GeezerVison is showing github.com — C API: Consider adding a public PyList_Extend() function · Issue #111138 · python/cpython
C API: Consider adding a public PyList_Extend() function #111138DescriptionFeature or enhancement
The private _PyList_Extend() function has been removed in Python 3.13: see PR #108451.
@scoder asked what is the replacement for this removed function.
The obvious replacement is PyObject_CallMethod(list, "extend", "O", arg): call the list.extend() method. But it's slower, PyObject_CallMethod() has to get the method and decode the "O" format string.
Another replacement is PyList_SetSlice(L, PY_SSIZE_T_MAX, PY_SSIZE_T_MAX, arg) which is less straightforward, but it's efficient.
I propose adding a public PyList_Extend() function. The list type is commonly used in C extensions, it's a convenient API to create a collection when the length is not known is advance.
I don't think that performance is really the problem here since PyList_SetSlice(L, PY_SSIZE_T_MAX, PY_SSIZE_T_MAX, arg) is available. The problem is more to more the function easier to discover and make the API easier to use. Also, it should help people to migrate away from the removed function.
There is already a public PyDict_Update() API for the dict type, it is part of the limited C API.
If we add a function for the list type, should we also add PySet_Update() "for completeness"? The private _PySet_Update() was removed in Python 3.13.
cc @serhiy-storchaka @encukou
A code search on _PyList_Extend in PyPI top 5,000 projects (2023-07-04) found 4 projects using this function:
- Cython (0.29.36)
- catboost (1.2)
- mypy (1.4.1)
- pyrsistent (0.19.3)
Logs:
Details
PYPI-2023-07-04/catboost-1.2.tar.gz: catboost-1.2/catboost_all_src/contrib/tools/cython/Cython/Compiler/Builtin.py: BuiltinMethod("extend", "TO", "r", "__Pyx_PyList_Extend",
PYPI-2023-07-04/catboost-1.2.tar.gz: catboost-1.2/catboost_all_src/contrib/tools/cython/Cython/Compiler/ExprNodes.py: extend_func = "__Pyx_PyList_Extend"
PYPI-2023-07-04/catboost-1.2.tar.gz: catboost-1.2/catboost_all_src/contrib/tools/cython/Cython/Utility/Optimize.c: static CYTHON_INLINE int __Pyx_PyList_Extend(PyObject* L, PyObject* v) {
PYPI-2023-07-04/catboost-1.2.tar.gz: catboost-1.2/catboost_all_src/contrib/tools/cython/Cython/Utility/Optimize.c: PyObject* none = _PyList_Extend((PyListObject*)L, v);
PYPI-2023-07-04/Cython-0.29.36.tar.gz: Cython-0.29.36/Cython/Compiler/Builtin.py: BuiltinMethod("extend", "TO", "r", "__Pyx_PyList_Extend",
PYPI-2023-07-04/Cython-0.29.36.tar.gz: Cython-0.29.36/Cython/Compiler/ExprNodes.py: extend_func = "__Pyx_PyList_Extend"
PYPI-2023-07-04/Cython-0.29.36.tar.gz: Cython-0.29.36/Cython/Utility/Optimize.c: static CYTHON_INLINE int __Pyx_PyList_Extend(PyObject* L, PyObject* v) {
PYPI-2023-07-04/Cython-0.29.36.tar.gz: Cython-0.29.36/Cython/Utility/Optimize.c: PyObject* none = _PyList_Extend((PyListObject*)L, v);
PYPI-2023-07-04/mypy-1.4.1.tar.gz: mypy-1.4.1/mypyc/lib-rt/dict_ops.c: PyObject *res = _PyList_Extend((PyListObject *)list, view);
PYPI-2023-07-04/mypy-1.4.1.tar.gz: mypy-1.4.1/mypyc/lib-rt/dict_ops.c: PyObject *res = _PyList_Extend((PyListObject *)list, view);
PYPI-2023-07-04/mypy-1.4.1.tar.gz: mypy-1.4.1/mypyc/lib-rt/dict_ops.c: PyObject *res = _PyList_Extend((PyListObject *)list, view);
PYPI-2023-07-04/mypy-1.4.1.tar.gz: mypy-1.4.1/mypyc/lib-rt/list_ops.c: return _PyList_Extend((PyListObject *)o1, o2);
PYPI-2023-07-04/pyrsistent-0.19.3.tar.gz: pyrsistent-0.19.3/pvectorcmodule.c: PyObject *retVal = _PyList_Extend((PyListObject *)self->appendList, args);
Cython uses the function to implement its own __Pyx_PyList_Extend() API:
static CYTHON_INLINE int __Pyx_PyList_Extend(PyObject* L, PyObject* v) {
#if CYTHON_COMPILING_IN_CPYTHON
PyObject* none = _PyList_Extend((PyListObject*)L, v);
if (unlikely(!none))
return -1;
Py_DECREF(none);
return 0;
#else
return PyList_SetSlice(L, PY_SSIZE_T_MAX, PY_SSIZE_T_MAX, v);
#endif
}Linked PRs
Reacted by Kirill Podoprigora Activitychanged the title [-]C API: Consider adding a public PyList_Extend() function to replace removed private _PyList_Extend()[/-] [+]C API: Consider adding a public PyList_Extend() function[/+] on Oct 20, 2023serhiy-storchaka commented on Oct 21, 2023 PyDict_Update() is not trivial. PyList_Extend() is trivial, literally one-liner.
#define PyList_Extend(list, arg) PyList_SetSlice((list), PY_SSIZE_T_MAX, PY_SSIZE_T_MAX, (arg)) I don't mind adding it as a macro. Together with PyList_Clear().
#define PyList_Clear(list) PyList_SetSlice((list), 0, PY_SSIZE_T_MAX, NULL) musicinmybrain commented on Oct 26, 2023 Just noting that the proposed PyList_Extend macro is not an exact drop-in replacement for the removed _PyList_Extend() function, since the necessary error-checking is a bit different: _PyList_Extend() returns PyObject *, but PyList_SetSlice() returns int. vstinner commented on Oct 27, 2023 vstinner commented on Oct 27, 2023
_PyList_Extend() returns PyObject *
For a method used in Python, it makes sense. For a C API, it looks inefficient to return None on success. musicinmybrain commented on Oct 27, 2023
For a method used in Python, it makes sense. For a C API, it looks inefficient to return None on success.
I agree, and I think returning int is the correct signature for a C API.
It’s just worth pointing out that extension authors porting to Python 3.13 will need to be aware that they cannot simply search-and-replace _PyList_Extend with the proposed PyList_Extend, despite the similar names. tobgu/pyrsistent#284 is a good example. vstinner commented on Nov 1, 2023 First step: I wrote PR #111582 to test the existing public PyList C API. vstinner commented on Nov 8, 2023 PR gh-111562 added tests on the PyList C API.
I wrote PR gh-111862 to add PyList_Extend() and PyList_Clear() to the public C API. vstinner commented on Nov 13, 2023 I close the issue.
Commit babb787 adds PyList_Extend() and PyList_Clear(). I wrote python/pythoncapi-compat#80 to add these functions to pythoncapi-compat.
@serhiy-storchaka's macros can also be used on Python 3.12 and older:
#define PyList_Extend(list, arg) PyList_SetSlice((list), PY_SSIZE_T_MAX, PY_SSIZE_T_MAX, (arg))
#define PyList_Clear(list) PyList_SetSlice((list), 0, PY_SSIZE_T_MAX, NULL)
If we add a function for the list type, should we also add PySet_Update() "for completeness"? The private _PySet_Update() was removed in Python 3.13.
I didn't add this function.
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