Merge fffe32e into 4585059

Author: stephenworsley

lib/iris/experimental/ugrid/cf.py

@@ -1275,6 +1275,42 @@ def __repr__(self):
         return "<%s of %s>" % (self.__class__.__name__, ", ".join(result))
 
 
+class CFUGridGroup(CFGroup):
+    """Represents a collection of CF Metadata Conventions variables and netCDF global attributes.
+
+    Represents a collection of 'NetCDF Climate and Forecast (CF) Metadata
+    Conventions' variables and netCDF global attributes.
+
+    Specialisation of :class:`~iris.fileformats.cf.CFGroup` that includes extra
+    collections for CF-UGRID-specific variable types.
+
+    """
+
+    @property
+    def connectivities(self):
+        """Collection of CF-UGRID connectivity variables."""
+        return self._cf_getter(CFUGridConnectivityVariable)
+
+    @property
+    def ugrid_coords(self):
+        """Collection of CF-UGRID-relevant auxiliary coordinate variables."""
+        return self._cf_getter(CFUGridAuxiliaryCoordinateVariable)
+
+    @property
+    def meshes(self):
+        """Collection of CF-UGRID mesh variables."""
+        return self._cf_getter(CFUGridMeshVariable)
+
+    @property
+    def non_data_variable_names(self):
+        """:class:`set` of names of the CF-netCDF/CF-UGRID variables that are not the data pay-load."""
+        extra_variables = (self.connectivities, self.ugrid_coords, self.meshes)
+        extra_result = set()
+        for variable in extra_variables:
+            extra_result |= set(variable)
+        return super().non_data_variable_names | extra_result
+
+
 ################################################################################
 class CFReader:
     """Allows the contents of a netCDF file to be interpreted.
@@ -1299,7 +1335,7 @@ class CFReader:
         CFUGridMeshVariable,
     )
 
-    CFGroup = CFGroup
+    CFGroup = CFUGridGroup
 
     def __init__(self, file_source, warn=False, monotonic=False):
         # Ensure safe operation for destructor, should init fail.
@@ -1331,6 +1367,11 @@ def __init__(self, file_source, warn=False, monotonic=False):
 
         self._check_monotonic = monotonic
 
+        self._with_ugrid = True
+        if not self._has_meshes():
+            self._trim()
+            self._with_ugrid = False
+
         self._translate()
         self._build_cf_groups()
         self._reset()
@@ -1348,6 +1389,26 @@ def __exit__(self, exc_type, exc_value, traceback):
         # When used as a context-manager, **always** close the file on exit.
         self._close()
 
+    def _has_meshes(self):
+        result = False
+        for _, variable in self._dataset.variables.items():
+            if hasattr(variable, "mesh"):
+                result = True
+                break
+        return result
+
+    def _trim(self):
+        self._variable_types = (
+            CFAncillaryDataVariable,
+            CFAuxiliaryCoordinateVariable,
+            CFBoundaryVariable,
+            CFClimatologyVariable,
+            CFGridMappingVariable,
+            CFLabelVariable,
+            CFMeasureVariable,
+        )
+        self.CFGroup = CFGroup
+
     @property
     def filename(self):
         """The file that the CFReader is reading."""

lib/iris/fileformats/cf.py

@@ -56,6 +56,8 @@ def _meshes_from_cf(cf_reader):
     # Mesh instances are shared between file phenomena.
     # TODO: more sophisticated Mesh sharing between files.
     # TODO: access external Mesh cache?
+    if not cf_reader._with_ugrid:
+        return {}
     mesh_vars = cf_reader.cf_group.meshes
     meshes = {
         name: _build_mesh(cf_reader, var, cf_reader.filename)