[C++-sig] libboost-python: how to create module in C++-runtime and use it from python code, in python 3.x

[Nikolay Shaplov]

I have a piece of software that embeds python using libboost-python. It 
creates a python module in C++ runtime, and then imports it from python code.

For python2.7 this works well.
See test_module.py and test27.cpp attached.
On Debian Stretch it can be build using command 

g++  test27.cpp  -I /usr/include/x86_64-linux-gnu/python2.7 -I /usr/include/
python2.7 -lboost_python -lboost_system -lpython2.7

As I've said It works as expected. Python code imports _hello_provider module 
that have been created in C++ code, and uses provide_hello() for printing.

I've tried to port it to python 3.x, and did not succeed.

I managed to write test3x.cpp, it build well on Debian Buster using command

g++ test3x.cpp  -I /usr/include/x86_64-linux-gnu/python3.9 -I /usr/include/
python3.9 -lpython3.9  -lboost_python39 -lboost_system

but it fails when I try to run it.

Traceback (most recent call last):
  File "/usr/lib/python3.9/imp.py", line 169, in load_source
    module = _exec(spec, sys.modules[name])
  File "<frozen importlib._bootstrap>", line 613, in _exec
  File "<frozen importlib._bootstrap_external>", line 790, in exec_module
  File "<frozen importlib._bootstrap>", line 228, in _call_with_frames_removed
  File "test_module.py", line 1, in <module>
    import _hello_provider
ModuleNotFoundError: No module named '_hello_provider'

for some reason "native" python module does not see _hello_provider that have 
been created in runtime.

I guess I am doing something wrong, but I am no expert in python and python 
embedding, and I did not managed to find any good example in the internet.

So I guess I need some help here.

P.S. If remove "import" related part from examples, it also works well, so the 
only task here is to make in properly visible...



-- 
Nikolay Shaplov aka Nataraj
Fuzzing Engineer at Postgres Professional
Matrix IM: @dhyan:nataraj.su

#include <boost/python.hpp>

std::string provide_hello() {
    return "hello world provided";
}

BOOST_PYTHON_MODULE(_hello_provider) {
    using namespace boost::python;
    def("provide_hello", &provide_hello);
}


int main()
{
  Py_InitializeEx(0);
  try {
    boost::python::object modImp = boost::python::import("imp");
    PyInit__hello_provider();

    PyImport_AddModule("test_module");

    modImp.attr("load_source")("test_module", "test_module.py");

    boost::python::exec("print(test_var)", boost::python::import("test_module").attr("__dict__"));
    boost::python::exec("print(hello_static())", boost::python::import("test_module").attr("__dict__"));
    boost::python::exec("print(hello_provided())", boost::python::import("test_module").attr("__dict__"));
  }
  catch (const boost::python::error_already_set&)
  {
    PyErr_Print();
  }
  Py_Finalize();
}

#include <boost/python.hpp>
 
std::string provide_hello() {
    return "hello world provided";
}
 
BOOST_PYTHON_MODULE(_hello_provider) {
    using namespace boost::python;
    def("provide_hello", &provide_hello);
}


int main()
{
  Py_InitializeEx(0);
  try {
    boost::python::object modImp = boost::python::import("imp");
    init_hello_provider();


    PyImport_AddModule("test_module");

    modImp.attr("load_source")("test_module", "test_module.py");

    boost::python::exec("print(test_var)", boost::python::import("test_module").attr("__dict__"));
    boost::python::exec("print(hello_static())", boost::python::import("test_module").attr("__dict__"));
    boost::python::exec("print(hello_provided())", boost::python::import("test_module").attr("__dict__"));
  }
  catch (const boost::python::error_already_set&)
  {
    PyErr_Print();
  }
  Py_Finalize();
}
import _hello_provider

test_var = 42

def test_out(expr):
	print(expr)


def hello_static():
	return "Hello world static"

def hello_provided():
	return _hello_provider.provide_hello()

#test_out('lalala')

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