On 23/10/14 03:38 AM, Erik Türke wrote:

Hi Python-Experts :-)

i am starting to get really frustrated trying to expose a simple C++ polymorphism to python with boost::python. I do have the following structure in C++:

struct Base { int typeID; };

struct Derived : public Base { int derivedProperty; }

//and some more from base derived types....

Base *returnSomethingDerivedFromBase(...) { Derived *ret = new Derived; ret->derivedProperty = 1234; return ret; }

BOOST_PYTHON_MODULE(foo) { class_<Base>("Base") .add_property("baseProperty", &Base::baseProperty);

class_<Derived, bases<Base> >("Derived") .add_property("derivedProperty", &Derived::derivedProperty);

def("returnSomethingDerivedFromBase", returnSomethingDerivedFromBase); }

And in Python i just want to have the following:

object = returnSomethingFromDerived() #object is of type Base if object.typeID = 1:

Typo: you likely mean '==', not '='.

#here i want to cast to Derived and access "derivedProperty" #but this is not working :-( : object.__class__ = Derived

That's indeed not possible. But I also don't know why you want that. The objet presumably already is of type 'Derived', so there is nothing to cast. (In Python all attribute lookups happen at call-time.)

Is there a way to accomplish this at all? Or isn´t this possible as it would be in C++ ?

Both C++ and Python are strongly typed (i.e., all objects have a specific type, with specific methods bound to them). But contrary to C++, Python also is dynamically typed, so you can reassign a new object with a different type to an existing variable. In that context, casting isn't of any use. Or perhaps a more complete example would be useful to demonstrate your needs, so we can look into how best to support that in Python. HTH, Stefan -- ...ich hab' noch einen Koffer in Berlin...