[ctml_writer] Fix handling of units in some fields

When using Python 3, using -1 as the dummy value for density and site_density
causes probems, because the (ordered) comparison between a density specified
with units (as a tuple) and 0 is not allowed. Instead, use None as the
placeholder value.

interfaces/cython/cantera/ctml_writer.py

@@ -2055,7 +2055,7 @@ class incompressible_solid(phase):
                  elements = '',
                  species = '',
                  note = '',
-                 density = -1.0,
+                 density = None,
                  transport = 'None',
                  initial_state = None,
                  options = []):
@@ -2064,7 +2064,7 @@ class incompressible_solid(phase):
                        initial_state, options)
         self._dens = density
         self._pure = 0
-        if self._dens < 0.0:
+        if self._dens is None:
             raise CTI_Error('density must be specified.')
         self._tr = transport
 
@@ -2093,7 +2093,7 @@ class lattice(phase):
                  transport = 'None',
                  initial_state = None,
                  options = [],
-                 site_density = -1.0,
+                 site_density = None,
                  vacancy_species = ''):
         phase.__init__(self, name, 3, elements, species, note, 'none',
                         initial_state, options)
@@ -2105,7 +2105,7 @@ class lattice(phase):
             raise CTI_Error('sublattice name must be specified')
         if species == '':
             raise CTI_Error('sublattice species must be specified')
-        if site_density < 0.0:
+        if site_density is None:
             raise CTI_Error('sublattice '+name
                             +' site density must be specified')
 

interfaces/cython/cantera/test/test_convert.py

@@ -304,3 +304,19 @@ class chemkinConverterTest(utilities.CanteraTest):
                         quiet=True)
 
         self.assertRaises(ck2cti.InputParseError, convert)
+
+class CtmlConverterTest(utilities.CanteraTest):
+    def test_sofc(self):
+        gas_a, anode_bulk, oxide_a = ct.import_phases(
+            '../../interfaces/cython/cantera/examples/surface_chemistry/sofc.cti',
+            ['gas', 'metal', 'oxide_bulk'])
+
+        self.assertNear(gas_a.P, ct.one_atm)
+        self.assertNear(anode_bulk['electron'].X, 1.0)
+        self.assertNear(oxide_a.density, 700)
+
+    def test_diamond(self):
+        gas, solid = ct.import_phases('diamond.cti', ['gas','diamond'])
+        face = ct.Interface('diamond.cti', 'diamond_100', [gas, solid])
+
+        self.assertNear(face.site_density, 3e-8)