[C++-sig] boost::python embedding return value

[Stefan Seefeld]

On 03/04/2010 11:59 AM, Neal Becker wrote:
> int main () {
>    Py_Initialize();
>
>    object main_module = import("__main__");
>    object main_namespace = main_module.attr("__dict__");
>
>    try {
>      object result = exec ("import sys\n"
> 			  "sys.path.append('./')\n"
> 			  "import test_embed\n"
> 			  "test_embed.five_square()\n",
> 			  main_namespace);
>      int five_squared = extract<int>  (result);
>      std::cout<<  five_squared<<  '\n';
>    }
>    catch (error_already_set const&) {
>      PyErr_Print();
>    }
> }
>
>
> test_embed.py:
> --------------------
> def five_square ():
>      return 5 ** 2
>
> I get:
> ./test_embed
> TypeError: No registered converter was able to produce a C++ rvalue of type
> int from this Python object of type NoneType
>
> Why did exec return None?  I expected it to return the result of
> "test_embed.five_squared()", which is the int 25.  What is the meaning of
> the return of exec_file?  A python module can't return a result.
>    

This is a straight wrapping of Python's C API. AFAICT, the return value 
is only useful to determine whether the call was successful. Thus, None 
may indicate an internal error, which you can check for with 
PyErr_Occured(). (I'm actually surprised we don't catch this internally 
and then raise an err_already_set exception !)

The real error could be that the interpreter wasn't able to import one 
of the modules (wrong PYTHONPATH ?), or something similar.

Regards,
         Stefan


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