Make MeshCoords immutable and sync updates with the attached mesh

docs/src/whatsnew/latest.rst

@@ -40,6 +40,12 @@ This document explains the changes made to Iris for this release
    now preserved when it was not previously. See also: :ref:`load-problems`.
    (:pull:`6465`, :pull:`6529`)
 
+#. `@ESadek-MO`_ made MeshCoords immutable. :class:`iris.MeshCoord`s are now updated automatically when
+   changing the attached mesh. All changes to the :class:`iris.MeshCoord` should instead be done to
+   the relevant :class:`iris.Coord` located on the attached :class:`iris.MeshXY`. This change also affects 
+   the behaviour when calling :attr:`iris.MeshCoord.points` and :attr:`MeshCoord.bounds`, which will return
+   real data but will leave the :class:`iris.MeshCoord` (and attached mesh) lazy. (:issue:`4757`, :pull:`6405`)
+
 
 🐛 Bugs Fixed
 =============

lib/iris/coords.py

@@ -98,6 +98,8 @@ def __init__(
         if not hasattr(self, "_metadata_manager"):
             self._metadata_manager = metadata_manager_factory(BaseMetadata)
 
+        self._mesh_timestamps = []
+
         #: CF standard name of the quantity that the metadata represents.
         self.standard_name = standard_name
 
@@ -569,6 +571,12 @@ def reindent_data_string(text, n_indent):
 
         return "\n".join(output_lines)
 
+    def __setattr__(self, key, value):
+        if getattr(self, "_mesh_timestamps", None) is not None:
+            for timestamp in self._mesh_timestamps:
+                timestamp.update()
+        object.__setattr__(self, key, value)
+
     def __str__(self):
         return self.summary()