boost::python: expose a C++ class to a python script embedded in a C++ app

Question

I am successfully able to load a python script file and call a function within using boost::python in a C++ app.

In the boost python EmbeddingPython wiki there is a tip on how to load a python module.

namespace bp = boost::python;

bp::object import(const std::string& module, const std::string& path, bp::object& globals)
{
    bp::dict locals;
    locals["module_name"] = module;
    locals["path"]        = path;

    bp::exec("import imp\n"
             "new_module = imp.load_module(module_name, open(path), path, ('py', 'U', imp.PY_SOURCE))\n",
             globals,
             locals);

    return locals["new_module"];
}

I can successfully use this to import a python module (test.py)

int main()
{
    Py_Initialize();

    bp::object main    = bp::import("__main__");
    bp::object globals = main.attr("__dict__");
    bp::object module  = import("test", "test.py", globals);
    bp::object run     = module.attr("run");

    run();

    return 0;
}

Running the above code with a hello-world test.py script works fine:

test.py:

def run():
    print "hello world"

Output:

hello world

Exposing a C++ class to python:

However, I now want to expose a C++ class to that script.

struct Foo
{
    void f() {}
};

As per the boost::python documentation, I expose this class as follows:

BOOST_PYTHON_MODULE(FooModule)
{
    bp::class_<Foo>("Foo")
        .def("f", &Foo::f)
        ;
}

As per the instructions in the above-linked wiki, I can then import my FooModule, and store it in my globals:

PyImport_AppendInittab("FooModule", &initFooModule); 

...

bp::object Foo = bp::import("FooModule");
globals["Foo"] = Foo;

This import is done prior to importing my test.py script, and this globals object is the one passed to bp::exec when importing my script (ie: Foo should be in the globals dict which bp::exec exposes to my script when importing).

However, for some reason my Foo module is not visible to test.py

Question:

How can I expose my Foo class to the test.py python script I am loading?

---

Full working example:

test.py:

def run():
    foo = Foo()
    foo.f()

main.cpp:

#include <iostream>
#include <boost/python.hpp>

namespace bp = boost::python;

bp::object import(const std::string& module, const std::string& path, bp::object& globals)
{
    bp::dict locals;
    locals["module_name"] = module;
    locals["path"]        = path;

    bp::exec("import imp\n"
             "new_module = imp.load_module(module_name, open(path), path, ('py', 'U', imp.PY_SOURCE))\n",
             globals,
             locals);
    return locals["new_module"];
}

struct Foo
{
    void f() {}
};

BOOST_PYTHON_MODULE(FooModule)
{
    bp::class_<Foo>("Foo")
        .def("f", &Foo::f)
        ;
}

int main()
try
{
    PyImport_AppendInittab("FooModule", &initFooModule);
    Py_Initialize();

    // get a handle to the globals dict    
    bp::object main = bp::import("__main__");
    bp::object globals = main.attr("__dict__");

    // import FooModule, and store it in the globals dict
    bp::object Foo = bp::import("FooModule");
    globals["Foo"] = Foo;

    // import the test script, passing the populated globals dict
    bp::object module = import("test", "test.py", globals);
    bp::object run = module.attr("run");

    // run the script
    run();

    return 0;
}
catch(const bp::error_already_set&)
{
    std::cerr << ">>> Error! Uncaught exception:\n";
    PyErr_Print();
    return 1;
}

Output:

>>> Error! Uncaught exception:
Traceback (most recent call last):
  File "test.py", line 2, in run
    foo = Foo()
NameError: global name 'Foo' is not defined

Pilsen

Answer 1

Instead of trying to inject FooModule into the Python script from the C++ side, just register the module with PyImport_AppendInittab from the C++ side and then import it from the Python side:

import FooModule

def run():
    foo = FooModule.Foo()
    foo.f()

Acarology

Note to anyone else looking at this, in doing the above, bp::import("FooModule"); is no longer necessary. The call to PyImport_AppendInittab is all that is required to allow the script to import FooModule – Pilsen