Somewhere between Python 3.0a3 and Python 3.0b3, a call to PySys_SetObject() 
after having used Py_NewInterpreter() to create a sub interpreter causes a 
crash. This appears to be due to interp->sysdict being NULL after 
Py_NewInterpreter() called.

As illustration of problem, consider program for Python 2.X.

#include <Python.h>

int
main(int argc, char *argv[])
{
    Py_Initialize();
    PySys_SetObject("xxx", PyLong_FromLongLong(1));
    fprintf(stderr, "sysdict=%d\n", !!PyThreadState_Get()->interp->sysdict);
    fflush(stderr);
    PyRun_SimpleString("import sys\n"
                       "print >> sys.stderr, 'xxx =', sys.xxx\n");

    Py_NewInterpreter();
    fprintf(stderr, "sysdict=%d\n", !!PyThreadState_Get()->interp->sysdict);
    fflush(stderr);
    PySys_SetObject("yyy", PyLong_FromLongLong(2));
    PyRun_SimpleString("import sys\n"
                       "print >> sys.stderr, 'yyy =', sys.yyy\n");

    Py_Finalize();
    return 0;
}

This when run yields:

sysdict=1
xxx = 1
sysdict=1
yyy = 2

Now, for Python 3.0 variant of same program:

#include <Python.h>

int
main(int argc, char *argv[])
{
    Py_Initialize();
    fprintf(stderr, "sysdict=%d\n", !!PyThreadState_Get()->interp->sysdict);
    fflush(stderr);
    PySys_SetObject("xxx", PyLong_FromLongLong(1));
    PyRun_SimpleString("import sys\n"
                       "print('xxx =',sys.xxx, file=sys.stderr)\n");

    Py_NewInterpreter();
    fprintf(stderr, "sysdict=%d\n", !!PyThreadState_Get()->interp->sysdict);
    fflush(stderr);
    PySys_SetObject("yyy", PyLong_FromLongLong(2));
    PyRun_SimpleString("import sys\n"
                       "print('yyy =',sys.yyy, file=sys.stderr)\n");

    Py_Finalize();
    return 0;
}

I get for Python 3.0a3:

sysdict=1
xxx = 1
sysdict=1
object  : AttributeError("'module' object has no attribute 'stderr'",)
type    : AttributeError
refcount: 4
address : 0xf1180
lost sys.stderr

I am not concerned here about loss of sys.stderr, although that could be a 
separate issue for all I know.

The important bit here is that sysdict is set after Py_NewInterpreter().

In Python 3.0b3/3.0rc1 I instead get:

sysdict=1
xxx = 1
sysdict=0
Bus error

This is because PySys_SetObject() is presumably crashing because sysdict is 
not set in interp object.

I tried to ask about this on python-3000 Google group, but that message ended 
up in some moderation queue and has vanished. Thus quote part of that message 
below.

"""
From what I can tell so far the problem is that 'interp->sysdict' is
NULL after calling Py_NewInterpreter() to create a secondary sub
interpreter.

Reading through code and using a debugger, at this point this seems to
be due to condition if code:

       sysmod = _PyImport_FindExtension("sys", "sys");
       if (bimod != NULL && sysmod != NULL) {
               interp->sysdict = PyModule_GetDict(sysmod);
               if (interp->sysdict == NULL)
                       goto handle_error;
               Py_INCREF(interp->sysdict);
               PySys_SetPath(Py_GetPath());
               PyDict_SetItemString(interp->sysdict, "modules",
                                    interp->modules);
               _PyImportHooks_Init();
               initmain();
               if (!Py_NoSiteFlag)
                       initsite();
       }

in Py_NewInterpreter() not executing due to
_PyImport_FindExtension("sys", "sys") returning NULL.

Down in _PyImport_FindExtension(), it appears that the reason it fails
is because of following returning with NULL.

       def = (PyModuleDef*)PyDict_GetItemString(extensions,
filename);

       .....

               if (def->m_base.m_init == NULL)
                       return NULL;

In other words, whatever m_base.m_init is meant to be is NULL when
perhaps it isn't meant to be.

(gdb) call ((PyModuleDef*)PyDict_GetItemString(extensions,"builtins"))-
>m_base.m_init
$9 = (PyObject *(*)()) 0
(gdb) call ((PyModuleDef*)PyDict_GetItemString(extensions,"sys"))-
>m_base.m_init
$10 = (PyObject *(*)()) 0

I am going to keep tracking through to try and work out why, but
posting this initial information in case this rings a bell with
anyone.
"""

Is this expected behaviour? Or, is it necessary now to perform some special 
initialisation after having called Py_NewInterpreter() to get builtins and 
sys modules setup?

This problem originally came up with mod_wsgi, which worked fine with Python 
3.0a3, but fails on more recent releases because of this.

I know sub interpreters may be seen as evil by some and the target of 
removal, but are they still meant to work for now at least?