can't import most modules (but no exception)

[David F. <Dav...@hu...>]

I’m trying to take the example from section 1.3 of the python embedding documentation (https://docs.python.org/3/extending/embedding.html#pure-embedding), with usage


Usage: call pythonfile funcname [args]

and rewrite it using CXX (with some extra diagnostics, and modified to use str() so it handles functions which return anything, not just long):

#include <Python.h>
#include <CXX/Objects.hxx>
#include <string>
#include <iostream>

int
main(int argc, char *argv[])
{
    int i;

    if (argc < 3) {
        fprintf(stderr,"Usage: call pythonfile funcname [args]\n");
        return 1;
    }

    std::string name(argv[1]);

    std::string funcname(argv[2]);
    Py_Initialize();
    std::string status;
    try {
        Py::Module module(name);

        std::cout << module.repr() << '\n';
        if (!module.isTrue()) {
            std::cout << "module was null/false\n";
            throw Py::Exception();
        }

        status = "finding function " + funcname;
        if (!module.hasAttr(funcname)) {
            std::cout << "function " << funcname + " not found\n";
            throw Py::Exception();
        }
        Py::Object obj = module.getAttr(funcname);
        if (!obj.isTrue()) {
            std::cout << "function was null/false\n";
            throw Py::Exception();
        }
        if (!obj.isCallable()) {
            std::cout << "got function but not callable?\n";
            throw Py::Exception();
        }
        status = "casting function " + funcname;
        Py::Callable func(obj);
        status = "creating arg tuple";
        Py::Tuple fargs(argc - 3);
        status = "filling arg tuple";
        for (i = 0; i < argc - 3; ++i) {
            fargs[i] = Py::Long(atoi(argv[i + 3]));
        }
        status = "calling";
        Py::Object res(func.apply(fargs));
        //        status = "converting to long";
        //        Py::Long v(res);
        std::cout << "Result of call: " << res.str() << '\n';
    } catch (const Py::Exception &) {
        std::cout << "call failed with status " << status << '\n';
    }

    Py_Finalize();
    return 0;
}


The original version compiles and runs correctly.   For example

$ ./call math exp 2
Result of call: 7

$ PYTHONPATH=`pwd` ./call multiply multiply 2 3
Will compute 2 times 3
Result of call: 6

My CXX version works as long as the module I specify is sys or os:

./pure_cxx sys exc_info
 <module 'sys' (built-in)>
 Result of call: (None, None, None)

$ ./pure_cxx os getcwd
<module 'os' from '/Library/Frameworks/Python.framework/Versions/3.4/lib/python3.4/os.py'>
Result of call: /Users/dfox/python/test/embed

but if I use math or a custom module, it fails to find the function in the given module:

$ ./pure_cxx math exp 2
<module 'math'>
function exp not found
call failed with status finding function exp

$ PYTHONPATH=`pwd` ./pure_cxx multiply multiply 2 3
<module 'multiply'>
function multiply not found
call failed with status finding function multiply

Oddly, it doesn’t raise an exception when I create the Py::Module, but that module seems to be a dummy module, with the right name and a few other attributes all with value None, as can be seen by converting the module to a Py::List and iterating over it:


0  __name__ : math

1  __loader__ : None

2  __spec__ : None

3  __package__ : None

4  __doc__ : None

Any idea what I’m doing wrong?