[Python] Fix setting mass/mole fractions for single-species phases

interfaces/cython/cantera/test/test_thermo.py

@@ -210,6 +210,12 @@ class TestThermoPhase(utilities.CanteraTest):
         self.assertNear(X[0], 0.1)
         self.assertNear(X[3], 0.9)
 
+    def test_setCompositionSingleSpecies(self):
+        gas = ct.Solution('argon.xml')
+        gas.X = [1]
+        gas.Y = np.array([[1.001]])
+        self.assertEqual(gas.Y[0], 1.0)
+
     def test_setCompositionSlice_bad(self):
         with self.assertRaises(ValueError):
             self.phase['H2','O2'].Y = [0.1, 0.2, 0.3]

interfaces/cython/cantera/thermo.pyx

@@ -492,6 +492,9 @@ cdef class ThermoPhase(_SolutionBase):
         cdef np.ndarray[np.double_t, ndim=1] data
 
         values = np.squeeze(values)
+        if values.ndim == 0:
+            values = values[np.newaxis] # corner case for single-species phases
+
         if len(values) == self.n_species:
             data = np.ascontiguousarray(values, dtype=np.double)
         elif len(values) == len(self._selected_species):