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PEP 757: C API to import-export Python integers (#3961)

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PEP: 757
Title: C API to import-export Python integers
Author: Sergey B Kirpichev <skirpichev@gmail.com>,
Victor Stinner <vstinner@python.org>
PEP-Delegate: C API Working Group
Status: Draft
Type: Standards Track
Created: 13-Sep-2024
Python-Version: 3.14
.. highlight:: c
Abstract
========
Add a new C API to import and export Python integers, ``int`` objects:
especially ``PyLongWriter_Create()`` and ``PyLong_AsDigitArray()``
functions.
Rationale
=========
Projects such as `gmpy2 <https://github.com/aleaxit/gmpy>`_, `SAGE
<https://www.sagemath.org/>`_ and `Python-FLINT
<https://github.com/flintlib/python-flint>`_ access directly Python
"internals" (the ``PyLongObject`` structure) or use an inefficient
temporary format (hex strings for Python-FLINT) to import and
export Python ``int`` objects. The Python ``int`` implementation
changed in Python 3.12 to add a tag and "compact values".
In the 3.13 alpha 1 release, the private undocumented ``_PyLong_New()``
function had been removed, but it is being used by these projects to
import Python integers. The private function has been restored in 3.13
alpha 2.
A public efficient abstraction is needed to interface Python with these
projects without exposing implementation details. It would allow Python
to change its internals without breaking these projects. For example,
implementation for gmpy2 was changed recently for CPython 3.9 and
for CPython 3.12.
Specification
=============
Layout API
----------
API::
typedef struct PyLongLayout {
// Bits per digit
uint8_t bits_per_digit;
// Digit size in bytes
uint8_t digit_size;
// Digits order:
// * 1 for most significant digit first
// * -1 for least significant digit first
int8_t digits_order;
// Endian:
// * 1 for most significant byte first (big endian)
// * -1 for least significant byte first (little endian)
int8_t endian;
} PyLongLayout;
PyAPI_FUNC(const PyLongLayout*) PyLong_GetNativeLayout(void);
Data needed by `GMP <https://gmplib.org/>`_-like import-export functions.
PyLong_GetNativeLayout()
^^^^^^^^^^^^^^^^^^^^^^^^
API: ``const PyLongLayout* PyLong_GetNativeLayout(void)``.
Get the native layout of Python ``int`` objects.
Export API
----------
Export a Python integer as a digits array::
typedef struct PyLong_DigitArray {
// Strong reference to the Python int object.
PyObject *obj;
// 1 if the number is negative, 0 otherwise.
int negative;
// Number of digits in the 'digits' array.
Py_ssize_t ndigits;
// Read-only array of unsigned digits.
const void *digits;
} PyLong_DigitArray;
PyAPI_FUNC(int) PyLong_AsDigitArray(
PyObject *obj,
PyLong_DigitArray *array);
PyAPI_FUNC(void) PyLong_FreeDigitArray(
PyLong_DigitArray *array);
On CPython 3.14, no memory copy is needed, it's just a thin wrapper to
expose Python int internal digits array.
``PyLong_DigitArray.obj`` stores a strong reference to the Python
``int`` object to make sure that that structure remains valid until
``PyLong_FreeDigitArray()`` is called.
PyLong_AsDigitArray()
^^^^^^^^^^^^^^^^^^^^^
API: ``int PyLong_AsDigitArray(PyObject *obj, PyLong_DigitArray *array)``.
Export a Python ``int`` object as a digits array.
On success, set *\*array* and return 0.
On error, set an exception and return -1.
This function always succeeds if *obj* is a Python ``int`` object or a
subclass.
``PyLong_FreeDigitArray()`` must be called once done with using
*array*.
PyLong_FreeDigitArray()
^^^^^^^^^^^^^^^^^^^^^^^
API: ``void PyLong_FreeDigitArray(PyLong_DigitArray *array)``.
Release the export *array* created by ``PyLong_AsDigitArray()``.
Import API
----------
Import a Python integer from a digits array::
// A Python integer writer instance.
// The instance must be destroyed by PyLongWriter_Finish().
typedef struct PyLongWriter PyLongWriter;
PyAPI_FUNC(PyLongWriter*) PyLongWriter_Create(
int negative,
Py_ssize_t ndigits,
void **digits);
PyAPI_FUNC(PyObject*) PyLongWriter_Finish(PyLongWriter *writer);
PyAPI_FUNC(void) PyLongWriter_Discard(PyLongWriter *writer);
On CPython 3.14, the implementation is a thin wrapper to the private
``_PyLong_New()`` function.
``PyLongWriter_Finish()`` takes care of normalizing the digits and
convert the object to a compact integer if needed.
PyLongWriter_Create()
^^^^^^^^^^^^^^^^^^^^^
API: ``PyLongWriter* PyLongWriter_Create(int negative, Py_ssize_t ndigits, void **digits)``.
Create a ``PyLongWriter``.
On success, set *\*digits* and return a writer.
On error, set an exception and return ``NULL``.
*negative* is ``1`` if the number is negative, or ``0`` otherwise.
*ndigits* is the number of digits in the *digits* array. It must be
greater than or equal to 0.
The caller must initialize the digits array *digits* and then call
``PyLongWriter_Finish()`` to get a Python ``int``. Digits must be
in the range [``0``; ``PyLong_BASE - 1``]. Unused digits must be set to
``0``.
PyLongWriter_Finish()
^^^^^^^^^^^^^^^^^^^^^
API: ``PyObject* PyLongWriter_Finish(PyLongWriter *writer)``.
Finish a ``PyLongWriter`` created by ``PyLongWriter_Create()``.
On success, return a Python ``int`` object.
On error, set an exception and return ``NULL``.
PyLongWriter_Discard()
^^^^^^^^^^^^^^^^^^^^^^
API: ``void PyLongWriter_Discard(PyLongWriter *writer)``.
Discard the internal object and destroy the writer instance.
Optimize small integers
=======================
Proposed API are efficient for large integers. Compared to accessing
directly Python internals, the proposed API can have a significant
performance overhead on small integers.
For small integers of a few digits (for example, 1 or 2 digits), existing APIs
can be used. Examples to import / export:
* ``PyLong_FromUInt64()`` / ``PyLong_AsUInt64()``;
* ``PyLong_FromLong()`` / ``PyLong_AsLong()`` or ``PyLong_AsInt()``;
* ``PyUnstable_PyLong_IsCompact()`` and
``PyUnstable_PyLong_CompactValue()``;
* ``PyLong_FromNativeBytes()`` / ``PyLong_AsNativeBytes()``;
* etc.
Implementation
==============
* CPython:
* https://github.com/python/cpython/pull/121339
* https://github.com/vstinner/cpython/pull/5
* gmpy:
* https://github.com/aleaxit/gmpy/pull/495
Benchmarks
==========
Export: PyLong_AsDigitArray() with gmpy2
----------------------------------------
Code::
static void
mpz_set_PyLong(mpz_t z, PyObject *obj)
{
int overflow;
long val = PyLong_AsLongAndOverflow(obj, &overflow);
if (overflow) {
const PyLongLayout* layout = PyLong_GetNativeLayout();
static PyLong_DigitArray long_export;
PyLong_AsDigitArray(obj, &long_export);
mpz_import(z, long_export.ndigits, layout->endian,
layout->digit_size, layout->digits_order,
layout->digit_size*8 - layout->bits_per_digit,
long_export.digits);
if (long_export.negative) {
mpz_neg(z, z);
}
PyLong_FreeDigitArray(&long_export);
}
else {
mpz_set_si(z, val);
}
}
Benchmark:
.. code-block:: py
import pyperf
from gmpy2 import mpz
runner = pyperf.Runner()
runner.bench_func('1<<7', mpz, 1 << 7)
runner.bench_func('1<<38', mpz, 1 << 38)
runner.bench_func('1<<300', mpz, 1 << 300)
runner.bench_func('1<<3000', mpz, 1 << 3000)
Results on Linux Fedora 40 with CPU isolation, Python built in release
mode:
+----------------+---------+-----------------------+
| Benchmark | ref | pep757 |
+================+=========+=======================+
| 1<<7 | 94.3 ns | 96.8 ns: 1.03x slower |
+----------------+---------+-----------------------+
| 1<<38 | 127 ns | 99.7 ns: 1.28x faster |
+----------------+---------+-----------------------+
| 1<<300 | 209 ns | 222 ns: 1.06x slower |
+----------------+---------+-----------------------+
| 1<<3000 | 955 ns | 963 ns: 1.01x slower |
+----------------+---------+-----------------------+
| Geometric mean | (ref) | 1.04x faster |
+----------------+---------+-----------------------+
Import: PyLongWriter_Create() with gmpy2
----------------------------------------
Code::
static PyObject *
GMPy_PyLong_From_MPZ(MPZ_Object *obj, CTXT_Object *context)
{
if (mpz_fits_slong_p(obj->z)) {
return PyLong_FromLong(mpz_get_si(obj->z));
}
const PyLongLayout *layout = PyLong_GetNativeLayout();
size_t size = (mpz_sizeinbase(obj->z, 2) +
layout->bits_per_digit - 1) / layout->bits_per_digit;
void *digits;
PyLongWriter *writer = PyLongWriter_Create(mpz_sgn(obj->z) < 0, size,
&digits);
if (writer == NULL) {
return NULL;
}
mpz_export(digits, NULL, layout->endian,
layout->digit_size, layout->digits_order,
layout->digit_size*8 - layout->bits_per_digit,
obj->z);
return PyLongWriter_Finish(writer);
}
Benchmark:
.. code-block:: py
import pyperf
from gmpy2 import mpz
runner = pyperf.Runner()
runner.bench_func('1<<7', int, mpz(1 << 7))
runner.bench_func('1<<38', int, mpz(1 << 38))
runner.bench_func('1<<300', int, mpz(1 << 300))
runner.bench_func('1<<3000', int, mpz(1 << 3000))
Results on Linux Fedora 40 with CPU isolation, Python built in release
mode:
+----------------+--------+----------------------+
| Benchmark | ref | pep757 |
+================+========+======================+
| 1<<300 | 193 ns | 215 ns: 1.11x slower |
+----------------+--------+----------------------+
| 1<<3000 | 927 ns | 943 ns: 1.02x slower |
+----------------+--------+----------------------+
| Geometric mean | (ref) | 1.03x slower |
+----------------+--------+----------------------+
Benchmark hidden because not significant (2): 1<<7, 1<<38.
Backwards Compatibility
=======================
There is no impact on the backward compatibility, only new APIs are
added.
Open Question
=============
* Should we add *digits_order* and *endian* members to ``sys.int_info``
and remove ``PyLong_GetNativeLayout()``? The
``PyLong_GetNativeLayout()`` function returns a C structure
which is more convenient to use in C than ``sys.int_info`` which uses
Python objects.
Rejected Ideas
==============
Support arbitrary layout
------------------------
It would be convenient to support arbitrary layout to import-export
Python integers.
For example, it was proposed to add a *layout* parameter to
``PyLongWriter_Create()`` and a *layout* member to the
``PyLong_DigitArray`` structure.
The problem is that it's more complex to implement and not really
needed. What's strictly needed is only an API to import-export using the
Python "native" layout.
If later there are use cases for arbitrary layouts, new APIs can be
added.
Discussions
===========
* https://github.com/capi-workgroup/decisions/issues/35
* https://github.com/python/cpython/pull/121339
* https://github.com/python/cpython/issues/102471
* `Add public function PyLong_GetDigits()
<https://github.com/capi-workgroup/decisions/issues/31>`_
* `Consider restoring _PyLong_New() function as public
<https://github.com/python/cpython/issues/111415>`_
* `gh-106320: Remove private _PyLong_New() function
<https://github.com/python/cpython/pull/108604>`_
Copyright
=========
This document is placed in the public domain or under the
CC0-1.0-Universal license, whichever is more permissive.