[Python-Dev] when is path==NULL? (original) (raw)

Thomas Lee tom at vector-seven.com
Tue Sep 30 14:42:11 CEST 2008

Ulrich Eckhardt wrote:

Hi!

I'm looking at trunk/Python/sysmodule.c, function PySysSetArgv(). In that function, there is code like this: PyObject* path = PySysGetObject("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 PyFatalError("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

More information about the Python-Dev mailing list