minor optimisations (#4549)

* minor optimisations

* avoid unncecessary dict update

* whatsnew entry

docs/src/whatsnew/latest.rst

@@ -193,6 +193,10 @@ This document explains the changes made to Iris for this release
    :class:`benchmarks.HorizontalChunkedRegridding` shows a time decrease
    from >10s to 625ms. (:issue:`4280`, :pull:`4400`)
 
+#. `@bjlittle`_ included an optimisation to :class:`~iris.cube.Cube.coord_dims`
+   to avoid unnecessary processing whenever a coordinate instance that already
+   exists within the cube is provided. (:pull:`4549`)
+
 
 🔥 Deprecations
 ===============

lib/iris/_merge.py

@@ -1809,7 +1809,8 @@ def key_func(coord):
 
         # Order the coordinates by hints, axis, and definition.
         for coord in sorted(coords, key=key_func):
-            if not cube.coord_dims(coord) and coord.shape == (1,):
+            dims = tuple(cube.coord_dims(coord))
+            if not dims and coord.shape == (1,):
                 # Extract the scalar coordinate data and metadata.
                 scalar_defns.append(coord.metadata)
                 # Because we know there's a single Cell in the
@@ -1834,11 +1835,11 @@ def key_func(coord):
                 # Extract the vector coordinate and metadata.
                 if id(coord) in cube_aux_coord_ids:
                     vector_aux_coords_and_dims.append(
-                        _CoordAndDims(coord, tuple(cube.coord_dims(coord)))
+                        _CoordAndDims(coord, dims)
                     )
                 else:
                     vector_dim_coords_and_dims.append(
-                        _CoordAndDims(coord, tuple(cube.coord_dims(coord)))
+                        _CoordAndDims(coord, dims)
                     )
 
         factory_defns = []

lib/iris/cube.py

@@ -1451,39 +1451,53 @@ def coord_dims(self, coord):
             The (name of the) coord to look for.
 
         """
-
-        coord = self.coord(coord)
-
-        # Search for existing coordinate (object) on the cube, faster lookup
-        # than equality - makes no functional difference.
-        matches = [
-            (dim,)
-            for coord_, dim in self._dim_coords_and_dims
-            if coord_ is coord
-        ]
-        if not matches:
-            matches = [
-                dims
-                for coord_, dims in self._aux_coords_and_dims
-                if coord_ is coord
-            ]
-
-        # Search derived aux coords
-        if not matches:
+        name_provided = False
+        if isinstance(coord, str):
+            # Forced to look-up the coordinate if we only have the name.
+            coord = self.coord(coord)
+            name_provided = True
+
+        coord_id = id(coord)
+
+        # Dimension of dimension coordinate by object id
+        dims_by_id = {id(c): (d,) for c, d in self._dim_coords_and_dims}
+        # Check for id match - faster than equality check
+        match = dims_by_id.get(coord_id)
+
+        if match is None:
+            # Dimension/s of auxiliary coordinate by object id
+            aux_dims_by_id = {id(c): d for c, d in self._aux_coords_and_dims}
+            # Check for id match - faster than equality
+            match = aux_dims_by_id.get(coord_id)
+            if match is None:
+                dims_by_id.update(aux_dims_by_id)
+
+        if match is None and not name_provided:
+            # We may have an equivalent coordinate but not the actual
+            # cube coordinate instance - so forced to perform coordinate
+            # lookup to attempt to retrieve it
+            coord = self.coord(coord)
+            # Check for id match - faster than equality
+            match = dims_by_id.get(id(coord))
+
+        # Search derived aux coordinates
+        if match is None:
             target_metadata = coord.metadata
 
-            def match(factory):
+            def matcher(factory):
                 return factory.metadata == target_metadata
 
-            factories = filter(match, self._aux_factories)
+            factories = filter(matcher, self._aux_factories)
             matches = [
                 factory.derived_dims(self.coord_dims) for factory in factories
             ]
+            if matches:
+                match = matches[0]
 
-        if not matches:
+        if match is None:
             raise iris.exceptions.CoordinateNotFoundError(coord.name())
 
-        return matches[0]
+        return match
 
     def cell_measure_dims(self, cell_measure):
         """