Error when no UGRID topology is present in IO functions.

Fixes #180

docs/changelog.rst

@@ -9,6 +9,13 @@ The format is based on `Keep a Changelog`_, and this project adheres to
 Unreleased
 ----------
 
+Changed
+-------
+
+- :func:`xugrid.open_dataarray`, :func:`xugrid.load_dataarray`,
+  :func:`xugrid.open_dataset`, :func:`xugrid.load_dataset` now error when no
+  UGRID conventions data is present in the file or object.
+
 Added
 ~~~~~
 

tests/test_ugrid_dataset.py

@@ -1014,6 +1014,13 @@ def test_open_dataset(tmp_path):
     assert "mesh2d_face_nodes" in back.ugrid.grids[0].to_dataset()
     assert "mesh2d_face_nodes" not in back.ugrid.obj
 
+    path = tmp_path / "no-topology-dataset.nc"
+    uds.to_netcdf(path)
+    with pytest.raises(
+        ValueError, match="The file or object does not contain UGRID conventions data."
+    ):
+        xugrid.open_dataset(path)
+
 
 def test_load_dataset(tmp_path):
     path = tmp_path / "ugrid-dataset.nc"
@@ -1042,7 +1049,7 @@ def test_open_dataarray_roundtrip(tmp_path):
     path = tmp_path / "ugrid-dataset.nc"
     uds = xugrid.UgridDataset(UGRID_DS())
     uds.ugrid.to_netcdf(path)
-    with pytest.raises(ValueError, match="Given file dataset contains more than one"):
+    with pytest.raises(ValueError, match="The file or object contains more than one"):
         xugrid.open_dataarray(path)
 
     path = tmp_path / "ugrid-dataarray.nc"
@@ -1052,11 +1059,21 @@ def test_open_dataarray_roundtrip(tmp_path):
     assert back.name == "a"
 
 
+def test_open_dataarray_ugrid_errors(tmp_path):
+    uds = ugrid1d_ds()
+    path = tmp_path / "no-topology-dataarray.nc"
+    uds.to_netcdf(path)
+    with pytest.raises(
+        ValueError, match="The file or object does not contain UGRID conventions data."
+    ):
+        xugrid.open_dataarray(path)
+
+
 def test_load_dataarray_roundtrip(tmp_path):
     path = tmp_path / "ugrid-dataset.nc"
     uds = xugrid.UgridDataset(UGRID_DS())
     uds.ugrid.to_netcdf(path)
-    with pytest.raises(ValueError, match="Given file dataset contains more than one"):
+    with pytest.raises(ValueError, match="The file or object contains more than one"):
         xugrid.load_dataarray(path)
 
     path = tmp_path / "ugrid-dataarray.nc"

xugrid/core/common.py

@@ -9,21 +9,32 @@
 DATAARRAY_VARIABLE = "__xarray_dataarray_variable__"
 
 
+def _dataset_helper(ds: xr.Dataset):
+    n_topology = len(ds.ugrid_roles.topology)
+    if n_topology == 0:
+        raise ValueError(
+            "The file or object does not contain UGRID conventions data. "
+            "One or more UGRID topologies are required. Perhaps you wrote "
+            "the file using `data.to_netcdf()` instead of `data.ugrid.to_netcdf()`?"
+        )
+    return UgridDataset(ds)
+
+
 def open_dataset(*args, **kwargs):
     ds = xr.open_dataset(*args, **kwargs)
-    return UgridDataset(ds)
+    return _dataset_helper(ds)
 
 
 def load_dataset(*args, **kwargs):
     ds = xr.load_dataset(*args, **kwargs)
-    return UgridDataset(ds)
+    return _dataset_helper(ds)
 
 
 def _dataarray_helper(ds: xr.Dataset):
-    dataset = UgridDataset(ds)
+    dataset = _dataset_helper(ds)
     if len(dataset.data_vars) != 1:
         raise ValueError(
-            "Given file dataset contains more than one data "
+            "The file or object contains more than one data "
             "variable. Please read with xarray.open_dataset and "
             "then select the variable you want."
         )