Change Cell equality to equate nans (fixes #4681) (#4701)

* nans in cells satisfy equality

* [pre-commit.ci] auto fixes from pre-commit.com hooks

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* address review comments

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docs/src/whatsnew/latest.rst

@@ -84,6 +84,12 @@ This document explains the changes made to Iris for this release
  to ``stderr`` when a :class:`~iris.fileformats.cf.CFReader` that fails to
  initialise is garbage collected. (:issue:`3312`, :pull:`4646`)
 
+#. `@stephenworsley`_ aligned the behaviour of :obj:`~iris.coords.Cell` equality
+ to match :obj:`~iris.coords.Coord` equality with respect to NaN values.
+ Two NaN valued Cells are now considered equal. This fixes :issue:`4681` and
+ causes NaN valued scalar coordinates to be able to merge be preserved during
+ cube merging. (:pull:`4701`)
+
 
 💣 Incompatible Changes
 =======================

lib/iris/coords.py

@@ -1353,14 +1353,27 @@ def __eq__(self, other):
  compared.
 
  """
+
+ def nan_equality(x, y):
+ return (
+ isinstance(x, (float, np.number))
+ and np.isnan(x)
+ and isinstance(y, (float, np.number))
+ and np.isnan(y)
+ )
+
  if isinstance(other, (int, float, np.number)) or hasattr(
  other, "timetuple"
  ):
  if self.bound is not None:
  return self.contains_point(other)
+ elif nan_equality(self.point, other):
+ return True
  else:
  return self.point == other
  elif isinstance(other, Cell):
+ if nan_equality(self.point, other.point):
+ return True
  return (self.point == other.point) and (
  self.bound == other.bound or self.bound == other.bound[::-1]
  )

lib/iris/tests/integration/merge/test_merge.py

@@ -12,7 +12,9 @@
 # before importing anything else.
 import iris.tests as tests # isort:skip
 
-from iris.coords import DimCoord
+import numpy as np
+
+from iris.coords import AuxCoord, DimCoord
 from iris.cube import Cube, CubeList
 
 
@@ -33,5 +35,30 @@ def test_form_contiguous_dimcoord(self):
  self.assertArrayEqual(coord2.bounds, coord1.bounds[::-1, ::-1])
 
 
+class TestNaNs(tests.IrisTest):
+ def test_merge_nan_coords(self):
+ # Test that nan valued coordinates merge together.
+ cube1 = Cube(np.ones([3, 4]), "air_temperature", units="K")
+ coord1 = DimCoord([1, 2, 3], long_name="x")
+ coord2 = DimCoord([0, 1, 2, 3], long_name="y")
+ nan_coord1 = AuxCoord(np.nan, long_name="nan1")
+ nan_coord2 = AuxCoord([np.nan] * 4, long_name="nan2")
+ cube1.add_dim_coord(coord1, 0)
+ cube1.add_dim_coord(coord2, 1)
+ cube1.add_aux_coord(nan_coord1)
+ cube1.add_aux_coord(nan_coord2, 1)
+ cubes = CubeList(cube1.slices_over("x"))
+ cube2 = cubes.merge_cube()
+
+ self.assertArrayEqual(
+ np.isnan(nan_coord1.points),
+ np.isnan(cube2.coord("nan1").points),
+ )
+ self.assertArrayEqual(
+ np.isnan(nan_coord2.points),
+ np.isnan(cube2.coord("nan2").points),
+ )
+
+
 if __name__ == "__main__":
  tests.main()

lib/iris/tests/unit/coords/test_Cell.py

@@ -146,6 +146,14 @@ def test_PartialDateTime_other(self):
  self.assertNotEqual(cell, PartialDateTime(month=3, hour=12))
  self.assertNotEqual(cell, PartialDateTime(month=4))
 
+ def test_nan_other(self):
+ # Check that nans satisfy equality.
+ cell1 = Cell(np.nan)
+ cell2 = Cell(np.nan)
+ self.assertEqual(cell1, np.nan)
+ self.assertEqual(np.nan, cell1)
+ self.assertEqual(cell1, cell2)
+
 
 class Test_contains_point(tests.IrisTest):
  def test_datetimelike_bounded_cell(self):