Repository: ecmwf/earthkit-data (python)

Issue:
`to_xarray` from GRIB dropping units
### What happened?

When calling `to_xarray()` on `GRIBReader` objects in earthkit-data, somnetimes the data units are lost. This seems to happen when there is a single variable in the GRIB input - but when there are multiple variables, the units are correctly carried into the xarray object.

### What are the steps to reproduce the bug?

```python
import earthkit.data

data = earthkit.data.from_source(
    'cds',
    'reanalysis-era5-single-levels',
    {
        'product_type': 'reanalysis',
        'variable': '2m_temperature',
        'year': '2024',
        'month': '08',
        'day': '09',
        'time': '18:00',
    },
)

ds = data.to_xarray()
```

If you look at the global and variable attributes of `ds`, there are no units in either. However, if we slightly tweak the above request to as for two variables as below, then the two variables in `ds` both have their correct units in the variable attributes.

```python
import earthkit.data

data = earthkit.data.from_source(
    'cds',
    'reanalysis-era5-single-levels',
    {
        'product_type': 'reanalysis',
        'variable': ['2m_temperature', 'total_precipitation'],
        'year': '2024',
        'month': '08',
        'day': '09',
        'time': '18:00',
    },
)

ds = data.to_xarray()
```

### Version

0.11.4

### Platform (OS and architecture)

MacOS

### Relevant log output

_No response_

### Accompanying data

_No response_

### Organisation

ECMWF

Pull request description:
No units attribute for single variable in xarray engine
Fixes #571 

Patch:
diff --git a/src/earthkit/data/readers/grib/xarray.py b/src/earthkit/data/readers/grib/xarray.py
index 9d6e8a2..2d71698 100644
--- a/src/earthkit/data/readers/grib/xarray.py
+++ b/src/earthkit/data/readers/grib/xarray.py
@@ -231,10 +231,12 @@ class XarrayMixIn:
                 - "fixed": Use the attributes defined in ``variable_attrs`` as variables
                   attributes and ``global_attrs`` as global attributes.
                 - "unique": Use all the attributes defined in ``attrs``, ``variable_attrs``
-                  and ``global_attrs``. When an attribute has unique a value for a dataset
+                  and ``global_attrs``.  When an attribute has unique value for a dataset
                   it will be a global attribute, otherwise it will be a variable attribute.
-                  However keys in ``variable_attrs`` are always used as variable attributes,
-                  while keys in ``global_attrs`` are always used as global attributes.
+                  However, this logic is only applied if a unique variable attribute can be
+                  a global attribute according to the CF conventions Appendix A. (e.g. "units" cannot
+                  be a global attribute). Additionally, keys in ``variable_attrs`` are always used as
+                  variable attributes, while keys in ``global_attrs`` are always used as global attributes.
             * attrs: str, number, callable, dict or list of these, None
                 Attribute or list of attributes. Only used when ``attrs_mode`` is ``unique``.
                 Its default value (None) expands to [] unless the ``profile`` overwrites it.
diff --git a/src/earthkit/data/utils/xarray/attrs.py b/src/earthkit/data/utils/xarray/attrs.py
index 2515016..6762fd0 100644
--- a/src/earthkit/data/utils/xarray/attrs.py
+++ b/src/earthkit/data/utils/xarray/attrs.py
@@ -8,9 +8,11 @@
 #
 
 import logging
+import os
 from abc import ABCMeta
 from abc import abstractmethod
 from collections import defaultdict
+from functools import cached_property
 
 from earthkit.data.utils import ensure_dict
 from earthkit.data.utils import ensure_iterable
@@ -18,6 +20,36 @@ from earthkit.data.utils import ensure_iterable
 LOG = logging.getLogger(__name__)
 
 
+class CFAttrs:
+    def _load(self):
+        here = os.path.dirname(__file__)
+        path = os.path.join(here, "cf_attrs.yaml")
+        if os.path.exists(path):
+            import yaml
+
+            try:
+                with open(path, "r") as f:
+                    return yaml.safe_load(f)
+            except Exception as e:
+                LOG.exception(f"Failed read CF attributes file {path}. {e}")
+                raise
+        else:
+            raise ValueError(f"CF attributes file not found! path={path}")
+
+    @cached_property
+    def attrs(self):
+        return self._load()
+
+    def can_be_global(self, name):
+        item = self.attrs.get(name, None)
+        if item:
+            return "G" in item["use"]
+        return True
+
+
+CF_ATTRS = CFAttrs()
+
+
 class Attr:
     """Generic attribute class.
 
@@ -238,7 +270,7 @@ class AttrsBuilder(metaclass=ABCMeta):
                 global_attrs[item.name] = item.value()
 
         # TODO: make it optional
-        global_attrs.pop("units", None)
+        # global_attrs.pop("units", None)
 
         return global_attrs
 
@@ -257,13 +289,24 @@ class UniqueAttrBuilder(AttrsBuilder):
             if len(v) == 1 and k not in self.attrs.variable_attrs:
                 global_attrs[k] = list(v)[0]
 
-        for var_obj in t_vars.values():
-            var_obj.adjust_attrs(drop_keys=global_attrs.keys(), rename=rename)
+        # Some attrs cannot be global according to the CF convention.
+        # These are removed from global attrs and kept as variable attrs.
+        global_attrs_keys = list(global_attrs.keys())
+        global_attrs_renamed_keys = global_attrs_keys
+        if rename:
+            global_attrs_renamed_keys = list(rename(global_attrs).keys())
+
+        for k1, k2 in zip(global_attrs_keys, global_attrs_renamed_keys):
+            if not CF_ATTRS.can_be_global(k1) or not CF_ATTRS.can_be_global(k2):
+                global_attrs.pop(k1)
 
         for k in self.attrs.variable_attrs:
             if k in global_attrs:
                 global_attrs.pop(k)
 
+        for var_obj in t_vars.values():
+            var_obj.adjust_attrs(drop_keys=global_attrs.keys(), rename=rename)
+
         global_attrs = {k: v for k, v in global_attrs.items() if v is not None}
 
         return global_attrs
diff --git a/src/earthkit/data/utils/xarray/cf_attrs.yaml b/src/earthkit/data/utils/xarray/cf_attrs.yaml
new file mode 100644
index 0000000..03767eb
--- /dev/null
+++ b/src/earthkit/data/utils/xarray/cf_attrs.yaml
@@ -0,0 +1,195 @@
+# Based on CF Conventions Appendix A
+# All CF attributes are listed here except for those that are used to describe grid mappings. See Appendix F for the grid mapping attributes.
+# The 'Type' values are S for string, N for numeric, and D for the type of the data variable.
+# The 'Use' values are G for global, C for variables containing coordinate data, and D for variables containing non-coordinate data
+Conventions:
+  type: S
+  use: G
+_FillValue:
+  type: D
+  use:
+  - C
+  - D
+actual_range:
+  type: N
+  use:
+  - C
+  - D
+add_offset:
+  type: N
+  use:
+  - C
+  - D
+ancillary_variables:
+  type: S
+  use: D
+axis:
+  type: S
+  use: C
+bounds:
+  type: S
+  use: C
+calendar:
+  type: S
+  use: C
+cell_measures:
+  type: S
+  use: D
+cell_methods:
+  type: S
+  use: D
+cf_role:
+  type: S
+  use: C
+climatology:
+  type: S
+  use: C
+comment:
+  type: S
+  use:
+  - G
+  - C
+  - D
+compress:
+  type: S
+  use: C
+computed_standard_name:
+  type: S
+  use: C
+coordinates:
+  type: S
+  use:
+  - D
+  - M
+external_variables:
+  type: S
+  use: G
+featureType:
+  type: S
+  use: G
+flag_masks:
+  type: D
+  use: D
+flag_meanings:
+  type: S
+  use: D
+flag_values:
+  type: D
+  use: D
+formula_terms:
+  type: S
+  use: C
+geometry:
+  type: S
+  use:
+  - C
+  - D
+geometry_type:
+  type: S
+  use: M
+grid_mapping:
+  type: S
+  use:
+  - D
+  - M
+history:
+  type: S
+  use:
+  - G
+  - Gr
+instance_dimension:
+  type: S
+  use: "-"
+institution:
+  type: S
+  use:
+  - G
+  - D
+interior_ring:
+  type: S
+  use: M
+leap_month:
+  type: N
+  use: C
+leap_year:
+  type: N
+  use: C
+long_name:
+  type: S
+  use:
+  - C
+  - D
+missing_value:
+  type: D
+  use:
+  - C
+  - D
+month_lengths:
+  type: N
+  use: C
+node_coordinates:
+  type: S
+  use: M
+node_count:
+  type: S
+  use: M
+nodes:
+  type: S
+  use: C
+part_node_count:
+  type: S
+  use: M
+positive:
+  type: S
+  use: C
+references:
+  type: S
+  use:
+  - G
+  - D
+sample_dimension:
+  type: S
+  use: "-"
+scale_factor:
+  type: N
+  use:
+  - C
+  - D
+source:
+  type: S
+  use:
+  - G
+  - D
+standard_error_multiplier:
+  type: N
+  use: D
+standard_name:
+  type: S
+  use:
+  - C
+  - D
+title:
+  type: S
+  use:
+  - G
+  - Gr
+units:
+  type: S
+  use:
+  - C
+  - D
+valid_max:
+  type: N
+  use:
+  - C
+  - D
+valid_min:
+  type: N
+  use:
+  - C
+  - D
+valid_range:
+  type: N
+  use:
+  - C
+  - D
diff --git a/src/earthkit/data/utils/xarray/engine.py b/src/earthkit/data/utils/xarray/engine.py
index 67304b0..1dfaa20 100644
--- a/src/earthkit/data/utils/xarray/engine.py
+++ b/src/earthkit/data/utils/xarray/engine.py
@@ -188,10 +188,12 @@ class EarthkitBackendEntrypoint(BackendEntrypoint):
             - "fixed": Use the attributes defined in ``variable_attrs`` as variables
               attributes and ``global_attrs`` as global attributes.
             - "unique": Use all the attributes defined in ``attrs``, ``variable_attrs``
-              and ``global_attrs``. When an attribute has unique a value for a dataset
+              and ``global_attrs``. When an attribute has unique value for a dataset
               it will be a global attribute, otherwise it will be a variable attribute.
-              However keys in ``variable_attrs`` are always used as variable attributes,
-              while keys in ``global_attrs`` are always used as global attributes.
+              However, this logic is only applied if a unique variable attribute can be
+              a global attribute according to the CF conventions Appendix A. (e.g. "units" cannot
+              be a global attribute). Additionally, keys in ``variable_attrs`` are always used as
+              variable attributes, while keys in ``global_attrs`` are always used as global attributes.
         attrs: str, number, callable, dict or list of these, None
             Attribute or list of attributes. Only used when ``attrs_mode`` is ``unique``.
             Its default value (None) expands to [] unless the ``profile`` overwrites it.


Test patch:
diff --git a/tests/xr_engine/test_xr_engine.py b/tests/xr_engine/test_xr_engine.py
index 4aa3794..59a9e45 100644
--- a/tests/xr_engine/test_xr_engine.py
+++ b/tests/xr_engine/test_xr_engine.py
@@ -469,10 +469,14 @@ def test_xr_engine_single_field():
     lats = np.linspace(90, -90, 19)
     lons = np.linspace(0, 350, 36)
 
-    attrs_ref = {
-        "param": "t",
+    var_attrs_ref = {
         "standard_name": "air_temperature",
         "long_name": "Temperature",
+        "units": "K",
+    }
+
+    global_attrs_ref = {
+        "param": "t",
         "paramId": 130,
         "class": "od",
         "stream": "oper",
@@ -488,7 +492,7 @@ def test_xr_engine_single_field():
         "institution": "ECMWF",
     }
 
-    assert ds.attrs == attrs_ref
+    assert ds.attrs == global_attrs_ref
 
     data_vars = ["t"]
 
@@ -510,6 +514,9 @@ def test_xr_engine_single_field():
 
     da = ds["t"]
 
+    for k, v in var_attrs_ref.items():
+        assert da.attrs[k] == v
+
     r = da[:, :]
     r.shape == (19, 36)
     assert np.allclose(r.values, vals_ref)