[C++-sig] Exposing C++ data with Boost.Python

[Stefan Seefeld]

On 01/09/2010 01:52 PM, devin kelly wrote:
> Hello,
>
> I'm trying to expose some data that I develop in C++ to python.  
> Basically, the reverse of this sample code:
>
> #include <iostream>
> #include <python2.6/Python.h>
> #include <boost/python.hpp>
> #include <boost/python/exec.hpp>
>
> int main(){
>
>         Py_Initialize();
>         object main_module = import("__main__");
>         object main_namespace = main_module.attr("__dict__");
>         ignored = exec("result = 5 ** 2", main_namespace);
>         int five_squared = extract<int>(main_namespace["result"]);
>         std::cout << five_squared << std::endl;
>
>         return 0;
> }
>
>
> So this code starts the python interpreter, squares 5 (in python) and 
> then extracts the result to an int called five_squared.  This works 
> fine for me, it's basically an example straight out of the 
> boost.python webpage.
>
> What I'd really like to do though is have an int that I initialize in 
> C++ and then square in python.  So this would require me to pass (or 
> expose) that data to python.  I've been trying this for a while and 
> have had no luck whatsoever.  The best I can think of is code like this:
>
> int main(){
>         Py_Initialize();
>         object main_module = import("__main__");
>         object main_namespace = main_module.attr("__dict__");
>         main_namespace["num2square"] = 6;
>         ignored = exec("result = num2square ** 2", main_namespace);
>         int five_squared = extract<int>(main_namespace["result"]);
>         std::cout << five_squared << std::endl;
>         return 0;
> }
>
> This doesn't work.  Python throws an error.  What am I doing wrong??

It might help indicating what error Python actually throws.

Also, if all you want is to evaluate an expression, I'd suggest you use 
"eval()", not "exec()".

         Stefan

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