[Python] Raise exception when setting composition with empty array

interfaces/cython/cantera/test/test_thermo.py

@@ -217,8 +217,16 @@ def test_setCompositionSingleSpecies(self):
         self.assertEqual(gas.Y[0], 1.0)
 
     def test_setCompositionSlice_bad(self):
+        X0 = self.phase.X
         with self.assertRaises(ValueError):
             self.phase['H2','O2'].Y = [0.1, 0.2, 0.3]
+        self.assertArrayNear(self.phase.X, X0)
+
+    def test_setCompositionEmpty_bad(self):
+        X0 = self.phase.X
+        with self.assertRaises(ValueError):
+            self.phase.Y = np.array([])
+        self.assertArrayNear(self.phase.X, X0)
 
     def test_set_equivalence_ratio_stoichiometric(self):
         gas = ct.Solution('gri30.xml')

interfaces/cython/cantera/thermo.pyx

@@ -497,14 +497,15 @@ cdef class ThermoPhase(_SolutionBase):
 
         if len(values) == self.n_species:
             data = np.ascontiguousarray(values, dtype=np.double)
-        elif len(values) == len(self._selected_species):
+        elif len(values) == len(self._selected_species) != 0:
             data = np.zeros(self.n_species, dtype=np.double)
             for i,k in enumerate(self._selected_species):
                 data[k] = values[i]
         else:
-            raise ValueError("Array has incorrect length."
-                " Got {}. Expected {} or {}.".format(
-                    len(values), self.n_species, len(self._selected_species)))
+            msg = "Got {}. Expected {}".format(len(values), self.n_species)
+            if len(self._selected_species):
+                msg += ' or {}'.format(len(self._selected_species))
+            raise ValueError('Array has incorrect length. ' + msg + '.')
         method(self.thermo, &data[0])
 
     property molecular_weights: