added is_dataless, and corrected data manager exceptions to not be cu…

…be specific

lib/iris/_data_manager.py

@@ -25,12 +25,18 @@ def __init__(self, data, shape=None):
         data :
             The :class:`~numpy.ndarray` or :class:`~numpy.ma.core.MaskedArray`
             real data, or :class:`~dask.array.core.Array` lazy data to be
-            managed.
+            managed. If a value of None is given, the data manager will be
+            considered dataless.
+
+        shape :
+            A tuple, representing the shape of the data manager. This can only
+            be used in the case of `data=None`, and will render the data manager
+            dataless.
 
         """
         if (shape is not None) and (data is not None):
-            msg = "A cube may not be created with both data and a custom shape."
-            raise iris.exceptions.InvalidCubeError(msg)
+            msg = "`shape` should only be provided if `data is None`"
+            raise ValueError(msg)
 
         # Initialise the instance.
         self._lazy_array = None
@@ -134,12 +140,12 @@ def _assert_axioms(self):
         overfilled = self._lazy_array is not None and self._real_array is not None
         if overfilled:
             msg = "Unexpected data state, got both lazy and real data."
-            raise iris.exceptions.InvalidCubeError(msg)
+            raise ValueError(msg)
         elif (
             empty and self._shape is None
         ):  # if I remove the second check, allows empty arrays, like old behaviour
             msg = "Unexpected data state, got no lazy or real data, and no shape."
-            raise iris.exceptions.InvalidCubeError(msg)
+            raise ValueError(msg)
 
     def _deepcopy(self, memo, data=None):
         """Perform a deepcopy of the :class:`~iris._data_manager.DataManager` instance.
@@ -290,6 +296,9 @@ def shape(self):
             result = self.core_data().shape
         return result
 
+    def is_dataless(self):
+        return (self.core_data() is None) and (self.shape is not None)
+
     def copy(self, data=None):
         """Return a deep copy of this :class:`~iris._data_manager.DataManager` instance.
 

lib/iris/cube.py

@@ -2887,6 +2887,16 @@ def has_lazy_data(self) -> bool:
         """
         return self._data_manager.has_lazy_data()
 
+    def is_dataless(self) -> bool:
+        """Detail whether this :class:`~iris.cube.Cube` is dataless.
+
+        Returns
+        -------
+        bool
+
+        """
+        return self._data_manager.is_dataless()
+
     @property
     def dim_coords(self) -> tuple[DimCoord, ...]:
         """Return a tuple of all the dimension coordinates, ordered by dimension.

lib/iris/tests/unit/data_manager/test_DataManager.py

@@ -647,6 +647,19 @@ def test_with_lazy_array(self):
         self.assertTrue(dm.has_lazy_data())
         self.assertIs(result, dm._lazy_array)
 
+class Test_is_dataless(tests.IrisTest):
+    def setUp(self):
+        self.data = np.array(0)
+        self.shape = (0)
+
+    def test_with_data(self):
+        dm = DataManager(self.data)
+        self.assertFalse(dm.is_dataless())
+
+    def test_without_data(self):
+        dm = DataManager(None, self.shape)
+        self.assertTrue(dm.is_dataless())
+
 
 if __name__ == "__main__":
     tests.main()