fix: error handling decode_cf_var exceptions - use add_note (#10886)

Author: jonaslb

doc/whats-new.rst

@@ -39,6 +39,9 @@ Bug Fixes
   ``engine=`` argument. (:pull:`10804`). By `Ian Hunt-Isaak <https://github.com/ianhi>`_.
 - Fix indexing with empty arrays for scipy & h5netcdf backends which now resolves to empty slices (:issue:`10867`, :pull:`10870`).
   By `Kai Mühlbauer <https://github.com/kmuehlbauer>`_
+- Fix error handling issue in ``decode_cf_variables`` when decoding fails - the exception is now re-raised
+  correctly, with a note added about the variable name that caused the error (:issue:`10873`, :pull:`10886`).
+  By `Jonas L. Bertelsen <https://github.com/jonaslb>`_
 
 Performance
 ~~~~~~~~~~~

xarray/conventions.py

@@ -422,7 +422,8 @@ def stackable(dim: Hashable) -> bool:
                 decode_timedelta=_item_or_default(decode_timedelta, k, None),
             )
         except Exception as e:
-            raise type(e)(f"Failed to decode variable {k!r}: {e}") from e
+            e.add_note(f"Raised while decoding variable {k!r} with value {v!r}")
+            raise
         if decode_coords in [True, "coordinates", "all"]:
             var_attrs = new_vars[k].attrs
             if "coordinates" in var_attrs:

xarray/tests/test_conventions.py

@@ -658,8 +658,11 @@ def test_scalar_units() -> None:
 
 
 def test_decode_cf_error_includes_variable_name():
-    ds = Dataset({"invalid": ([], 1e36, {"units": "days since 2000-01-01"})})
-    with pytest.raises(ValueError, match="Failed to decode variable 'invalid'"):
+    ds = Dataset({"my_invalid_var": ([], 1e36, {"units": "days since 2000-01-01"})})
+    with pytest.raises(
+        ValueError,
+        match=r"unable to decode(?s:.*)my_invalid_var",
+    ):
         decode_cf(ds)