[Python] Fix display of information from errors in callback functions

include/cantera/cython/funcWrapper.h

@@ -20,13 +20,13 @@ class CallbackError : public Cantera::CanteraError
 {
 public:
     CallbackError(void* type, void* value) :
+        CanteraError("Python callback function"),
         m_type((PyObject*) type),
         m_value((PyObject*) value)
     {
     }
-    const char* what() const throw() {
-        formattedMessage_ = "\n" + std::string(71, '*') + "\n";
-        formattedMessage_ += "Exception raised in Python callback function:\n";
+    std::string getMessage() const {
+        std::string msg;
 
         PyObject* name = PyObject_GetAttrString(m_type, "__name__");
         PyObject* value_str = PyObject_Str(m_value);
@@ -40,26 +40,28 @@ public:
         #endif
 
         if (name_bytes) {
-            formattedMessage_ += PyBytes_AsString(name_bytes);
+            msg += PyBytes_AsString(name_bytes);
             Py_DECREF(name_bytes);
         } else {
-            formattedMessage_ += "<error determining exception type>";
+            msg += "<error determining exception type>";
         }
 
-        formattedMessage_ += ": ";
+        msg += ": ";
 
         if (value_bytes) {
-            formattedMessage_ += PyBytes_AsString(value_bytes);
+            msg += PyBytes_AsString(value_bytes);
             Py_DECREF(value_bytes);
         } else {
-            formattedMessage_ += "<error determining exception message>";
+            msg += "<error determining exception message>";
         }
 
         Py_XDECREF(name);
         Py_XDECREF(value_str);
+        return msg;
+    }
 
-        formattedMessage_ += "\n" + std::string(71, '*') + "\n";
-        return formattedMessage_.c_str();
+    virtual std::string getClass() const {
+        return "Exception";
     }
 
     PyObject* m_type;

interfaces/cython/cantera/test/test_reactor.py

@@ -364,6 +364,16 @@ class TestReactor(utilities.CanteraTest):
         self.assertNear(ma + 0.1, mb)
         self.assertArrayNear(ma * Ya + 0.1 * gas2.Y, mb * Yb)
 
+    def test_user_function_error(self):
+        # Make sure Python error message actually gets displayed
+        self.make_reactors(n_reactors=2)
+        mfc = ct.MassFlowController(self.r1, self.r2)
+        mfc.set_mass_flow_rate(lambda t: eggs)
+
+        # TODO: replace with 'assertRasesRegex' after dropping Python 2.7 support
+        with self.assertRaisesRegexp(Exception, 'eggs'):
+            self.net.step()
+
     def test_valve1(self):
         self.make_reactors(P1=10*ct.one_atm, X1='AR:1.0', X2='O2:1.0')
         self.net.rtol = 1e-12

src/numerics/FuncEval.cpp

@@ -15,7 +15,7 @@ int FuncEval::eval_nothrow(double t, double* y, double* ydot)
         eval(t, y, ydot, m_sens_params.data());
     } catch (CanteraError& err) {
         if (suppressErrors()) {
-            m_errors.push_back(err.getMessage());
+            m_errors.push_back(err.what());
         } else {
             writelog(err.what());
         }