New submission from STINNER Victor <vstin...@python.org>:
While working on the PEP 674 implementation, I noticed that PyDescr_NAME() and
PyDescr_TYPE() macros are used as l-value by two projects: M2Crypto and
mecab-python3. Both are code generated by SWIG.
M2Crypto-0.38.0/src/SWIG/_m2crypto_wrap.c and
mecab-python3-1.0.4/src/MeCab/MeCab_wrap.cpp contain the function:
SWIGINTERN PyGetSetDescrObject *
SwigPyStaticVar_new_getset(PyTypeObject *type, PyGetSetDef *getset) {
PyGetSetDescrObject *descr;
descr = (PyGetSetDescrObject *)PyType_GenericAlloc(SwigPyStaticVar_Type(), 0);
assert(descr);
Py_XINCREF(type);
PyDescr_TYPE(descr) = type;
PyDescr_NAME(descr) = PyString_InternFromString(getset->name);
descr->d_getset = getset;
if (PyDescr_NAME(descr) == NULL) {
Py_DECREF(descr);
descr = NULL;
}
return descr;
}
ref: https://bugs.python.org/issue45476#msg407410
SwigPyStaticVar_new_getset() is more of less an outdated copy of Python 2.7
descr_copy() of Python Objects/descrobject.c.
I tried to prevent using the PyDescr_NAME() and PyDescr_TYPE() macros as
l-value in two ways:
* Add PyDescr_SET_NAME() and PyDescr_SET_Type()
* Add PyDescr_New()
The problem of the PyDescr_SET_NAME() and PyDescr_SET_Type() approach is that
SWIG code is outdated: it doesn't initialized the PyDescrObject.d_qualname
member added to Python 3.3 (bpo-13577, commit
9d57481f043cb9b94bfc45c1ee041415d915cf8a).
So I implemented PyDescr_New() function: in CPython, it means basically to
rename descr_new() to PyDescr_New(). Problem: implementating this function for
older Python versions is non trivial. Code that I wrote for the
pythoncapi_compat project:
// bpo-45476 added PyDescr_New() to Python 3.11.0a5
#if PY_VERSION_HEX < 0x030B00A5
PyDescrObject *
PyDescr_New(PyTypeObject *descr_type, PyTypeObject *type, const char *name)
{
assert(descr_type != NULL);
assert(type != NULL);
assert(name != NULL);
PyObject *name_obj;
#if PY_MAJOR_VERSION >= 3
name_obj = PyUnicode_InternFromString(name);
#else
name_obj = PyString_InternFromString(name);
#endif
if (name_obj == NULL) {
return NULL;
}
PyDescrObject *descr = (PyDescrObject *)PyType_GenericAlloc(descr_type, 0);
if (descr == NULL) {
Py_DECREF(name_obj);
return NULL;
}
descr->d_type = (PyTypeObject*)Py_NewRef(type);
descr->d_name = name_obj;
// PyDescrObject.d_qualname was added to Python 3.3.0a1
#if PY_VERSION_HEX >= 0x030300A1
descr->d_qualname = NULL;
#endif
return descr;
}
#endif
The even more complicated part is to write unit tests for this function in
pythoncapi_compat. It requires to implement a whole type with tp_traverse
tp_dealloc functions.
In Python, this function must be at least documented.
At the end, IMO it's not really worth it to add so much complexity to Python
and pythoncapi_compat just for 2 projects using SWIG. I created this issue to
keep a track of my analysis, if someone else tries to do something similar
tomorrow.
I attached my WIP patches to keep a copy of this work, in case if we need to
revisit this idea later.
For the PEP 674, IMO it's better to leave the two PyDescr_NAME() and
PyDescr_TYPE() macros unchanged. There is no need in the short term to make
PyDescrObject structure and structures using it opaque in the short term. Well,
SWIG code looks incorrect (it doesn't initialize d_qualname), but a fix just
for SWIG should be enough.
----------
components: C API
messages: 411765
nosy: vstinner
priority: normal
severity: normal
status: open
title: [C API] Make the PyDescrObject structure opaque
versions: Python 3.11
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Python tracker <rep...@bugs.python.org>
<https://bugs.python.org/issue46538>
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