Programmatically define a package structure in embedded python 3

Question

I would like to create an embedded python 3 interpreter and let python scripts import modules created via the C Python API.

No problem to create a "top-level" module, but now I would like to organized my modules in packages ... But I failed.

Here is my current (simple) code :

#include <Python.h>

//// Definition of 'emb.sub' module
static PyObject* emb_sub_foo(PyObject *self, PyObject *args)
{
    char const* n = "I am sub foo";
    return Py_BuildValue("s", n);
}
static PyMethodDef EmbSubMethods[] = {
    {"foo", emb_sub_foo, METH_VARARGS, "Returns sub foo"},
    {NULL, NULL, 0, NULL}
};
static PyModuleDef EmbSubModule = {
    PyModuleDef_HEAD_INIT, "emb.sub", NULL, -1, EmbSubMethods,
    NULL, NULL, NULL, NULL
};
static PyObject* PyInit_emb_sub(void)
{
    return PyModule_Create(&EmbSubModule);
}

//// Embedded Python
int main()
{
    PyImport_AppendInittab("emb.emb", &PyInit_emb_sub);
    Py_Initialize();

    PyRun_SimpleString("import emb.sub\n");

    Py_Finalize();
    return 0;
}

When I executes the program, I get :

Traceback (most recent call last):
  File "<string>", line 1, in <module>
ImportError: No module named 'emb'

So I create an empty emb module, and I set its __path__ like this :

#include <Python.h>

//// Definition of 'emb' module
static PyModuleDef EmbModule = {
    PyModuleDef_HEAD_INIT, "emb", NULL, -1, NULL,
    NULL, NULL, NULL, NULL
};
static PyObject* PyInit_emb(void)
{
    PyObject *mod = PyModule_Create(&EmbModule);
    PyModule_AddObject(mod, "__path__", Py_BuildValue("()"));

    return mod;
}

//// Definition of 'emb.sub' module
static PyObject* emb_sub_foo(PyObject *self, PyObject *args)
{
    char const* n = "I am sub foo";
    return Py_BuildValue("s", n);
}
static PyMethodDef EmbSubMethods[] = {
    {"foo", emb_sub_foo, METH_VARARGS, "Returns sub foo"},
    {NULL, NULL, 0, NULL}
};
static PyModuleDef EmbSubModule = {
    PyModuleDef_HEAD_INIT, "emb.sub", NULL, -1, EmbSubMethods,
    NULL, NULL, NULL, NULL
};
static PyObject* PyInit_emb_sub(void)
{
    return PyModule_Create(&EmbSubModule);
}

//// Embedded Python
int main()
{
    PyImport_AppendInittab("emb", &PyInit_emb);
    PyImport_AppendInittab("emb.sub", &PyInit_emb_sub);
    Py_Initialize();

    PyRun_SimpleString("import emb.sub\n");

    Py_Finalize();
    return 0;
}

And now I get this error :

Traceback (most recent call last):
  File "<string>", line 1, in <module>
ImportError: No module named 'emb.sub'

I would like to know if it is possible to create a hierarchy of packages and modules in embedded python ?

Thanks !

@Ha11owed Finally I did not used packages but accepted response works. — merovingien
– merovingien, Commented Mar 7, 2018 at 8:16

dgym · Accepted Answer · 2016-11-11 16:12:12Z

As far as I can tell the default builtin module handling fails to match against package+module names. This can be worked around by adding a custom import hook that does, just run this after Py_Initialize():

PyRun_SimpleString(
    "import importlib.abc\n" \
    "import importlib.machinery\n" \
    "import sys\n" \
    "\n" \
    "\n" \
    "class Finder(importlib.abc.MetaPathFinder):\n" \
    "    def find_spec(self, fullname, path, target=None):\n" \
    "        if fullname in sys.builtin_module_names:\n" \
    "            return importlib.machinery.ModuleSpec(\n" \
    "                fullname,\n" \
    "                importlib.machinery.BuiltinImporter,\n" \
    "            )\n" \
    "\n" \
    "\n" \
    "sys.meta_path.append(Finder())\n" \
);

I tried this with your complete code with success:

#include <Python.h>

//// Definition of 'emb' module
static PyModuleDef EmbModule = {
    PyModuleDef_HEAD_INIT, "emb", NULL, -1, NULL,
    NULL, NULL, NULL, NULL
};
static PyObject* PyInit_emb(void)
{
    PyObject *mod = PyModule_Create(&EmbModule);
    PyModule_AddObject(mod, "__path__", Py_BuildValue("()"));

    return mod;
}

//// Definition of 'emb.sub' module
static PyObject* emb_sub_foo(PyObject *self, PyObject *args)
{
    char const* n = "I am sub foo";
    return Py_BuildValue("s", n);
}
static PyMethodDef EmbSubMethods[] = {
    {"foo", emb_sub_foo, METH_VARARGS, "Returns sub foo"},
    {NULL, NULL, 0, NULL}
};
static PyModuleDef EmbSubModule = {
    PyModuleDef_HEAD_INIT, "emb.sub", NULL, -1, EmbSubMethods,
    NULL, NULL, NULL, NULL
};
static PyObject* PyInit_emb_sub(void)
{
    return PyModule_Create(&EmbSubModule);
}

//// Embedded Python
int main()
{
    PyImport_AppendInittab("emb", &PyInit_emb);
    PyImport_AppendInittab("emb.sub", &PyInit_emb_sub);
    Py_Initialize();

    PyRun_SimpleString(
        "import importlib.abc\n" \
        "import importlib.machinery\n" \
        "import sys\n" \
        "\n" \
        "\n" \
        "class Finder(importlib.abc.MetaPathFinder):\n" \
        "    def find_spec(self, fullname, path, target=None):\n" \
        "        if fullname in sys.builtin_module_names:\n" \
        "            return importlib.machinery.ModuleSpec(\n" \
        "                fullname,\n" \
        "                importlib.machinery.BuiltinImporter,\n" \
        "            )\n" \
        "\n" \
        "\n" \
        "sys.meta_path.append(Finder())\n" \
    );

    PyRun_SimpleString("import emb.sub\n");
    PyRun_SimpleString("print(emb.sub.foo())\n");

    Py_Finalize();
    return 0;
}

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