From 8637dcd366852cbe55e47595a7435d42dbdd4d8e Mon Sep 17 00:00:00 2001
From: daflack <david.flack1@metoffice.gov.uk>
Date: Fri, 17 Nov 2023 15:56:37 +0000
Subject: [PATCH] Fix filter operator for filtering cube

fixes #256
---
 src/CSET/operators/filters.py   | 4 +++-
 tests/operators/test_filters.py | 4 ++++
 2 files changed, 7 insertions(+), 1 deletion(-)

diff --git a/src/CSET/operators/filters.py b/src/CSET/operators/filters.py
index 432fd46a7..48c12c9cd 100644
--- a/src/CSET/operators/filters.py
+++ b/src/CSET/operators/filters.py
@@ -45,7 +45,9 @@ def filter_cubes(
         If the constraint doesn't produce a single cube.
     """
     filtered_cubes = cube.extract(constraint)
-
+    # Return directly if already a cube.
+    if isinstance(filtered_cubes, iris.cube.Cube):
+        return filtered_cubes
     # Check filtered cubes is a CubeList containing one cube.
     if len(filtered_cubes) == 1:
         return filtered_cubes[0]
diff --git a/tests/operators/test_filters.py b/tests/operators/test_filters.py
index abd912fe9..41470653c 100644
--- a/tests/operators/test_filters.py
+++ b/tests/operators/test_filters.py
@@ -26,6 +26,7 @@ def test_filters_operator():
         constraints.generate_stash_constraint("m01s03i236"),
         a=constraints.generate_cell_methods_constraint([]),
     )
+    # Test filtering a CubeList.
     cube = filters.filter_cubes(cubes, constraint)
     assert cube.cell_methods == ()
     expected_cube = "<iris 'Cube' of air_temperature / (K) (time: 3; grid_latitude: 17; grid_longitude: 13)>"
@@ -34,3 +35,6 @@ def test_filters_operator():
     constraint = constraints.generate_stash_constraint("m01s03i236")
     with pytest.raises(ValueError):
         cube = filters.filter_cubes(cubes, constraint)
+    # Test filtering a Cube.
+    single_cube = filters.filter_cubes(cube, constraint)
+    assert repr(cube) == repr(single_cube)