Revert Cell.__hash__ fix

docs/src/whatsnew/latest.rst

@@ -111,12 +111,6 @@ 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`)
-
 #. `@wjbenfold`_ fixed plotting of circular coordinates to extend kwarg arrays
    as well as the data. (:issue:`466`, :pull:`4649`)
 
@@ -249,7 +243,7 @@ This document explains the changes made to Iris for this release
 
 #. `@bjlittle`_ and `@jamesp`_ (reviewer) and `@lbdreyer`_ (reviewer) extended
    the GitHub Continuous-Integration to cover testing on ``py38``, ``py39``,
-   and ``py310``. (:pull:`4840` and :pull:`4852`)
+   and ``py310``. (:pull:`4840`)
 
 #. `@bjlittle`_ and `@trexfeathers`_ (reviewer) adopted `setuptools-scm`_ for
    automated ``iris`` package versioning. (:pull:`4841`)

lib/iris/coords.py

@@ -1266,13 +1266,7 @@ def _get_2d_coord_bound_grid(bounds):
     return result
 
 
-class _Hash:
-    """Mutable hash property"""
-
-    slots = ("_hash",)
-
-
-class Cell(namedtuple("Cell", ["point", "bound"]), _Hash):
+class Cell(namedtuple("Cell", ["point", "bound"])):
     """
     An immutable representation of a single cell of a coordinate, including the
     sample point and/or boundary position.
@@ -1332,18 +1326,6 @@ def __new__(cls, point=None, bound=None):
 
         return super().__new__(cls, point, bound)
 
-    def __init__(self, *args, **kwargs):
-        # Pre-compute the hash value of this instance at creation time based
-        # on the Cell.point alone. This results in a significant performance
-        # gain, as Cell.__hash__ is reduced to a minimalist attribute lookup
-        # for each invocation.
-        try:
-            value = 0 if np.isnan(self.point) else hash((self.point,))
-        except TypeError:
-            # Passing a string to np.isnan causes this exception.
-            value = hash((self.point,))
-        self._hash = value
-
     def __mod__(self, mod):
         point = self.point
         bound = self.bound
@@ -1363,48 +1345,22 @@ def __add__(self, mod):
         return Cell(point, bound)
 
     def __hash__(self):
-        # Required for >py39 and >np1.22.x due to changes in Cell behaviour for
-        # Cell.point=np.nan, as calling super().__hash__() returns a different
-        # hash each time and thus does not trigger the following call to
-        # Cell.__eq__ to determine equality.
-        # Note that, no explicit Cell.bound nan check is performed here.
-        # That is delegated to Cell.__eq__ instead. It's imperative we keep
-        # Cell.__hash__ light-weight to minimise performance degradation.
-        # Also see Cell.__init__ for Cell._hash assignment.
-        # Reference:
-        #   - https://bugs.python.org/issue43475
-        #   - https://github.com/numpy/numpy/issues/18833
-        #   - https://github.com/numpy/numpy/pull/18908
-        #   - https://github.com/numpy/numpy/issues/21210
-        return self._hash
+        return super().__hash__()
 
     def __eq__(self, other):
         """
         Compares Cell equality depending on the type of the object to be
         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

@@ -12,9 +12,7 @@
 # before importing anything else.
 import iris.tests as tests  # isort:skip
 
-import numpy as np
-
-from iris.coords import AuxCoord, DimCoord
+from iris.coords import DimCoord
 from iris.cube import Cube, CubeList
 
 
@@ -35,30 +33,5 @@ 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

@@ -146,32 +146,6 @@ 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___hash__(tests.IrisTest):
-    def test_nan__hash(self):
-        cell = Cell(np.nan, None)
-        self.assertEqual(cell._hash, 0)
-
-    def test_nan___hash__(self):
-        cell = Cell(np.nan, None)
-        self.assertEqual(cell.__hash__(), 0)
-
-    def test_non_nan__hash(self):
-        cell = Cell(1, None)
-        self.assertNotEqual(cell._hash, 0)
-
-    def test_non_nan___hash__(self):
-        cell = Cell("two", ("one", "three"))
-        self.assertNotEqual(cell.__hash__(), 0)
-
 
 class Test_contains_point(tests.IrisTest):
     def test_datetimelike_bounded_cell(self):