subspace query swapped operands

Author: davidhassell

Changelog.rst

@@ -12,6 +12,9 @@ Version NEXTVERSION
 * Fix bug in `cf.Field.collapse` that causes a ``ValueError`` to be raised
   for missing external cell measures data
   (https://github.com/NCAS-CMS/cf-python/issues/885)
+* Fix `cf.Field.subspace` for a cyclic axis so that a query range
+  provided by `cf.wi` or `cf.wo` is also treated cyclicly
+  (https://github.com/NCAS-CMS/cf-python/issues/887)
 * New dependency: ``distributed>=2025.5.1``
 
 ----

cf/mixin/fielddomain.py

@@ -455,25 +455,26 @@ def _indices(self, config, data_axes, ancillary_mask, kwargs):
                         if debug:
                             logger.debug("  1-d CASE 2:")  # pragma: no cover
 
-                        # Adjust value so it's in the range [c[0],
-                        # c[0]+period)
                         value0, value1 = value.value
-                        print (value.value, value0, value1)
                         if value1 < value0:
+                            # Query has swapped operands (i.e. arg0 >
+                            # arg1) => Create a new equivalant Query
+                            # that has the arg0 < arg1. E.g. for a
+                            # period of 360, cf.wi(355, 5) is
+                            # transformed to cf.wi(355, 365).
+                            #
+                            # This is done (effectively) by repeatedly
+                            # adding the cyclic period to arg1 until
+                            # it is greater than arg0, taking into
+                            # account any units that are in play.
                             period = item.period()
-                            d = self._Data(value.value)
-                            print(repr(d))
-                            d.Units= period.Units
-                            
-                            print (-d.diff())
-                            n = (-d.diff()/period).ceil()
-                            print(repr(n))
-                            value0 = (d[0]).datum()
-                            value1 = (d[1] + n * period).datum()
-                            print(repr(d))
-                            value = wi(value0, value1, units=d.Units)
-                            
-                            print(repr(value), item.period())
+                            value = value.copy()
+                            value.set_condition_units(period.Units)
+                            value0, value1 = value.value
+                            n = ((value0 - value1) / period).ceil()
+                            value1 = value1 + n * period
+                            value = wi(value0, value1)
+
                         size = item.size
                         if item.increasing:
                             anchor = value.value[0]

cf/test/test_Field.py

@@ -1286,6 +1286,14 @@ def test_Field_indices(self):
         a[..., [0, 1, 6, 7, 8]] = np.ma.masked
         self.assertTrue(cf.functions._numpy_allclose(g.array, a), g.array)
 
+        for q in (cf.wi(315, 45), cf.wi(-45, -675)):
+            # Cyclic cf.wi with swapped operands (increasing coords)
+            indices = f.indices(grid_longitude=q)
+            g = f[indices]
+            self.assertEqual(g.shape, (1, 10, 3))
+            x = g.dimension_coordinate("X").array
+            self.assertTrue((x == [-40, 0, 40]).all())
+
         # wi (decreasing)
         f.flip("X", inplace=True)
 
@@ -1342,6 +1350,14 @@ def test_Field_indices(self):
             (x == [0, 40, 80, 120, 160, 200, 240, 280, 320][::-1]).all()
         )
 
+        for q in (cf.wi(315, 45), cf.wi(-45, -675)):
+            # Cyclic cf.wi with swapped operands (decreasing coords)
+            indices = f.indices(grid_longitude=q)
+            g = f[indices]
+            self.assertEqual(g.shape, (1, 10, 3))
+            x = g.dimension_coordinate("X").array
+            self.assertTrue((x == [40, 0, -40]).all())
+
         # wo
         f = f0.copy()