[Python-Dev] when is path==NULL?

[Thomas Lee]

Ulrich Eckhardt wrote:
> Hi!
>
> I'm looking at trunk/Python/sysmodule.c, function PySys_SetArgv(). In that 
> function, there is code like this:
>
>   PyObject* path = PySys_GetObject("path");
>   ...
>   if (path != NULL) {
>     ...
>   }
>
> My intuition says that if path==NULL, something is very wrong. At least I 
> would expect to get 'None', but never NULL, except when out of memory. So, 
> for the case that path==NULL', I would simply invoke Py_FatalError("no mem 
> for sys.path"), similarly to the other call there.
>
> Sounds reasonable?
>
> Uli
>
>   
Maybe it's just being safe?

 From Python/sysmodule.c:

    PyThreadState *tstate = PyThreadState_GET();
    PyObject *sd = tstate->interp->sysdict;
    if (sd == NULL)
        return NULL;
    return PyDict_GetItemString(sd, name);


So if tstate->interp->sysdict is NULL, we return NULL. That's probably a 
bit unlikely.

However, PyDict_GetItemString attempts to allocate a new PyString from 
the given char* key. If that fails, PySys_GetObject will also return 
NULL -- just like most functions in the code base that hit an out of 
memory error:

PyObject *
PyDict_GetItemString(PyObject *v, const char *key)
{
    PyObject *kv, *rv;
    kv = PyString_FromString(key);
    if (kv == NULL)
        return NULL;
    rv = PyDict_GetItem(v, kv);
    Py_DECREF(kv);
    return rv;
}

Seems perfectly reasonable for it to return NULL in this situation.

Cheers,
T