[Boost-users] [python] nested attributes, style question

From: Malte Clasen (news_at_[hidden])
Date: 2005-12-10 16:32:29

Hi,

let's say we've got the following C++ classes:

class position {
public:
     float get_height() const;
     void set_height(float value);
private:
     float height_;
};

class object {
public:
     position get_position() const;
     void set_position(const position& value);
private:
     position pos_;
};

This design is not fixed, but right now this is the starting point. When 
I added the boost.python interface to it using add_property(), I quickly 
noticed that using these classes feels somehow unpythonic: I cannot 
modify the height of the position of an object using

     obj.pos.height = x

because get_position() returns the position by value, so the following 
set_height modifies a temporary instance of position. The work-around on 
the python side looks clumsy:

     position pos = obj.pos
     pos.height = x
     obj.pos = pos

Now I wonder whether I just just go for public member variables on the 
C++ side and expose these using def_readwrite(). But this would couple 
the implementation to the interface. Moving the object class to another 
kind of position handling would be easy with the get/set-based interface 
using on-the-fly conversions, but nearly impossible with the public 
member. That's why I think of some kind of attribute proxy object that 
hides the C++ get/set stuff in a way that allows attribute manipulation 
on the python side but keeps interface and implementation separated on 
the C++ side. I'm not sure how this proxy would look like, but the idea 
feels somehow right...

Is there a common solution to this problem? Does my proxy approach seem 
reasonable?

     Malte