Adding in a new `PyList_Pop` C-API

[Vizonex]

Feature or enhancement

Proposal:

So I maintain a python library called FrozenList and I have been trying to improve it's performance in cython significantly but there is still one final bottleneck that remains in my way and that is having a good PyList_Pop() implementation that I can use instead of Cython Generating a more expensive pure python call so I was wondering if I could go in and add a new function in the future to the current C-API that could enable popping an item out of a list object, I have made a full sample using the provided compatibility header file extension for my example here.

#ifndef __LIST_POP_H__
#define __LIST_POP_H__

#ifdef __cplusplus
extern "C" {
#endif

/* A Small File containing a compatible PyList_Pop C-API implementation for
 * Python. */
#include "Python.h"
#include "pythoncapi_compat.h" /* SEE: https://github.com/python/pythoncapi-compat */
#include <assert.h>

/* Inlined version of `valid_index` seen in `listobject.c`
It's just here only for the demonstrational purposes only... */
#define __PYLIST_VALID_INDEX(i, limit) (((size_t)(i)) < ((size_t)(limit)))

/* same api as `list.pop(...)` in python it accepts an index as well as negative
 * numbers. op: the object to pop from. index: the index to take an object out
 * of. returns: The object that has been removed, otherwise this will be NULL 
 * if an error is raised. */

PyObject *PyList_Pop(PyObject *op, Py_ssize_t index) 
{
  	/* Must be defined here due to needing a Critical Section. */
  	PyObject *v;

  	if (!PyList_Check(op)) {
  	  	PyErr_BadInternalCall();
  	  	return NULL;
  	}

  	if (Py_SIZE(op) == 0) {
  	  	PyErr_SetString(PyExc_IndexError, "pop from empty list");
  	  	return NULL;
  	}

  	if (index < 0)
    	index += Py_SIZE(op);

  	if (!__PYLIST_VALID_INDEX(index, Py_SIZE(op))) {
  	  	PyErr_SetString(PyExc_IndexError, "pop index out of range");
  	  	return NULL;
  	}

  /* We're not going to have the exact same access as python
   * has to stuff like ptr_wise_atomic_memmove so we have to
   * use a much different approach to ensure success, will obtain
   * a new ref immediately since both routes below both STEAL
   * a reference! */

    /* Protect from free-threaded manipulation */
    Py_BEGIN_CRITICAL_SECTION(op)

    v = Py_NewRef(PyList_GET_ITEM(op, index));
    if (Py_SIZE(op) == 1) {
      	assert(PyList_Clear(op) < 0);
      	return v;
    }

    /* Really this shouldn't fail... */
    assert((PySequence_DelItem(op, index) < 0));
    Py_END_CRITICAL_SECTION();
    return v;
};

#ifdef __cplusplus
}
#endif

#endif // __LIST_POP_H__

Has this already been discussed elsewhere?

No response given

Links to previous discussion of this feature:

I have looked around to see if this has been discussed before and I have not found anything worthwhile about this at all yet.

[picnixz]

Please open a thread in https://discuss.python.org/c/ideas/6 first, you will have more people seeing that. Adding somethjing to the C API also needs support from the C API working group: https://github.com/capi-workgroup