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

[Neal Becker]

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.