How do you use the python3 c api for a command line driven app?

Question

I've been using a custom build as a replacement for virtualenv for a while now, and it's brillant. It takes longer to build, but it actually works, and it never screws up.

Part of this in a simple python wrapper that adds some specific folders to the library path, which I've found very useful. The code for it is trivial:

#include <stdio.h>
#include <n/text/StringUtils.h>
#include <Python.h>

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

  /* Setup */
  Py_SetProgramName(argv[0]);
  Py_Initialize();
  PySys_SetArgv(argc, argv);

  /* Add local path */
  PyObject *sys = PyImport_ImportModule("sys");
  PyObject *path = PyObject_GetAttrString(sys, "path");

  /* Custom path */
  char *cwd = nrealpath(argv[0]);
  char *libdir = nstrpath(cwd, "python_lib", NULL);
  PyList_Append(path, PyString_FromString(libdir));
  free(cwd);
  free(libdir);

  /* Run the 'main' module */
  int rtn = Py_Main(argc, argv); // <-- Notice the command line arguments.
  Py_Finalize();

  return rtn;
}

So, moving to python3 is good right? So...

I dutifully replaced the call to PyString_FromString() with PyByte_FromString() and tried to recompile, but it raises errors:

/Users/doug/env/src/main.c:8:21: error: incompatible pointer types passing 'char *' to parameter of type 'wchar_t *' (aka 'int *')
      [-Werror,-Wincompatible-pointer-types]
  Py_SetProgramName(argv[0]);
                    ^~~~~~~
/Users/doug/projects/py-sdl2/py3/include/python3.3m/pythonrun.h:25:45: note: passing argument to parameter here
PyAPI_FUNC(void) Py_SetProgramName(wchar_t *);
                                            ^
/Users/doug/env/src/main.c:10:23: error: incompatible pointer types passing 'char **' to parameter of type 'wchar_t **' (aka 'int **')
      [-Werror,-Wincompatible-pointer-types]
  PySys_SetArgv(argc, argv);
                      ^~~~
/Users/doug/projects/py-sdl2/py3/include/python3.3m/sysmodule.h:12:47: note: passing argument to parameter here
PyAPI_FUNC(void) PySys_SetArgv(int, wchar_t **);
                                              ^
/Users/doug/env/src/main.c:24:27: error: incompatible pointer types passing 'char **' to parameter of type 'wchar_t **' (aka 'int **')
      [-Werror,-Wincompatible-pointer-types]
  int rtn = Py_Main(argc, argv);
                          ^~~~
/Users/doug/projects/py-sdl2/py3/include/python3.3m/pythonrun.h:148:45: note: passing argument to parameter 'argv' here
PyAPI_FUNC(int) Py_Main(int argc, wchar_t **argv);
                                            ^
3 errors generated.
make[2]: *** [CMakeFiles/python.dir/src/main.c.o] Error 1
make[1]: *** [CMakeFiles/python.dir/all] Error 2
make: *** [all] Error 2

As you can see from the error, wchar_t is used instead of char *.

How are you supposed to use this api?

I see there are a few examples of doing this, for example: http://svn.python.org/projects/python/tags/r32rc2/Python/frozenmain.c

seriously?

My 29 line program has to become a 110 line monster full of #ifdefs?

Am I misunderstanding, or has the python3 c api really become this ridiculously difficult to use?

Surely I'm missing some obvious convenience function which does this for you, in a simple, portable and cross platform way?

Answer 1

The official recommended way of converting from char to wchar_t is by using Py_DecodeLocale. Like this:

wchar_t *program = Py_DecodeLocale(argv[0], NULL);
Py_SetProgramName(program);