Thank you for the suggestion--unfortunately it does not seem to work for me. In my Python code, I verified that the list is composed of different types of objects. However, in C++ I tried std::cout << typeid(extract<baseclass&>(myList[j])).name(); and it shows that all of the extracted objects are of the same class (baseclass). Any other ideas? I want to be able to do the following in C++: for (i=0 to length of list) list[i].evaluate(); where the "evaluate" method is different depending on what kind of object is pulled from the list. Craig Finch --- Roman Yakovenko <roman.yakovenko@gmail.com> wrote:

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I have run into a problem that I don't know how to solve with Boost.python, and I can't find the answer online. I have a list of objects in Python, and the objects are of different classes. All

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classes are derived from a common base class, and the classes are defined in C++ and exposed to Python. The list of objects is created in Python, and I would like to get this list into C++, where the

On 6/27/07, Craig Finch <oanjao@yahoo.com> wrote: the the

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objects of different types will be handled differently. I can bring the list into C++ as a boost::python::list, but then I have to use extract <type> to get the individual objects from the list. The problem is that extract<baseclass> works, but then all of the extracted objects are the same type. The information contained in the derived types is lost.

Try to extract "baseclass&".

-- Roman Yakovenko C++ Python language binding http://www.language-binding.net/

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If you could post a minimal, complete example, it would be easier to diagnose the problem.

Here is a simple example of what I am trying to do. There is no problem if I know in advance the class of the objects to be imported. However, I don't know the class until runtime, so I don't know how set the appropriate class for extract <???>. ---------------- C++ CODE--------------- class BaseClass { public: virtual void evaluate(void); }; class A : public BaseClass { public: int index; virtual void evaluate(void); }; class B : public BaseClass { public: float value; virtual void evaluate(void); }; class Importer { public: void setList (boost::python::list l); }; void BaseClass::evaluate (void) { std::cout << "Useless base class" << std::endl; } void A::evaluate (void) { std::cout << "Index is " << index << std::endl; } void B::evaluate (void) { std::cout << "Value is " << value << std::endl; } using namespace boost::python; void BaseClass::evaluate (void) { std::cout << "Useless base class" << std::endl; } void A::evaluate (void) { std::cout << "Index is " << index << std::endl; } void B::evaluate (void) { std::cout << "Value is " << value << std::endl; } void Importer::setList (list l) { BaseClass o; int len = extract<int>(l.attr("__len__")()); for (int i=0; i<len; i++) { std::cout << typeid(extract <BaseClass&> (l[i])).name() << std::endl; o = extract <BaseClass&> (l[i]); o.evaluate(); } } BOOST_PYTHON_MODULE(C_test) { class_<BaseClass> ("BaseClass"); class_<A, bases<BaseClass> > ("A") .def("evaluate", &A::evaluate) .def_readwrite("index", &A::index) ; class_<B, bases<BaseClass> > ("B") .def("evaluate", &B::evaluate) .def_readwrite("value", &B::value) ; class_<Importer> ("Importer") .def("setList", &Importer::setList) ; } -------------- PYTHON CODE ------------------ import C_test # Derived Classes class A (C_test.A): def __init__ (self, int): C_test.A.__init__(self) self.index = int class B (C_test.B): def __init__ (self, float): C_test.B.__init__(self) self.value = float mixedList = [] mixedList.append(A(3)) mixedList.append(B(5.234)) imp = C_test.Importer() imp.setList(mixedList) --------------- OUTPUT ---------------- N5boost6python7extractIR9BaseClassEE Useless base class N5boost6python7extractIR9BaseClassEE Useless base class --------------------------------------- -------------- Please reply to cfinch@ieee.org ____________________________________________________________________________________ Never miss an email again! Yahoo! Toolbar alerts you the instant new Mail arrives. http://tools.search.yahoo.com/toolbar/features/mail/

Chris, Your suggestion worked for me. I constructed a simple but complete example of how to handle objects of different types when bringing a Boost.python list into C++ for processing. An explanation and the necessary files are posted on my software development blog at http://www.shocksolution.com/blog/2007/08/08/mixing-objects-of-different-typ... for anyone else who wants to know how to do this. Thanks guys! Craig --- Christopher Woods <chryswoods@gmail.com> wrote:

Hi Craig,

I've played with your example and I've got it to work by changing your extract<BaseClass> and extract<BaseClass&> into extract<BaseClass*> and extract<const BaseClass*>. In the first case (extract<BaseClass>) you were extracting the object in the list into a *copy*, which was of type BaseClass, so the "Useless base class" function of that copy was then called. By extracting into a pointer (via extract<BaseClass*>) you aren't performing a copy, and you obtain a polymorphic pointer to the original object.

As for the typeid part of the example, I don't know why typeid gets it wrong when you use extract<BaseClass&>. I had to change this to extract<const BaseClass*> and then do a typeid of the dereference of the resulting polymorphic pointer.

Here is the modified version of 'setList' that worked on my box (linux, gcc 3.4, python 2.3);

============================== void Importer::setList (list l) { BaseClass *o; int len = extract<int>(l.attr("__len__")());

for (int i=0; i<len; i++) { std::cout << typeid( *(extract <const BaseClass*> (l[i])) ).name() << std::endl;

o = extract <BaseClass*> (l[i]); o->evaluate(); } } ==============================

This gave output;

============================== 1A Index is 3 1B Value is 5.234 ==============================

I hope this helps,

Christopher

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