Create derived python class from a c++ class using Boost Python

Question

I have a shared lib that I've created with boost python with a bunch of classes and I want to be able to inherit from these classes in python. The inheritance bit seems to work fine but I am unable to call methods in a super class.

c++ class definition:

class Game {
    vector<pair<object, Listener*> > _listeners;

public:
    Game();

    virtual void assignListener(object listener);
    vector<pair<object, Listener*> >& listeners();
};

my boost python wrapper for this class looks like this:

BOOST_PYTHON_MODULE(libgla) {
    using namespace boost::python;

    ...

    class_<Game>("Game", init<>())
            .def("assign_listener", &Game::assignListener);

    ...
};

my python test code looks like this:

from libgla import Engine, Game, Window, ErrorListener, KeyListener


class CustomGame(Game):
    def __init__(self):
        self.assign_listener(KeyListener())

engine = Engine(ErrorListener())
engine.set_game(CustomGame())
engine.set_window(Window(1280, 1024, "test"))
engine.start()

this code terminates on the assign_listener function with the following error:

Traceback (most recent call last):
  File "app.py", line 9, in <module>
    engine.set_game(CustomGame())
  File "app.py", line 6, in __init__
    self.assign_listener(KeyListener())
Boost.Python.ArgumentError: Python argument types in
    Game.assign_listener(CustomGame, KeyListener)
did not match C++ signature:
    assign_listener(Game {lvalue}, boost::python::api::object)

Am I doing something wrong or is this boost python limitation?

Note: the following python code works as it should

from libgla import Engine, Game, Window, ErrorListener, KeyListener

engine = Engine(ErrorListener())
game = Game()
game.assign_listener(KeyListener())
engine.set_game(game)
engine.set_window(Window(1280, 1024, "test"))
engine.start()

Edit 1

I think instance_holder described here is the answer however I can't figure out how to implement it and the documentation doesn't really give a clear example.

mhh no, would that cause a problem? Do you mean that I should inherit from the boost::python::api::object? Because I was under the impression that all I need to do is implement class_<KeyListener> and boost python should know about it. — Dovydas Rupšys
– Dovydas Rupšys, Commented Dec 21, 2015 at 21:20
i did this far away (maybe do not remember), try to do this or method "assignListener" must receive KeyListener, not object. It cannot apply your call to your method — Elena NNN
– Elena NNN, Commented Dec 21, 2015 at 21:23
Actually that can't be a problem because It works fine if do : game = Game() game.assign_listener(KeyListener()) — Dovydas Rupšys
– Dovydas Rupšys, Commented Dec 21, 2015 at 21:25
My thoughts on the problem are that c++ Boost::Python Game object doesn't recognize CustomGame as a derived class and therefore fails. It's just that the documentation is a bit sparse and I can't find a solution. — Dovydas Rupšys
– Dovydas Rupšys, Commented Dec 21, 2015 at 21:44

John Zwinck · Accepted Answer · 2015-12-22 10:07:24Z

1

Try this:

class CustomGame(Game):
    def __init__(self):
        Game.__init__(self)
        self.assign_listener(KeyListener())

You were missing the superclass initialization. Without it, use of your base class is not really valid.

edited Dec 22, 2015 at 10:07

answered Dec 22, 2015 at 4:07

John Zwinck

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2 Comments

Nathan Tuggy Over a year ago

Could you please edit in an explanation of why this code answers the question?

Dovydas Rupšys Over a year ago

Thanks it worked, I can't believe I didn't even consider this. I haven't been programming in python for long, is this normal do you always have to run the super constructor even if it is just a default one?

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