Often, in C type implementations, you want to set a PyObject * slot to NULL. Often this goes like: Py_XDECREF(self->foo); self->foo = NULL; However, if your type participates in GC and self->foo is something that might get traversed, this can lead to traversing self->foo after it has been DECREFed, but before it's set to NULL. The correct way to do this is something like: tmp = self->foo; self->foo = NULL; Py_XDECREF(tmp); I suggest that there should be a standard macro to automate this. Py_CLEAR(self->foo) This would be defined to be the same result as Py_XDECREF except that the argument will be set to NULL. Thoughts? If there are no objections I'll add the following definition to object.h, after the definition for Py_DECREF: #define Py_CLEAR(op) \ do { \ if (op) { \ PyObject *tmp = (op); \ (op) = NULL; \ Py_DECREF(tmp); \ } \ } while (0) and update the docs and the tutorial on creating types in C. Jim -- Jim Fulton mailto:jim@zope.com Python Powered! CTO (540) 361-1714 http://www.python.org Zope Corporation http://www.zope.com http://www.zope.org