refactored DatalessError (#6865)

Co-authored-by: Chris Bunney <[email protected]>

Author: ESadek-MO

lib/iris/_lazy_data.py

@@ -370,7 +370,7 @@ def _co_realise_lazy_arrays(arrays):
         if real_out is not None:
             real_out = np.asanyarray(real_out)
         else:
-            raise iris.exceptions.DatalessError("realising")
+            raise iris.exceptions.DatalessError("Data is None; cannot be realised.")
         if isinstance(real_out, ma.core.MaskedConstant):
             # Convert any masked constants into NumPy masked arrays.
             # NOTE: in this case, also apply the original lazy-array dtype, as

lib/iris/cube.py

@@ -5374,7 +5374,9 @@ def interpolate(
 
         """
         if self.is_dataless():
-            raise iris.exceptions.DatalessError("interpolate")
+            raise iris.exceptions.DatalessError(
+                "Dataless cubes cannot be interpolated."
+            )
         coords, points = zip(*sample_points)
         interp = scheme.interpolator(self, coords)  # type: ignore[arg-type]
         return interp(points, collapse_scalar=collapse_scalar)
@@ -5421,7 +5423,7 @@ def regrid(self, grid: Cube, scheme: iris.analysis.RegriddingScheme) -> Cube:
 
         """
         if self.is_dataless():
-            raise iris.exceptions.DatalessError("regrid")
+            raise iris.exceptions.DatalessError("Dataless cubes cannot be regridded.")
         regridder = scheme.regridder(self, grid)
         return regridder(self)
 

lib/iris/exceptions.py

@@ -170,14 +170,9 @@ class CannotAddError(ValueError):
 
 
 class DatalessError(ValueError):
-    """Raised when an method cannot be performed on a dataless :class:`~iris.cube.Cube`."""
+    """Raised when a method cannot be performed on a dataless :class:`~iris.cube.Cube`."""
 
-    def __str__(self):
-        msg = (
-            "Dataless cubes are still early in implementation, and dataless {} "
-            "operations are not currently supported."
-        )
-        return msg.format(super().__str__())
+    pass
 
 
 class CFParseError(IrisError):