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

[Stefan Seefeld]

Neal,

I should have read your mail more carefully before replying. Sorry for that.


On 03/04/2010 01:35 PM, Neal Becker wrote:
> Stefan Seefeld wrote:
>
>    
>> 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
>>>        

The problem is that you shouldn't attempt to extract anything from the 
return value of an exec() call. It will contain None if everything goes 
well. If not, the C API returns a null-pointer, which boost.python 
translates into an err_already_set exception.

I'm not exactly sure what you attempt to do. If you really want to 
evaluate an expression (inclusively a function call), you should use 
eval(), instead of exec().

The typical usage of exec() will be to run some code, then inspect the 
global namespace to inspect any "side-effects" of this execution.
Notably, you may have your Python module store objects in the namespace, 
and then let your C++ code extract and use them.
For example

   std::string code =
   "import test_embed\n"
   "result = test_embed.five_square()";

   object = exec(code, global, global);
   int result = extract<int>(global["result"]);

Hope this helps,
         Stefan

-- 

       ...ich hab' noch einen Koffer in Berlin...