[Python] Allow setting composition using a dict

The following syntax makes it easier to set the composition programmatically,
e.g. inside a for loop:

    phase.Y = {'H2':0.1, 'O2':0.4, 'AR':0.5}

interfaces/cython/cantera/_cantera.pxd

@@ -1,5 +1,6 @@
 from libcpp.vector cimport vector
 from libcpp.string cimport string
+from libcpp.map cimport map as stdmap
 from libcpp cimport bool as cbool
 from cpython cimport bool as pybool
 
@@ -84,10 +85,12 @@ cdef extern from "cantera/thermo/ThermoPhase.h" namespace "Cantera":
 
         # composition
         void setMassFractionsByName(string) except +
+        void setMassFractionsByName(stdmap[string,double]&) except +
         double massFraction(size_t) except +
         double massFraction(string) except +
 
         void setMoleFractionsByName(string) except +
+        void setMoleFractionsByName(stdmap[string,double]&) except +
         void getMoleFractions(double*) except +
         double moleFraction(size_t) except +
         double moleFraction(string) except +

interfaces/cython/cantera/test/test_thermo.py

@@ -83,6 +83,27 @@ class TestThermoPhase(utilities.CanteraTest):
             self.phase.X = 'H2:1e-1.4'
         self.assertArrayNear(X0, self.phase.X)
 
+    def test_setCompositionDict(self):
+        self.phase.X = {'H2':1.0, 'O2':3.0}
+        X = self.phase.X
+        self.assertNear(X[0], 0.25)
+        self.assertNear(X[3], 0.75)
+
+        self.phase.Y = {'H2':1.0, 'O2':3.0}
+        Y = self.phase.Y
+        self.assertNear(Y[0], 0.25)
+        self.assertNear(Y[3], 0.75)
+
+    @unittest.expectedFailure
+    def test_setCompositionDict_bad1(self):
+        # Non-existent species should raise an exception
+        with self.assertRaises(Exception):
+            self.phase.X = {'H2':1.0, 'HCl':3.0}
+
+    def test_setCompositionDict_bad2(self):
+        with self.assertRaises(Exception):
+            self.phase.Y = {'H2':1.0, 'O2':'xx'}
+
     def test_report(self):
         report = self.phase.report()
         self.assertTrue(self.phase.name in report)

interfaces/cython/cantera/thermo.pyx

@@ -2,6 +2,16 @@ cdef enum Thermasis:
     mass_basis = 0
     molar_basis = 1
 
+
+cdef stdmap[string,double] comp_map(dict X) except *:
+    cdef stdmap[string,double] m
+    cdef str species
+    cdef float val
+    for species,value in X.items():
+        m[stringify(species)] = value
+    return m
+
+
 cdef class ThermoPhase(_SolutionBase):
     """
     A phase with an equation of state.
@@ -255,10 +265,11 @@ cdef class ThermoPhase(_SolutionBase):
 
     property Y:
         """
-        Get/Set the species mass fractions. Can be set as either an array or
-        as a string. Always returns an array::
+        Get/Set the species mass fractions. Can be set as an array, as a dict,
+        or as a string. Always returns an array::
 
             >>> phase.Y = [0.1, 0, 0, 0.4, 0, 0, 0, 0, 0.5]
+            >>> phase.Y = {'H2':0.1, 'O2':0.4, 'AR':0.5}
             >>> phase.Y = 'H2:0.1, O2:0.4, AR:0.5'
             >>> phase.Y
             array([0.1, 0, 0, 0.4, 0, 0, 0, 0, 0.5])
@@ -268,25 +279,29 @@ cdef class ThermoPhase(_SolutionBase):
         def __set__(self, Y):
             if isinstance(Y, (str, unicode)):
                 self.thermo.setMassFractionsByName(stringify(Y))
+            elif isinstance(Y, dict):
+                self.thermo.setMassFractionsByName(comp_map(Y))
             else:
                 self._setArray1(thermo_setMassFractions, Y)
 
     property X:
         """
-        Get/Set the species mole fractions. Can be set as either an array or
-        as a string. Always returns an array::
+        Get/Set the species mole fractions. Can be set as an array, as a dict,
+        or as a string. Always returns an array::
 
             >>> phase.X = [0.1, 0, 0, 0.4, 0, 0, 0, 0, 0.5]
+            >>> phase.X = {'H2':0.1, 'O2':0.4, 'AR':0.5}
             >>> phase.X = 'H2:0.1, O2:0.4, AR:0.5'
             >>> phase.X
             array([0.1, 0, 0, 0.4, 0, 0, 0, 0, 0.5])
-
         """
         def __get__(self):
             return self._getArray1(thermo_getMoleFractions)
         def __set__(self, X):
             if isinstance(X, (str, unicode)):
                 self.thermo.setMoleFractionsByName(stringify(X))
+            elif isinstance(X, dict):
+                self.thermo.setMoleFractionsByName(comp_map(X))
             else:
                 self._setArray1(thermo_setMoleFractions, X)