CPython engine marshals python classes to lists when the expectation is to get a handle.

src/Libraries/DSCPython/CPythonEvaluator.cs

@@ -147,6 +147,7 @@ internal delegate void EvaluationEventHandler(EvaluationState state,
     [IsVisibleInDynamoLibrary(false)]
     public static class CPythonEvaluator
     {
+        private const string DynamoSkipAttributeName = "__dynamoskipconversion__";
         static PyScope globalScope;
         internal static readonly string globalScopeName = "global";
         static CPythonEvaluator()
@@ -387,6 +388,12 @@ public static DataMarshaler OutputMarshaler
                             {
                                 return outputMarshaler.Marshal(clrObj);
                             }
+
+                            if (IsMarkedToSkipConversion(pyObj))
+                            {
+                                return GetDynamoCPythonHandle(pyObj);
+                            }
+
                             // Dictionaries are iterable, so they should come first
                             if (PyDict.IsDictType(pyObj))
                             {
@@ -439,11 +446,7 @@ public static DataMarshaler OutputMarshaler
                                 {
                                     if (unmarshalled.Equals(pyObj))
                                     {
-                                        var globalScope = PyScopeManager.Global.Get(globalScopeName);
-                                        //try moving object to global scope
-                                        globalScope.Set(pyObj.Handle.ToString(), pyObj);
-
-                                        return new DynamoCPythonHandle(pyObj.Handle);
+                                        return GetDynamoCPythonHandle(pyObj);
                                     }
                                     else
                                     {
@@ -459,6 +462,18 @@ public static DataMarshaler OutputMarshaler
             }
         }
 
+        private static DynamoCPythonHandle GetDynamoCPythonHandle(PyObject pyObj)
+        {
+            var globalScope = PyScopeManager.Global.Get(globalScopeName);
+            globalScope.Set(pyObj.Handle.ToString(), pyObj);
+            return new DynamoCPythonHandle(pyObj.Handle);
+        }
+
+        private static bool IsMarkedToSkipConversion(PyObject pyObj)
+        {
+            return pyObj.HasAttr(DynamoSkipAttributeName);
+        }
+
         private static DataMarshaler inputMarshaler;
         private static DataMarshaler outputMarshaler;
 

test/Libraries/DynamoPythonTests/PythonEvalTests.cs

@@ -2,6 +2,8 @@
 using System.Collections;
 using System.Collections.Generic;
 using NUnit.Framework;
+using System.Linq;
+using ProtoCore.Lang;
 
 namespace DSPythonTests
 {
@@ -212,6 +214,48 @@ def __str__(self):
             });
         }
 
+        [Test]
+        public void PythonObjectWithDynamoSkipisNotMarshaled()
+        {
+            var code = @"
+
+class iterable:
+    def __str__(self):
+        return 'I want to participate in conversion'
+    def __iter__(self):
+        return iter([0,1,2,3])
+    def __getitem__(self,key):
+        return key
+
+o = iterable()
+OUT = o
+";
+
+            var code2 = @"
+
+class notiterable:
+    def __dynamoskipconversion__(self):
+        pass
+    def __str__(self):
+        return 'I want to skip in conversion'
+    def __iter__(self):
+        return iter([0,1,2,3])
+    def __getitem__(self,key):
+        return key
+
+o = notiterable()
+OUT = o
+";
+            var empty = new ArrayList();
+            var result1 = DSCPython.CPythonEvaluator.EvaluatePythonScript(code, empty, empty);
+            var result2 = DSCPython.CPythonEvaluator.EvaluatePythonScript(code2, empty, empty);
+            Assert.IsInstanceOf(typeof(IList), result1);
+            Assert.IsTrue(new List<object>() { 0L, 1L, 2L, 3L }
+                .SequenceEqual((IEnumerable<Object>)result1));
+            Assert.IsInstanceOf(typeof(DSCPython.DynamoCPythonHandle),result2 );
+
+        }
+
         [Test]
         public void NonListIterablesCanBeOutput()
         {