) == 1
-
-
 @requires_kerchunk
 @pytest.mark.parametrize(
     "inline_threshold, vars_to_inline",

virtualizarr/translators/kerchunk.py

@@ -43,42 +43,40 @@ def virtual_vars_and_metadata_from_kerchunk_refs(
     return virtual_vars, ds_attrs, coord_names
 
 
-def extract_group(vds_refs: KerchunkStoreRefs, group: str | None) -> KerchunkStoreRefs:
-    """Extract only the part of the kerchunk reference dict that is relevant to a single HDF group"""
+def extract_group(vds_refs: KerchunkStoreRefs, group: str) -> KerchunkStoreRefs:
+    """
+    Extract only the part of the kerchunk reference dict that is relevant to a single HDF group.
+
+    group : str
+        Should be a non-empty string
+    """
     hdf_groups = [
         k.removesuffix(".zgroup") for k in vds_refs["refs"].keys() if ".zgroup" in k
     ]
-    if len(hdf_groups) == 1:
-        return vds_refs
-    else:
-        if group is None:
-            raise ValueError(
-                f"Multiple HDF Groups found. Must specify group= keyword to select one of {hdf_groups}"
-            )
-        else:
-            # Ensure supplied group kwarg is consistent with kerchunk keys
-            if not group.endswith("/"):
-                group += "/"
-            if group.startswith("/"):
-                group = group.removeprefix("/")
-
-        if group not in hdf_groups:
-            raise ValueError(f'Group "{group}" not found in {hdf_groups}')
-
-        # Filter by group prefix and remove prefix from all keys
-        groupdict = {
-            k.removeprefix(group): v
-            for k, v in vds_refs["refs"].items()
-            if k.startswith(group)
-        }
-        # Also remove group prefix from _ARRAY_DIMENSIONS
-        for k, v in groupdict.items():
-            if isinstance(v, str):
-                groupdict[k] = v.replace("\\/", "/").replace(group, "")
 
-        vds_refs["refs"] = groupdict
+    # Ensure supplied group kwarg is consistent with kerchunk keys
+    if not group.endswith("/"):
+        group += "/"
+    if group.startswith("/"):
+        group = group.removeprefix("/")
 
-        return KerchunkStoreRefs(vds_refs)
+    if group not in hdf_groups:
+        raise ValueError(f'Group "{group}" not found in {hdf_groups}')
+
+    # Filter by group prefix and remove prefix from all keys
+    groupdict = {
+        k.removeprefix(group): v
+        for k, v in vds_refs["refs"].items()
+        if k.startswith(group)
+    }
+    # Also remove group prefix from _ARRAY_DIMENSIONS
+    for k, v in groupdict.items():
+        if isinstance(v, str):
+            groupdict[k] = v.replace("\\/", "/").replace(group, "")
+
+    vds_refs["refs"] = groupdict
+
+    return KerchunkStoreRefs(vds_refs)
 
 
 def virtual_vars_from_kerchunk_refs(
@@ -222,9 +220,17 @@ def find_var_names(ds_reference_dict: KerchunkStoreRefs) -> list[str]:
     """Find the names of zarr variables in this store/group."""
 
     refs = ds_reference_dict["refs"]
-    found_var_names = {key.split("/")[0] for key in refs.keys() if "/" in key}
 
-    return list(found_var_names)
+    found_var_names = []
+    for key in refs.keys():
+        # has to capture "foo/.zarray", but ignore ".zgroup", ".zattrs", and "subgroup/bar/.zarray"
+        # TODO this might be a sign that we should introduce a KerchunkGroupRefs type and cut down the references before getting to this point...
+        if key not in (".zgroup", ".zattrs", ".zmetadata"):
+            first_part, second_part, *_ = key.split("/")
+            if second_part == ".zarray":
+                found_var_names.append(first_part)
+
+    return found_var_names
 
 
 def extract_array_refs(