add comment about clearing Py::Dict when changing C++ data

src/Mod/Path/App/CommandPyImp.cpp

@@ -56,6 +56,11 @@ std::string CommandPy::representation(void) const
     return str.str();
 }
 
+// 
+// Py::Dict parameters_copy_dict is now a class member to avoid delete/create/copy on every read access from python code
+// Now the pre-filled Py::Dict is returned which is more consistent with normal python behaviour.
+// It should be cleared whenever the c++ Parameters object is changed eg setParameters() or other objects invalidate its content, eg setPlacement()
+// https://forum.freecadweb.org/viewtopic.php?f=15&t=50583
 
 PyObject *CommandPy::PyMake(struct _typeobject *, PyObject *, PyObject *)  // Python wrapper
 {
@@ -158,7 +163,7 @@ void CommandPy::setName(Py::String arg)
 
 Py::Dict CommandPy::getParameters(void) const
 {
-    // dict now a class member , https://forum.freecadweb.org/viewtopic.php?f=15&t=50583&
+    // dict now a class member , https://forum.freecadweb.org/viewtopic.php?f=15&t=50583
     if (parameters_copy_dict.length()==0) {    
       for(std::map<std::string,double>::iterator i = getCommandPtr()->Parameters.begin(); i != getCommandPtr()->Parameters.end(); ++i) {
           parameters_copy_dict.setItem(i->first, Py::Float(i->second));