From c9bcf7388c936c9c2f41dbcc064b3bedb1ae4f74 Mon Sep 17 00:00:00 2001
From: David Hassell <david.hassell@ncas.ac.uk>
Date: Mon, 4 Mar 2024 12:36:24 +0000
Subject: [PATCH 1/2] Fix error message when can't create area weights

---
 Changelog.rst | 13 +++++++++++++
 cf/field.py   | 23 +++++++++++------------
 2 files changed, 24 insertions(+), 12 deletions(-)

diff --git a/Changelog.rst b/Changelog.rst
index e0346890b2..1d8a4bcb34 100644
--- a/Changelog.rst
+++ b/Changelog.rst
@@ -1,3 +1,14 @@
+version NEXT
+------------
+
+**2024-??-??**
+
+* Fix misleading error message when it is not possible to create area
+  weights requested from `cf.Field.collapse`
+  (https://github.com/NCAS-CMS/cf-python/issues/731)
+
+----
+
 version 3.16.1
 --------------
 
@@ -29,6 +40,8 @@ version 3.16.1
   (https://github.com/NCAS-CMS/cf-python/issues/713)
 * Changed dependency: ``1.11.1.0<=cfdm<1.11.2.0``
 
+----
+
 version 3.16.0
 --------------
 
diff --git a/cf/field.py b/cf/field.py
index 75b32313f7..c9456f27a2 100644
--- a/cf/field.py
+++ b/cf/field.py
@@ -3836,18 +3836,17 @@ def weights(
                         # Found area weights from X and Y dimension
                         # coordinates
                         area_weights = True
-                    else:
-                        Weights.polygon_area(
-                            self,
-                            None,
-                            comp,
-                            weights_axes,
-                            measure=measure,
-                            radius=radius,
-                            great_circle=great_circle,
-                            methods=methods,
-                            auto=False,
-                        )
+                    elif Weights.polygon_area(
+                        self,
+                        None,
+                        comp,
+                        weights_axes,
+                        measure=measure,
+                        radius=radius,
+                        great_circle=great_circle,
+                        methods=methods,
+                        auto=True,
+                    ):
                         # Found area weights from UGRID/geometry cells
                         area_weights = True
 

From 3c2802f42f51c8fad667f257c9c6c4a2dd5f3d19 Mon Sep 17 00:00:00 2001
From: David Hassell <david.hassell@ncas.ac.uk>
Date: Tue, 5 Mar 2024 15:26:32 +0000
Subject: [PATCH 2/2] weights error test

---
 cf/test/test_weights.py | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/cf/test/test_weights.py b/cf/test/test_weights.py
index e72c3399da..bd202672a3 100644
--- a/cf/test/test_weights.py
+++ b/cf/test/test_weights.py
@@ -326,6 +326,16 @@ def test_weights_cell_measures_coordinates(self):
         with self.assertRaises(ValueError):
             w = f.weights("area", cell_measures=False, coordinates=False)
 
+    def test_weights_exceptions(self):
+        f = cf.example_field(0)
+        f.coordinate("X").del_bounds()
+        f.coordinate("Y").del_bounds()
+
+        with self.assertRaisesRegex(
+            ValueError, "Can't create weights: Unable to find cell areas"
+        ):
+            f.weights("area")
+
 
 if __name__ == "__main__":
     print("Run date:", datetime.datetime.now())