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?

Sorry, should also mention that this was on MacOS X 10.4 (Tiger).

This seems a duplicate of issue3723.