Meshcoord metadata follows face/edge coords where present. (#5020)

* Meshcoords take metadata from face/edge coords.

* Test fixes.

* Added tests for missing-vs-present standard-names and units.

* Slight code tidying.

* Review changes.

* Added whatsnew.

Author: pp-mo

docs/src/whatsnew/latest.rst

@@ -38,6 +38,15 @@ This document explains the changes made to Iris for this release
 #. `@trexfeathers`_ and `Julian Heming`_ added new mappings between CF
    standard names and UK Met Office LBFC codes. (:pull:`4859`)
 
+#. `@pp-mo`_ changed the metadata of a face/edge-type
+   :class:`~iris.experimental.ugrid.mesh.MeshCoord`, to be same as the face/edge
+   coordinate in the mesh from which it takes its ``.points``.  Previously, all MeshCoords
+   took their metadata from the node coord, but only a node-type MeshCoord now does
+   that.  Also, the MeshCoord ``.var_name`` is now that of the underlying coord, whereas
+   previously this was always None.  These changes make MeshCoord more like an ordinary
+   :class:`~iris.coords.AuxCoord`, which avoids some specific known usage problems.
+   (:issue:`4860`, :pull:`5020`)
+
 
 🐛 Bugs Fixed
 =============

lib/iris/experimental/ugrid/mesh.py

@@ -2841,16 +2841,60 @@ def __init__(
 
         # Get the 'coord identity' metadata from the relevant node-coordinate.
         node_coord = self.mesh.coord(include_nodes=True, axis=self.axis)
+        node_metadict = node_coord.metadata._asdict()
+        # Use node metadata, unless location is face/edge.
+        use_metadict = node_metadict.copy()
+        if location != "node":
+            # Location is either "edge" or "face" - get the relevant coord.
+            kwargs = {f"include_{location}s": True, "axis": axis}
+            location_coord = self.mesh.coord(**kwargs)
+
+            # Take the MeshCoord metadata from the 'location' coord.
+            use_metadict = location_coord.metadata._asdict()
+            unit_unknown = Unit(None)
+
+            # N.B. at present, coords in a Mesh are stored+accessed by 'axis', which
+            # means they must have a standard_name.  So ...
+            # (a) the 'location' (face/edge) coord *always* has a useable phenomenon
+            #     identity.
+            # (b) we still want to check that location+node coords have the same
+            #     phenomenon (i.e. physical meaning identity + units), **but** ...
+            # (c) we will accept/ignore some differences : not just "var_name", but
+            #     also "long_name" *and* "attributes".  So it is *only* "standard_name"
+            #     and "units" that cause an error if they differ.
+            for key in ("standard_name", "units"):
+                bounds_value = use_metadict[key]
+                nodes_value = node_metadict[key]
+                if key == "units" and (
+                    bounds_value == unit_unknown or nodes_value == unit_unknown
+                ):
+                    # Allow "any" unit to match no-units (for now)
+                    continue
+                if bounds_value != nodes_value:
+
+                    def fix_repr(val):
+                        # Tidy values appearance by converting Unit to string, and
+                        # wrapping strings in '', but leaving other types as a
+                        # plain str() representation.
+                        if isinstance(val, Unit):
+                            val = str(val)
+                        if isinstance(val, str):
+                            val = repr(val)
+                        return val
+
+                    nodes_value, bounds_value = [
+                        fix_repr(val) for val in (nodes_value, bounds_value)
+                    ]
+                    msg = (
+                        f"Node coordinate {node_coord!r} disagrees with the "
+                        f"{location} coordinate {location_coord!r}, "
+                        f'in having a "{key}" value of {nodes_value} '
+                        f"instead of {bounds_value}."
+                    )
+                    raise ValueError(msg)
+
         # Call parent constructor to handle the common constructor args.
-        super().__init__(
-            points,
-            bounds=bounds,
-            standard_name=node_coord.standard_name,
-            long_name=node_coord.long_name,
-            var_name=None,  # We *don't* "represent" the underlying node var
-            units=node_coord.units,
-            attributes=node_coord.attributes,
-        )
+        super().__init__(points, bounds=bounds, **use_metadict)
 
     # Define accessors for MeshCoord-specific properties mesh/location/axis.
     # These are all read-only.

lib/iris/tests/results/experimental/ugrid/2D_1t_face_half_levels.cml

@@ -20,8 +20,8 @@
 		..., 
 		[-42.7342, -40.8934, -46.161, -48.912],
 		[-40.8934, -38.4268, -42.6612, -46.161],
-		[-38.4268, -35.2644, -38.4268, -42.6612]]" id="21594c35" long_name="Latitude of mesh nodes." points="[33.4328, 36.1226, 38.2012, ..., -44.791,
-		-42.1583, -38.815]" shape="(864,)" standard_name="latitude" units="Unit('degrees')" value_type="float32"/>
+		[-38.4268, -35.2644, -38.4268, -42.6612]]" id="72da1058" long_name="Characteristic latitude of mesh faces." points="[33.4328, 36.1226, 38.2012, ..., -44.791,
+		-42.1583, -38.815]" shape="(864,)" standard_name="latitude" units="Unit('degrees')" value_type="float32" var_name="Mesh2d_half_levels_face_y"/>
       </coord>
       <coord datadims="[1]">
         <meshCoord bounds="[[-45.0, -37.5, -37.5, -45.0],
@@ -30,8 +30,8 @@
 		..., 
 		[-112.5, -120.0, -126.958, -118.361],
 		[-120.0, -127.5, -135.0, -126.958],
-		[-127.5, -135.0, -142.5, -135.0]]" id="fac9555b" long_name="Longitude of mesh nodes." points="[-41.3152, -33.8068, -26.296, ..., -119.377,
-		-127.321, -135.0]" shape="(864,)" standard_name="longitude" units="Unit('degrees')" value_type="float32"/>
+		[-127.5, -135.0, -142.5, -135.0]]" id="b5c6bdeb" long_name="Characteristic longitude of mesh faces." points="[-41.3152, -33.8068, -26.296, ..., -119.377,
+		-127.321, -135.0]" shape="(864,)" standard_name="longitude" units="Unit('degrees')" value_type="float32" var_name="Mesh2d_half_levels_face_x"/>
       </coord>
       <coord datadims="[0]">
         <auxCoord bounds="[[21600.0, 21600.0]]" id="17a27d05" long_name="Time axis" points="[21600.0]" shape="(1,)" standard_name="time" units="Unit('seconds since 2016-01-01 15:00:00', calendar='standard')" value_type="float64" var_name="time_instant">

lib/iris/tests/results/experimental/ugrid/2D_72t_face_half_levels.cml

@@ -20,8 +20,8 @@
 		..., 
 		[-42.7342, -40.8934, -46.161, -48.912],
 		[-40.8934, -38.4268, -42.6612, -46.161],
-		[-38.4268, -35.2644, -38.4268, -42.6612]]" id="21594c35" long_name="Latitude of mesh nodes." points="[33.4328, 36.1226, 38.2012, ..., -44.791,
-		-42.1583, -38.815]" shape="(864,)" standard_name="latitude" units="Unit('degrees')" value_type="float32"/>
+		[-38.4268, -35.2644, -38.4268, -42.6612]]" id="72da1058" long_name="Characteristic latitude of mesh faces." points="[33.4328, 36.1226, 38.2012, ..., -44.791,
+		-42.1583, -38.815]" shape="(864,)" standard_name="latitude" units="Unit('degrees')" value_type="float32" var_name="Mesh2d_half_levels_face_y"/>
       </coord>
       <coord datadims="[1]">
         <meshCoord bounds="[[-45.0, -37.5, -37.5, -45.0],
@@ -30,8 +30,8 @@
 		..., 
 		[-112.5, -120.0, -126.958, -118.361],
 		[-120.0, -127.5, -135.0, -126.958],
-		[-127.5, -135.0, -142.5, -135.0]]" id="fac9555b" long_name="Longitude of mesh nodes." points="[-41.3152, -33.8068, -26.296, ..., -119.377,
-		-127.321, -135.0]" shape="(864,)" standard_name="longitude" units="Unit('degrees')" value_type="float32"/>
+		[-127.5, -135.0, -142.5, -135.0]]" id="b5c6bdeb" long_name="Characteristic longitude of mesh faces." points="[-41.3152, -33.8068, -26.296, ..., -119.377,
+		-127.321, -135.0]" shape="(864,)" standard_name="longitude" units="Unit('degrees')" value_type="float32" var_name="Mesh2d_half_levels_face_x"/>
       </coord>
       <coord datadims="[0]">
         <auxCoord bounds="[[300.0, 300.0],

lib/iris/tests/results/experimental/ugrid/3D_1t_face_full_levels.cml

@@ -31,8 +31,8 @@
 		..., 
 		[-42.7342, -40.8934, -46.161, -48.912],
 		[-40.8934, -38.4268, -42.6612, -46.161],
-		[-38.4268, -35.2644, -38.4268, -42.6612]]" id="21594c35" long_name="Latitude of mesh nodes." points="[33.4328, 36.1226, 38.2012, ..., -44.791,
-		-42.1583, -38.815]" shape="(864,)" standard_name="latitude" units="Unit('degrees')" value_type="float32"/>
+		[-38.4268, -35.2644, -38.4268, -42.6612]]" id="72da1058" long_name="Characteristic latitude of mesh faces." points="[33.4328, 36.1226, 38.2012, ..., -44.791,
+		-42.1583, -38.815]" shape="(864,)" standard_name="latitude" units="Unit('degrees')" value_type="float32" var_name="Mesh2d_full_levels_face_y"/>
       </coord>
       <coord datadims="[2]">
         <meshCoord bounds="[[-45.0, -37.5, -37.5, -45.0],
@@ -41,8 +41,8 @@
 		..., 
 		[-112.5, -120.0, -126.958, -118.361],
 		[-120.0, -127.5, -135.0, -126.958],
-		[-127.5, -135.0, -142.5, -135.0]]" id="fac9555b" long_name="Longitude of mesh nodes." points="[-41.3152, -33.8068, -26.296, ..., -119.377,
-		-127.321, -135.0]" shape="(864,)" standard_name="longitude" units="Unit('degrees')" value_type="float32"/>
+		[-127.5, -135.0, -142.5, -135.0]]" id="b5c6bdeb" long_name="Characteristic longitude of mesh faces." points="[-41.3152, -33.8068, -26.296, ..., -119.377,
+		-127.321, -135.0]" shape="(864,)" standard_name="longitude" units="Unit('degrees')" value_type="float32" var_name="Mesh2d_full_levels_face_x"/>
       </coord>
       <coord datadims="[0]">
         <auxCoord bounds="[[21600.0, 21600.0]]" id="17a27d05" long_name="Time axis" points="[21600.0]" shape="(1,)" standard_name="time" units="Unit('seconds since 2016-01-01 15:00:00', calendar='standard')" value_type="float64" var_name="time_instant">

lib/iris/tests/results/experimental/ugrid/3D_1t_face_half_levels.cml

@@ -31,8 +31,8 @@
 		..., 
 		[-42.7342, -40.8934, -46.161, -48.912],
 		[-40.8934, -38.4268, -42.6612, -46.161],
-		[-38.4268, -35.2644, -38.4268, -42.6612]]" id="21594c35" long_name="Latitude of mesh nodes." points="[33.4328, 36.1226, 38.2012, ..., -44.791,
-		-42.1583, -38.815]" shape="(864,)" standard_name="latitude" units="Unit('degrees')" value_type="float32"/>
+		[-38.4268, -35.2644, -38.4268, -42.6612]]" id="72da1058" long_name="Characteristic latitude of mesh faces." points="[33.4328, 36.1226, 38.2012, ..., -44.791,
+		-42.1583, -38.815]" shape="(864,)" standard_name="latitude" units="Unit('degrees')" value_type="float32" var_name="Mesh2d_half_levels_face_y"/>
       </coord>
       <coord datadims="[2]">
         <meshCoord bounds="[[-45.0, -37.5, -37.5, -45.0],
@@ -41,8 +41,8 @@
 		..., 
 		[-112.5, -120.0, -126.958, -118.361],
 		[-120.0, -127.5, -135.0, -126.958],
-		[-127.5, -135.0, -142.5, -135.0]]" id="fac9555b" long_name="Longitude of mesh nodes." points="[-41.3152, -33.8068, -26.296, ..., -119.377,
-		-127.321, -135.0]" shape="(864,)" standard_name="longitude" units="Unit('degrees')" value_type="float32"/>
+		[-127.5, -135.0, -142.5, -135.0]]" id="b5c6bdeb" long_name="Characteristic longitude of mesh faces." points="[-41.3152, -33.8068, -26.296, ..., -119.377,
+		-127.321, -135.0]" shape="(864,)" standard_name="longitude" units="Unit('degrees')" value_type="float32" var_name="Mesh2d_half_levels_face_x"/>
       </coord>
       <coord datadims="[0]">
         <auxCoord bounds="[[21600.0, 21600.0]]" id="17a27d05" long_name="Time axis" points="[21600.0]" shape="(1,)" standard_name="time" units="Unit('seconds since 2016-01-01 15:00:00', calendar='standard')" value_type="float64" var_name="time_instant">

lib/iris/tests/results/experimental/ugrid/3D_snow_pseudo_levels.cml

@@ -20,8 +20,8 @@
 		..., 
 		[-42.7342, -40.8934, -46.161, -48.912],
 		[-40.8934, -38.4268, -42.6612, -46.161],
-		[-38.4268, -35.2644, -38.4268, -42.6612]]" id="21594c35" long_name="Latitude of mesh nodes." points="[33.4328, 36.1226, 38.2012, ..., -44.791,
-		-42.1583, -38.815]" shape="(864,)" standard_name="latitude" units="Unit('degrees')" value_type="float32"/>
+		[-38.4268, -35.2644, -38.4268, -42.6612]]" id="72da1058" long_name="Characteristic latitude of mesh faces." points="[33.4328, 36.1226, 38.2012, ..., -44.791,
+		-42.1583, -38.815]" shape="(864,)" standard_name="latitude" units="Unit('degrees')" value_type="float32" var_name="Mesh2d_half_levels_face_y"/>
       </coord>
       <coord datadims="[2]">
         <meshCoord bounds="[[-45.0, -37.5, -37.5, -45.0],
@@ -30,8 +30,8 @@
 		..., 
 		[-112.5, -120.0, -126.958, -118.361],
 		[-120.0, -127.5, -135.0, -126.958],
-		[-127.5, -135.0, -142.5, -135.0]]" id="fac9555b" long_name="Longitude of mesh nodes." points="[-41.3152, -33.8068, -26.296, ..., -119.377,
-		-127.321, -135.0]" shape="(864,)" standard_name="longitude" units="Unit('degrees')" value_type="float32"/>
+		[-127.5, -135.0, -142.5, -135.0]]" id="b5c6bdeb" long_name="Characteristic longitude of mesh faces." points="[-41.3152, -33.8068, -26.296, ..., -119.377,
+		-127.321, -135.0]" shape="(864,)" standard_name="longitude" units="Unit('degrees')" value_type="float32" var_name="Mesh2d_half_levels_face_x"/>
       </coord>
       <coord datadims="[0]">
         <auxCoord bounds="[[21600.0, 21600.0]]" id="17a27d05" long_name="Time axis" points="[21600.0]" shape="(1,)" standard_name="time" units="Unit('seconds since 2016-01-01 15:00:00', calendar='standard')" value_type="float64" var_name="time_instant">

lib/iris/tests/results/experimental/ugrid/3D_soil_pseudo_levels.cml

@@ -20,8 +20,8 @@
 		..., 
 		[-42.7342, -40.8934, -46.161, -48.912],
 		[-40.8934, -38.4268, -42.6612, -46.161],
-		[-38.4268, -35.2644, -38.4268, -42.6612]]" id="21594c35" long_name="Latitude of mesh nodes." points="[33.4328, 36.1226, 38.2012, ..., -44.791,
-		-42.1583, -38.815]" shape="(864,)" standard_name="latitude" units="Unit('degrees')" value_type="float32"/>
+		[-38.4268, -35.2644, -38.4268, -42.6612]]" id="72da1058" long_name="Characteristic latitude of mesh faces." points="[33.4328, 36.1226, 38.2012, ..., -44.791,
+		-42.1583, -38.815]" shape="(864,)" standard_name="latitude" units="Unit('degrees')" value_type="float32" var_name="Mesh2d_half_levels_face_y"/>
       </coord>
       <coord datadims="[2]">
         <meshCoord bounds="[[-45.0, -37.5, -37.5, -45.0],
@@ -30,8 +30,8 @@
 		..., 
 		[-112.5, -120.0, -126.958, -118.361],
 		[-120.0, -127.5, -135.0, -126.958],
-		[-127.5, -135.0, -142.5, -135.0]]" id="fac9555b" long_name="Longitude of mesh nodes." points="[-41.3152, -33.8068, -26.296, ..., -119.377,
-		-127.321, -135.0]" shape="(864,)" standard_name="longitude" units="Unit('degrees')" value_type="float32"/>
+		[-127.5, -135.0, -142.5, -135.0]]" id="b5c6bdeb" long_name="Characteristic longitude of mesh faces." points="[-41.3152, -33.8068, -26.296, ..., -119.377,
+		-127.321, -135.0]" shape="(864,)" standard_name="longitude" units="Unit('degrees')" value_type="float32" var_name="Mesh2d_half_levels_face_x"/>
       </coord>
       <coord datadims="[0]">
         <auxCoord bounds="[[21600.0, 21600.0]]" id="17a27d05" long_name="Time axis" points="[21600.0]" shape="(1,)" standard_name="time" units="Unit('seconds since 2016-01-01 15:00:00', calendar='standard')" value_type="float64" var_name="time_instant">

lib/iris/tests/results/experimental/ugrid/3D_tile_pseudo_levels.cml

@@ -20,8 +20,8 @@
 		..., 
 		[-42.7342, -40.8934, -46.161, -48.912],
 		[-40.8934, -38.4268, -42.6612, -46.161],
-		[-38.4268, -35.2644, -38.4268, -42.6612]]" id="21594c35" long_name="Latitude of mesh nodes." points="[33.4328, 36.1226, 38.2012, ..., -44.791,
-		-42.1583, -38.815]" shape="(864,)" standard_name="latitude" units="Unit('degrees')" value_type="float32"/>
+		[-38.4268, -35.2644, -38.4268, -42.6612]]" id="72da1058" long_name="Characteristic latitude of mesh faces." points="[33.4328, 36.1226, 38.2012, ..., -44.791,
+		-42.1583, -38.815]" shape="(864,)" standard_name="latitude" units="Unit('degrees')" value_type="float32" var_name="Mesh2d_half_levels_face_y"/>
       </coord>
       <coord datadims="[2]">
         <meshCoord bounds="[[-45.0, -37.5, -37.5, -45.0],
@@ -30,8 +30,8 @@
 		..., 
 		[-112.5, -120.0, -126.958, -118.361],
 		[-120.0, -127.5, -135.0, -126.958],
-		[-127.5, -135.0, -142.5, -135.0]]" id="fac9555b" long_name="Longitude of mesh nodes." points="[-41.3152, -33.8068, -26.296, ..., -119.377,
-		-127.321, -135.0]" shape="(864,)" standard_name="longitude" units="Unit('degrees')" value_type="float32"/>
+		[-127.5, -135.0, -142.5, -135.0]]" id="b5c6bdeb" long_name="Characteristic longitude of mesh faces." points="[-41.3152, -33.8068, -26.296, ..., -119.377,
+		-127.321, -135.0]" shape="(864,)" standard_name="longitude" units="Unit('degrees')" value_type="float32" var_name="Mesh2d_half_levels_face_x"/>
       </coord>
       <coord datadims="[0]">
         <auxCoord bounds="[[21600.0, 21600.0]]" id="17a27d05" long_name="Time axis" points="[21600.0]" shape="(1,)" standard_name="time" units="Unit('seconds since 2016-01-01 15:00:00', calendar='standard')" value_type="float64" var_name="time_instant">

lib/iris/tests/results/experimental/ugrid/3D_veg_pseudo_levels.cml

@@ -20,8 +20,8 @@
 		..., 
 		[-42.7342, -40.8934, -46.161, -48.912],
 		[-40.8934, -38.4268, -42.6612, -46.161],
-		[-38.4268, -35.2644, -38.4268, -42.6612]]" id="21594c35" long_name="Latitude of mesh nodes." points="[33.4328, 36.1226, 38.2012, ..., -44.791,
-		-42.1583, -38.815]" shape="(864,)" standard_name="latitude" units="Unit('degrees')" value_type="float32"/>
+		[-38.4268, -35.2644, -38.4268, -42.6612]]" id="72da1058" long_name="Characteristic latitude of mesh faces." points="[33.4328, 36.1226, 38.2012, ..., -44.791,
+		-42.1583, -38.815]" shape="(864,)" standard_name="latitude" units="Unit('degrees')" value_type="float32" var_name="Mesh2d_half_levels_face_y"/>
       </coord>
       <coord datadims="[2]">
         <meshCoord bounds="[[-45.0, -37.5, -37.5, -45.0],
@@ -30,8 +30,8 @@
 		..., 
 		[-112.5, -120.0, -126.958, -118.361],
 		[-120.0, -127.5, -135.0, -126.958],
-		[-127.5, -135.0, -142.5, -135.0]]" id="fac9555b" long_name="Longitude of mesh nodes." points="[-41.3152, -33.8068, -26.296, ..., -119.377,
-		-127.321, -135.0]" shape="(864,)" standard_name="longitude" units="Unit('degrees')" value_type="float32"/>
+		[-127.5, -135.0, -142.5, -135.0]]" id="b5c6bdeb" long_name="Characteristic longitude of mesh faces." points="[-41.3152, -33.8068, -26.296, ..., -119.377,
+		-127.321, -135.0]" shape="(864,)" standard_name="longitude" units="Unit('degrees')" value_type="float32" var_name="Mesh2d_half_levels_face_x"/>
       </coord>
       <coord datadims="[0]">
         <auxCoord bounds="[[21600.0, 21600.0]]" id="17a27d05" long_name="Time axis" points="[21600.0]" shape="(1,)" standard_name="time" units="Unit('seconds since 2016-01-01 15:00:00', calendar='standard')" value_type="float64" var_name="time_instant">