fix convert_units from unknown

Author: rcomer

docs/iris/src/whatsnew/2.0a0.rst

@@ -91,6 +91,10 @@ Bugs Fixed
   :class:`~iris.cube.Cube` with :data:`integer` or :data:`boolean` data with
   a :data:`float` result will raise an :data:`ArithmeticError` exception.
 
+* Attempting to use the :meth:`iris.cube.Cube.convert_units` method on a cube
+  where the existing unit is unknown will raise a :data:`UnitConversionError`
+  exception.
+
 
 Incompatible Changes
 ====================

lib/iris/cube.py

@@ -880,8 +880,11 @@ def convert_units(self, unit):
 
         """
         # If the cube has units convert the data.
-        if not self.units.is_unknown():
-            self.data = self.units.convert(self.data, unit)
+        if self.units.is_unknown():
+            raise iris.exceptions.UnitConversionError(
+                'cannot convert from unknown units to {!s}, '
+                'set the "units" attribute instead'.format(unit))
+        self.data = self.units.convert(self.data, unit)
         self.units = unit
 
     def add_cell_method(self, cell_method):

lib/iris/exceptions.py

@@ -1,4 +1,4 @@
-# (C) British Crown Copyright 2010 - 2016, Met Office
+# (C) British Crown Copyright 2010 - 2017, Met Office
 #
 # This file is part of Iris.
 #
@@ -152,3 +152,8 @@ def __init__(self, msg):
 
 class LazyAggregatorError(Exception):
     pass
+
+
+class UnitConversionError(IrisError):
+    """Raised when Iris is unable to convert a unit."""
+    pass

lib/iris/tests/unit/cube/test_Cube.py

@@ -37,7 +37,8 @@
 from iris.analysis import MEAN
 from iris.cube import Cube
 from iris.coords import AuxCoord, DimCoord, CellMeasure
-from iris.exceptions import CoordinateNotFoundError, CellMeasureNotFoundError
+from iris.exceptions import (CoordinateNotFoundError, CellMeasureNotFoundError,
+                             UnitConversionError)
 from iris.tests import mock
 import iris.tests.stock as stock
 from iris._lazy_data import as_lazy_data
@@ -1008,6 +1009,11 @@ def test_wrap_radians(self):
         self.assertEqual(result.data[0, 0, 0], 303)
         self.assertEqual(result.data[0, 0, -1], 28)
 
+    def test_convert_unknown_units(self):
+        cube = iris.cube.Cube(1)
+        with self.assertRaises(UnitConversionError):
+            cube.convert_units('mm day-1')
+
     def test_tolerance_bug(self):
         # Floating point changes introduced by wrapping mean
         # the resulting coordinate values are not equal to their