From 3ea65654e36c88e83266ee8e79f1995929143135 Mon Sep 17 00:00:00 2001
From: David Hassell <david.hassell@ncas.ac.uk>
Date: Tue, 1 Mar 2022 15:31:20 +0000
Subject: [PATCH 1/2] error messages, tests

---
 cf/data/data.py      | 14 ++++++--------
 cf/test/test_Data.py | 21 +++++++++++++++------
 2 files changed, 21 insertions(+), 14 deletions(-)

diff --git a/cf/data/data.py b/cf/data/data.py
index 068b309d90..a6b3393e3e 100644
--- a/cf/data/data.py
+++ b/cf/data/data.py
@@ -10558,15 +10558,13 @@ def datum(self, *index):
                     index = tuple(index)
                 else:
                     raise ValueError(
-                        "Incorrect number of indices for {} array".format(
-                            self.__class__.__name__
-                        )
+                        f"Incorrect number of indices ({n_index}) for "
+                        f"{self.ndim}-d {self.__class__.__name__} data"
                     )
             elif n_index != self.ndim:
                 raise ValueError(
-                    "Incorrect number of indices for {} array".format(
-                        self.__class__.__name__
-                    )
+                    f"Incorrect number of indices ({n_index}) for "
+                    f"{self.ndim}-d {self.__class__.__name__} data"
                 )
 
             array = self[index].array
@@ -10576,8 +10574,8 @@ def datum(self, *index):
 
         else:
             raise ValueError(
-                "Can only convert a {} array of size 1 to a "
-                "Python scalar".format(self.__class__.__name__)
+                f"For size {self.size} data, must provide an index of "
+                "the element to be converted to a Python scalar"
             )
 
         if not np.ma.isMA(array):
diff --git a/cf/test/test_Data.py b/cf/test/test_Data.py
index 53eed8b5e6..2224b1f411 100644
--- a/cf/test/test_Data.py
+++ b/cf/test/test_Data.py
@@ -1771,12 +1771,7 @@ def test_Data_datum(self):
             self.assertEqual(d.datum(-1), 3)
             for index in d.ndindex():
                 self.assertEqual(d.datum(index), d.array[index].item())
-                self.assertEqual(
-                    d.datum(*index),
-                    d.array[index].item(),
-                    "{}, {}".format(d.datum(*index), d.array[index].item()),
-                )
-        # --- End: for
+                self.assertEqual(d.datum(*index), d.array[index].item())
 
         d = cf.Data(5, "metre")
         d[()] = cf.masked
@@ -1795,6 +1790,20 @@ def test_Data_datum(self):
         self.assertIs(d.datum([0, -1]), cf.masked)
         self.assertIs(d.datum(-1, -1), cf.masked)
 
+        d = cf.Data([1, 2])
+        with self.assertRaises(ValueError):
+            d.datum()
+
+        with self.assertRaises(ValueError):
+            d.datum(3)
+
+        with self.assertRaises(ValueError):
+            d.datum(0, 0)
+
+        d = cf.Data([[1, 2]])
+        with self.assertRaises(ValueError):
+            d.datum((0,))
+
     def test_Data_flip(self):
         if self.test_only and inspect.stack()[0][3] not in self.test_only:
             return

From adcec58fddd34a3b623cd982ec873101ad93aaa5 Mon Sep 17 00:00:00 2001
From: David Hassell <david.hassell@ncas.ac.uk>
Date: Tue, 1 Mar 2022 15:33:23 +0000
Subject: [PATCH 2/2] daskified

---
 cf/data/data.py | 1 +
 1 file changed, 1 insertion(+)

diff --git a/cf/data/data.py b/cf/data/data.py
index a6b3393e3e..3db1024372 100644
--- a/cf/data/data.py
+++ b/cf/data/data.py
@@ -10443,6 +10443,7 @@ def in_memory(self):
 
         return True
 
+    @daskified(_DASKIFIED_VERBOSE)
     def datum(self, *index):
         """Return an element of the data array as a standard Python
         scalar.