[Python-Dev] Use of PyObject_NEW

[Martin v. Loewis]

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