[Python-Dev] Use of PyObject_NEW (original) (raw)

Martin v. Loewis martin@v.loewis.de
Fri, 15 Mar 2002 08:29:50 +0100

I tried to understand the various memory allocation function and macros in Python, and found that there is probably a misunderstanding in what PyObject_NEW does.

For example, PyRange_New says

rangeobject *obj = PyObject_NEW(rangeobject, &PyRange_Type);

if (obj == NULL)
    return NULL;

The assumption apparently is that somebody will raise a MemoryError and return NULL when allocation fails. However, this code expands to

 rangeobject *obj = ((rangeobject*)PyObject_Init(
           (PyObject *) malloc(((&PyRange_Type)->tp_basicsize)),
           (&PyRange_Type)));

if (obj == ((void *)0) )
    return ((void *)0) ;

malloc will just return NULL in case of failure, and PyObject_Init starts with

if (op == NULL) {
    PyErr_SetString(PyExc_SystemError,
            "NULL object passed to PyObject_Init");
    return op;
  }

So instead of a MemoryError, you will get a SystemError if the system runs out of memory. Is that intentional?

The documentation says

Macro version of \cfunction{PyObject_New()}, to gain performance at the expense of safety. This does not check \var{type} for a \NULL{} value.

This is incorrect: It does check for NULL. It also does not help to gain performance - PyObject_New has three calls (_PyObject_New, malloc, _Py_NewReference), and so does PyObject_NEW (malloc, PyObject_Init, _Py_NewReference).

I recommend to deprecate PyObject_NEW (and correspondingly PyObject_NEW_VAR, PyObject_DEL).

Regards, Martin