Helpful error messages with open_dataset

On non-cartesian meshes.
Fixes #65

pymech/dataset.py

@@ -73,9 +73,18 @@ def _open_nek_dataset(path, drop_variables=None):
 
     elements = field.elem
     elem_stores = [_NekDataStore(elem) for elem in elements]
-    elem_dsets = [
-        xr.Dataset.load_store(store).set_coords(store.axes) for store in elem_stores
-    ]
+    try:
+        elem_dsets = [
+            xr.Dataset.load_store(store).set_coords(store.axes) for store in elem_stores
+        ]
+    except ValueError as err:
+        raise NotImplementedError(
+            "Opening dataset failed because you probably tried to open a field file "
+            "with an unsupported mesh. "
+            "The `pymech.open_dataset` function currently works only with cartesian "
+            "box meshes. For more details on this, see "
+            "https://github.com/eX-Mech/pymech/issues/31"
+        ) from err
 
     # See: https://github.com/MITgcm/xmitgcm/pull/200
     ds = xr.combine_by_coords(elem_dsets, combine_attrs="drop")

tests/test_dataset.py

@@ -19,14 +19,24 @@ def _check_channel_dset(ds):
     assert math.isclose(ds.time, 0.2, abs_tol=1e-6)
 
 
-def test_nekdataset(test_data_dir):
+def test_channel_dataset(test_data_dir):
     import pymech.dataset as pd
 
     fname = f"{test_data_dir}/nek/channel3D_0.f00001"
     ds = pd.open_dataset(fname)
     _check_channel_dset(ds)
 
 
+def test_phill_dataset(test_data_dir):
+    import pymech.dataset as pd
+
+    fname = f"{test_data_dir}/nek/phill0.f00010"
+    with pytest.raises(
+        NotImplementedError, match="currently works only with cartesian box meshes"
+    ):
+        pd.open_dataset(fname)
+
+
 @pytest.mark.parametrize("file_name", ["channel3D_0.f00001", "stsabl0.f00001"])
 def test_dataset_coords(test_data_dir, file_name):
     """Check if the 1D coordinates match with the original 3D coordinate arrays"""