Make the Coord.cell method lazy (#5693)

Co-authored-by: Elias <110238618+ESadek-MO@users.noreply.github.com>

docs/src/whatsnew/latest.rst

@@ -104,10 +104,14 @@ This document explains the changes made to Iris for this release
    lazy data from file. This will also speed up coordinate comparison.
    (:pull:`5610`)
 
+#. `@bouweandela`_ changed :func:`iris.coords.Coord.cell` so it does not realize
+   all coordinate data and only loads a single cell instead. (:pull:`5693`)
+
 #. `@rcomer`_ and `@trexfeathers`_ (reviewer) modified 
    :func:`~iris.analysis.stats.pearsonr` so it preserves lazy data in all cases
    and also runs a little faster.  (:pull:`5638`)
 
+
 🔥 Deprecations
 ===============
 

lib/iris/coords.py

@@ -2075,7 +2075,7 @@ def cell(self, index):
         """
         index = iris.util._build_full_slice_given_keys(index, self.ndim)
 
-        point = tuple(np.array(self.points[index], ndmin=1).flatten())
+        point = tuple(np.array(self.core_points()[index], ndmin=1).flatten())
         if len(point) != 1:
             raise IndexError(
                 "The index %s did not uniquely identify a single "
@@ -2084,7 +2084,7 @@ def cell(self, index):
 
         bound = None
         if self.has_bounds():
-            bound = tuple(np.array(self.bounds[index], ndmin=1).flatten())
+            bound = tuple(np.array(self.core_bounds()[index], ndmin=1).flatten())
 
         if self.units.is_time_reference():
             point = self.units.num2date(point)

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

@@ -240,9 +240,12 @@ def _mock_coord(self):
         coord = mock.Mock(
             spec=Coord,
             ndim=1,
-            points=np.array([mock.sentinel.time]),
-            bounds=np.array([[mock.sentinel.lower, mock.sentinel.upper]]),
         )
+        coord.core_points = lambda: np.array([mock.sentinel.time])
+        coord.core_bounds = lambda: np.array(
+            [[mock.sentinel.lower, mock.sentinel.upper]]
+        )
+
         return coord
 
     def test_time_as_object(self):