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

2010-01-09 Thread devin kelly 
Hello,

I'm trying to expose some data that I develop in C++ to python.  Basically,
the reverse of this sample code:

#include 
#include 
#include 
#include 

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(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(main_namespace["result"]);
std::cout << five_squared <<
std::endl;
return 0;
}

This doesn't work.  Python throws an error.  What am I doing wrong??

Thanks for any help,
Devin


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

2010-01-09 Thread 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 
#include 
#include 
#include 

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(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(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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