Revert Cell.__hash__ fix

docs/src/whatsnew/latest.rst

@@ -140,6 +140,10 @@ This document explains the changes made to Iris for this release
    attribute will be silently changed to "standard".  This may cause failures in
    code that explicitly checks the calendar attribute. (:pull:`4847`)
 
+#. `@bjlittle`_ accounted for the recent changes in ``python`` >=3.10.0 when
+   hashing a ``nan``. Specifically, a ``nan`` scalar coordinate will be
+   promoted to a vector ``nan`` coordinate during cube merge.
+
 
 🚀 Performance Enhancements
 ===========================
@@ -249,7 +253,7 @@ 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
@@ -1363,20 +1345,7 @@ 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):
         """

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

@@ -37,6 +37,10 @@ def test_form_contiguous_dimcoord(self):
 
 class TestNaNs(tests.IrisTest):
     def test_merge_nan_coords(self):
+        from sys import version_info
+
+        from pkg_resources import parse_version
+
         # 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")
@@ -50,9 +54,20 @@ def test_merge_nan_coords(self):
         cubes = CubeList(cube1.slices_over("x"))
         cube2 = cubes.merge_cube()
 
+        # Account for change in behaviour for py310+ when hashing a NaN
+        # Reference https://github.com/SciTools/iris/pull/4874
+        version = (
+            f"{version_info.major}.{version_info.minor}.{version_info.micro}"
+        )
+        if parse_version(version) >= parse_version("3.10.0"):
+            # vector coordinate
+            expected = np.array([np.nan] * cube1.shape[0])
+        else:
+            # scalar coordinate
+            expected = nan_coord1.points
+
         self.assertArrayEqual(
-            np.isnan(nan_coord1.points),
-            np.isnan(cube2.coord("nan1").points),
+            np.isnan(expected), np.isnan(cube2.coord("nan1").points)
         )
         self.assertArrayEqual(
             np.isnan(nan_coord2.points),

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

@@ -155,24 +155,6 @@ def test_nan_other(self):
         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):
         point = object()