Fix bug that caused wrong directions from cf.DimensionCoordinate.direction

Changelog.rst

@@ -16,6 +16,9 @@ version 3.17.0
 * Fix bug that caused `Data._axes` to be incorrect after a call to
   `cf.Field.collapse`
   (https://github.com/NCAS-CMS/cf-python/issues/857)
+* Fix bug that caused wrong directions from
+  `cf.DimensionCoordinate.direction`
+  (https://github.com/NCAS-CMS/cf-python/issues/859)
 * Changed dependency: ``Python>=3.9.0``
 * Changed dependency: ``numpy>=2.0.0``
 * Changed dependency: ``cfdm>=1.12.0.0, <1.12.1.0``

cf/dimensioncoordinate.py

@@ -131,9 +131,9 @@ def _infer_direction(self):
         """Return True if a coordinate is increasing, otherwise return
         False.
 
-        A dimension coordinate construct is considered to be increasing if
-        its data array values are increasing in index space, or if it has
-        no data nor bounds.
+        A dimension coordinate construct is considered to be
+        increasing if its data array values are not strictly
+        decreasing in index space, or if it has no data nor bounds.
 
         If the direction can not be inferred from the data not bounds then
         the coordinate's units are used.
@@ -168,12 +168,12 @@ def _infer_direction(self):
                 c = data._get_cached_elements()
                 if c:
                     try:
-                        return bool(c.get(0) < c.get(1))
+                        return bool(c.get(0) <= c.get(1))
                     except TypeError:
                         pass
 
                 data = data[:2].compute()
-                return bool(data.item(0) < data.item(1))
+                return bool(data.item(0) <= data.item(1))
 
         # Still here?
         data = self.get_bounds_data(None, _fill_value=False)
@@ -182,12 +182,12 @@ def _infer_direction(self):
             c = data._get_cached_elements()
             if c:
                 try:
-                    return bool(c.get(0) < c.get(1))
+                    return bool(c.get(0) <= c.get(1))
                 except TypeError:
                     pass
 
             b = data[0].compute()
-            return bool(b.item(0) < b.item(1))
+            return bool(b.item(0) <= b.item(1))
 
         # Still here? Then infer the direction from the units.
         return not self.Units.ispressure

cf/test/test_DimensionCoordinate.py

@@ -809,6 +809,30 @@ def test_DimensionCoordinate_anchor(self):
         self.assertEqual(e[0].array, d[0].array - 360)
 
 
+    def test_DimensionCoordinate_direction(self):
+        """Test DimensionCoordinate.direction"""
+        d = self.dim.copy()
+
+        # Test the use case of
+        # https://github.com/NCAS-CMS/cf-python/issues/859
+        #
+        # Create a coordinate with all equal values
+        d.data[...] = d[0].array
+        for i, x in enumerate(d.array):
+            d.bounds.data[i, :] = x
+
+        d._custom["direction"] = None  # Force a re-calculation of direction
+        self.assertTrue(d.direction())
+        d._custom["direction"] = None
+        self.assertTrue(d[0].direction())
+
+        d.del_bounds()
+        d._custom["direction"] = None
+        self.assertTrue(d.direction())
+        d._custom["direction"] = None
+        self.assertTrue(d[0].direction())
+
+
 if __name__ == "__main__":
     print("Run date:", datetime.datetime.now())
     cf.environment()