Add a simple ToWeakPtr return value policy

Improve WorkspaceGroup test to cover the use case.
Refs #9807

Code/Mantid/Framework/PythonInterface/inc/MantidPythonInterface/kernel/Policies/ToWeakPtr.h

@@ -0,0 +1,93 @@
+#ifndef MANTID_PYTHONINTERFACE_TOWEAKPTR_H_
+#define MANTID_PYTHONINTERFACE_TOWEAKPTR_H_
+/**
+    Copyright © 2015 ISIS Rutherford Appleton Laboratory, NScD Oak Ridge
+   National Laboratory & European Spallation Source
+
+    This file is part of Mantid.
+
+    Mantid is free software; you can redistribute it and/or modify
+    it under the terms of the GNU General Public License as published by
+    the Free Software Foundation; either version 3 of the License, or
+    (at your option) any later version.
+
+    Mantid is distributed in the hope that it will be useful,
+    but WITHOUT ANY WARRANTY; without even the implied warranty of
+    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+    GNU General Public License for more details.
+
+    You should have received a copy of the GNU General Public License
+    along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+    File change history is stored at: <https://github.com/mantidproject/mantid>
+    Code Documentation is available at: <http://doxygen.mantidproject.org>
+ */
+#include <boost/python/detail/prefix.hpp>
+#include <boost/python/to_python_value.hpp>
+
+#include <boost/shared_ptr.hpp>
+#include <boost/weak_ptr.hpp>
+
+namespace Mantid {
+namespace PythonInterface {
+namespace Policies {
+
+namespace {
+//-----------------------------------------------------------------------
+// MPL helper structs
+//-----------------------------------------------------------------------
+/// MPL struct to figure out if a type is a boost::shared_ptr<T>
+/// The general one inherits from boost::false_type
+template <typename T> struct IsSharedPtr : boost::false_type {};
+
+/// Specialization for boost::shared_ptr<const T> types to inherit from
+/// boost::true_type
+template <typename T>
+struct IsSharedPtr<boost::shared_ptr<T>> : boost::true_type {};
+
+//-----------------------------------------------------------------------
+// Policy implementation
+//-----------------------------------------------------------------------
+/**
+ * Constructs a boost::weak_ptr around the incoming boost::shared_ptr
+ */
+template <typename ArgType> struct ToWeakPtrImpl {
+  // Useful types
+  typedef typename ArgType::element_type PointeeType;
+  typedef boost::weak_ptr<PointeeType> WeakPtr;
+
+  inline PyObject *operator()(const ArgType &p) const {
+    if (!p)
+      Py_RETURN_NONE;
+    return boost::python::to_python_value<WeakPtr>()(WeakPtr(p));
+  }
+
+  inline PyTypeObject const *get_pytype() const {
+    return boost::python::converter::registered<WeakPtr>::converters
+        .to_python_target_type();
+  }
+};
+
+//-----------------------------------------------------------------------
+// Error handler
+//-----------------------------------------------------------------------
+template <typename T> struct ToWeakPtr_Requires_Shared_Ptr_Return_Value {};
+} // ends anonymous namespace
+
+/**
+ * Implements the ToWeakPtr policy as required by boost.python
+ */
+struct ToWeakPtr {
+  template <class T> struct apply {
+    // Deduce if type is correct for policy
+    typedef typename boost::mpl::if_c<
+        IsSharedPtr<T>::value, ToWeakPtrImpl<T>,
+        ToWeakPtr_Requires_Shared_Ptr_Return_Value<T>>::type type;
+  };
+};
+
+} // ends Policies namespace
+}
+} // ends Mantid::PythonInterface namespaces
+
+#endif /* MANTID_PYTHONINTERFACE_REMOVECONST_H_REMOVECONST_H_ */

Code/Mantid/Framework/PythonInterface/mantid/api/src/Exports/WorkspaceGroup.cpp

@@ -1,35 +1,49 @@
 #include "MantidAPI/WorkspaceGroup.h"
+#include "MantidPythonInterface/kernel/Policies/ToWeakPtr.h"
 #include "MantidPythonInterface/kernel/Registry/RegisterWorkspacePtrToPython.h"
 
 #include <boost/python/class.hpp>
 #include <boost/python/return_value_policy.hpp>
 
 using namespace Mantid::API;
-using Mantid::PythonInterface::Registry::RegisterWorkspacePtrToPython;
+using namespace Mantid::PythonInterface;
 using namespace boost::python;
 
 // clang-format off
-void export_WorkspaceGroup() 
+void export_WorkspaceGroup()
 // clang-format on
 {
-  class_< WorkspaceGroup, bases<Workspace>, boost::noncopyable >("WorkspaceGroup", no_init)
-    .def("getNumberOfEntries", &WorkspaceGroup::getNumberOfEntries, "Returns the number of entries in the group")
-    .def("getNames", &WorkspaceGroup::getNames, "Returns the names of the entries in the group")
-    .def("contains", (bool (WorkspaceGroup::*)(const std::string & wsName) const)&WorkspaceGroup::contains, "Returns true if the given name is in the group")
-    .def("add", &WorkspaceGroup::add, "Add a name to the group")
-    .def("size", &WorkspaceGroup::size, "Returns the number of workspaces contained in the group")
-    .def("remove", &WorkspaceGroup::remove, "Remove a name from the group")
-    .def("getItem", (Workspace_sptr (WorkspaceGroup::*)(const size_t) const)&WorkspaceGroup::getItem,
-         "Returns the item at the given index")
-    .def("isMultiPeriod", &WorkspaceGroup::isMultiperiod, "Retuns true if the workspace group is multi-period")
-    // ------------ Operators --------------------------------
-    .def("__len__", &WorkspaceGroup::getNumberOfEntries)
-    .def("__contains__", (bool (WorkspaceGroup::*)(const std::string & wsName) const)&WorkspaceGroup::contains)
-    .def("__getitem__", (Workspace_sptr (WorkspaceGroup::*)(const size_t) const)&WorkspaceGroup::getItem)
-  ;
+  class_<WorkspaceGroup, bases<Workspace>, boost::noncopyable>("WorkspaceGroup",
+                                                               no_init)
+      .def("getNumberOfEntries", &WorkspaceGroup::getNumberOfEntries,
+           "Returns the number of entries in the group")
+      .def("getNames", &WorkspaceGroup::getNames,
+           "Returns the names of the entries in the group")
+      .def("contains",
+           (bool (WorkspaceGroup::*)(const std::string &wsName) const) &
+               WorkspaceGroup::contains,
+           "Returns true if the given name is in the group")
+      .def("add", &WorkspaceGroup::add, "Add a name to the group")
+      .def("size", &WorkspaceGroup::size,
+           "Returns the number of workspaces contained in the group")
+      .def("remove", &WorkspaceGroup::remove, "Remove a name from the group")
+      .def("getItem", (Workspace_sptr (WorkspaceGroup::*)(const size_t) const) &
+                          WorkspaceGroup::getItem,
+           return_value_policy<Policies::ToWeakPtr>(),
+           "Returns the item at the given index")
+      .def("isMultiPeriod", &WorkspaceGroup::isMultiperiod,
+           "Retuns true if the workspace group is multi-period")
+      // ------------ Operators --------------------------------
+      .def("__len__", &WorkspaceGroup::getNumberOfEntries)
+      .def("__contains__",
+           (bool (WorkspaceGroup::*)(const std::string &wsName) const) &
+               WorkspaceGroup::contains)
+      .def("__getitem__",
+           (Workspace_sptr (WorkspaceGroup::*)(const size_t) const) &
+               WorkspaceGroup::getItem,
+           return_value_policy<Policies::ToWeakPtr>());
 
   //-----------------------------------------------------------------------------------------------
 
-  RegisterWorkspacePtrToPython<WorkspaceGroup>();
+  Registry::RegisterWorkspacePtrToPython<WorkspaceGroup>();
 }
-

Code/Mantid/Framework/PythonInterface/mantid/kernel/CMakeLists.txt

@@ -86,6 +86,8 @@ set ( INC_FILES
   ${HEADER_DIR}/kernel/Environment/Threading.h
   ${HEADER_DIR}/kernel/Environment/WrapperHelpers.h
   ${HEADER_DIR}/kernel/Policies/MatrixToNumpy.h
+  ${HEADER_DIR}/kernel/Policies/RemoveConst.h
+  ${HEADER_DIR}/kernel/Policies/ToWeakPtr.h
   ${HEADER_DIR}/kernel/Policies/VectorToNumpy.h
   ${HEADER_DIR}/kernel/Registry/PropertyValueHandler.h
   ${HEADER_DIR}/kernel/Registry/PropertyWithValueFactory.h

Code/Mantid/Framework/PythonInterface/test/python/mantid/api/WorkspaceGroupTest.py

@@ -42,6 +42,15 @@ def test_group_index_access_returns_correct_workspace(self):
             member = group[i]
             self.assertTrue(isinstance(member, MatrixWorkspace))
 
+        # Clearing the data should leave the handle unusable
+        member = group[0]
+        mtd.remove("First")
+        try:
+            member.name()
+            self.fail("Handle for item extracted from WorkspaceGroup is still usable after ADS has been cleared, it should be a weak reference and raise an error.")
+        except RuntimeError, exc:
+            self.assertEquals(str(exc), 'Variable invalidated, data has been deleted.')
+
     def test_SimpleAlgorithm_Accepts_Group_Handle(self):
         from mantid.simpleapi import Scale