Use partial to make cube pickleable

docs/src/whatsnew/latest.rst

@@ -124,10 +124,6 @@ This document explains the changes made to Iris for this release
 #. `@rcomer`_ fixed :meth:`~iris.cube.Cube.subset` to alway return ``None`` if
    no value match is found.  (:pull:`4417`)
 
-#. `@wjbenfold`_ resolved an issue that previously caused regridding with lazy
-   data to take significantly longer than with real data. Relevant benchmark
-   shows a time decrease from >10s to 625ms. (:issue:`4280`, :pull:`4400`)
-
 #. `@wjbenfold`_ changed :meth:`iris.util.points_step` to stop it from warning
    when applied to a single point (:issue:`4250`, :pull:`4367`)
 
@@ -140,6 +136,10 @@ This document explains the changes made to Iris for this release
    coordinate bounds using minimum and maximum for unordered coordinates,
    fixing :issue:`1528`. (:pull:`4315`)
 
+#. `@wjbenfold`_ changed how a delayed unit conversion is performed on a cube
+   so that a cube with lazy data awaiting a unit conversion can be pickled.
+   (:issue:`4354 `, :pull:`4377`)
+
 
 💣 Incompatible Changes
 =======================
@@ -150,7 +150,10 @@ This document explains the changes made to Iris for this release
 🚀 Performance Enhancements
 ===========================
 
-#. N/A
+#. `@wjbenfold`_ resolved an issue that previously caused regridding with lazy
+   data to take significantly longer than with real data. Benchmark
+   :class:`benchmarks.HorizontalChunkedRegridding` shows a time decrease
+   from >10s to 625ms. (:issue:`4280`, :pull:`4400`)
 
 
 🔥 Deprecations

lib/iris/cube.py

@@ -1051,9 +1051,7 @@ def convert_units(self, unit):
             old_unit = self.units
             new_unit = unit
 
-            # Define a delayed conversion operation (i.e. a callback).
-            def pointwise_convert(values):
-                return old_unit.convert(values, new_unit)
+            pointwise_convert = partial(old_unit.convert, other=new_unit)
 
             new_data = _lazy.lazy_elementwise(
                 self.lazy_data(), pointwise_convert

lib/iris/tests/test_pickling.py

@@ -16,9 +16,10 @@
 import pickle
 
 import cf_units
+import numpy as np
 
 import iris
-from iris._lazy_data import as_concrete_data
+from iris._lazy_data import as_concrete_data, as_lazy_data
 
 
 class TestPickle(tests.IrisTest):
@@ -76,6 +77,14 @@ def test_cube_with_coord_points(self):
         _, recon_cube = next(self.pickle_then_unpickle(cube))
         self.assertEqual(recon_cube, cube)
 
+    def test_cube_with_deferred_unit_conversion(self):
+        real_data = np.arange(12.0).reshape((3, 4))
+        lazy_data = as_lazy_data(real_data)
+        cube = iris.cube.Cube(lazy_data, units="m")
+        cube.convert_units("ft")
+        _, recon_cube = next(self.pickle_then_unpickle(cube))
+        self.assertEqual(recon_cube, cube)
+
     @tests.skip_data
     def test_cubelist_pickle(self):
         cubelist = iris.load(