GeezerVison is showing github.com — [C API] Add PyType_Lookup() function · Issue #139847 · python/cpython
[C API] Add PyType_Lookup() function #139847DescriptionFeature or enhancement
There is a private _PyType_Lookup() function which is used by 27 projects of the PyPI top 15,000 projects:
- Nuitka (2.7.16)
- ansys_pythonnet-3.1.0rc6
- casadi (3.7.2)
- catboost (1.2.8)
- cvxpy (1.7.2)
- cvxpy_base (1.7.2)
- cython (3.1.4)
- extensionclass (6.0)
- gamsapi (51.1.0)
- gdal (3.11.4)
- gevent (25.9.1)
- htseq (2.0.9)
- ihm (2.7)
- m2crypto (0.45.1)
- mecab-ko (1.0.1)
- mecab_python3 (1.0.10)
- msgspec (0.19.0)
- mupdf (1.21.1.20230112.1504)
- mypy (1.18.2)
- pybind11 (3.0.1)
- pycuda (2025.1.2)
- pygdal (3.6.4.11)
- pygraphviz (1.14)
- python_crfsuite (0.9.11)
- pythonnet (3.0.5)
- sentencepiece (0.2.1)
- yappi (1.6.10)
I propose to make the function public.
Linked PRs
Activitymarkshannon commented on Oct 10, 2025 This does seem to be filling a gap in the C API. encukou commented on Oct 10, 2025 To me, _PyType_Lookup sounds like a special case -- a useful special case, but it might be good to consider in together with more variations. Off the top of my head, things that you might want to enable/disable are:
- (not) looking in the instance, direct class, superclasses, metaclass
- (not) invoking descriptors
- (not) binding methods with
Py_TPFLAGS_METHOD_DESCRIPTOR
Also: when we walk the MRO we should return the class the attribute is on (the defining class). encukou commented on Oct 10, 2025 encukou commented on Oct 20, 2025 I guess the most important thing about this lookup is not the exact behaviour (though it's important to document that), but the fact that Python uses it to find (and cache) special methods.
Should we expose this, or _PyObject_LookupSpecial or _PyObject_LookupSpecialMethod?
Do we know how many uses those functions have, and how many of the _PyType_Lookup users resolve descriptors after the _PyType_Lookup call? vstinner commented on Dec 3, 2025 I looked at some projects to see how _PyType_Lookup() is used.
mypyc: C code emitted by mypyc:
PyObject *del_str = PyUnicode_FromString("__del__");
PyObject *del_method = (del_str == NULL) ? NULL : _PyType_Lookup(Py_TYPE(self), del_str);
PyErr_WriteUnraisable(del_method);
Py_XDECREF(del_method);
Py_XDECREF(del_str);msgspec:
PyObject *descr = _PyType_Lookup(type, name);
if (descr != NULL) {
descrgetfunc get = Py_TYPE(descr)->tp_descr_get;
descrsetfunc set = Py_TYPE(descr)->tp_descr_set;
if (get != NULL && set != NULL) {
Py_INCREF(descr);
val = get(descr, iter->obj, (PyObject *)type);
Py_DECREF(descr);
if (val != NULL) goto found_val;
PyErr_Clear();
}
}pythonnet (src/runtime/Types/MetaType.cs):
public static int tp_setattro(BorrowedReference tp, BorrowedReference name, BorrowedReference value)
{
BorrowedReference descr = Runtime._PyType_Lookup(tp, name);
if (descr != null)
{
BorrowedReference dt = Runtime.PyObject_TYPE(descr);
if (dt == Runtime.PyWrapperDescriptorType
|| dt == Runtime.PyMethodType
|| typeof(ExtensionType).IsInstanceOfType(GetManagedObject(descr))
)
{
IntPtr fp = Util.ReadIntPtr(dt, TypeOffset.tp_descr_set);
if (fp != IntPtr.Zero)
{
return NativeCall.Int_Call_3(fp, descr, name, value);
}
Exceptions.SetError(Exceptions.AttributeError, "attribute is read-only");
return -1;
}
}
int res = Runtime.PyObject_GenericSetAttr(tp, name, value);
Runtime.PyType_Modified(tp);
return res;
}sentencepiece: code generated by SWIG.
#ifdef SWIGPYTHON_BUILTIN
SWIGRUNTIME int
SWIG_Python_NonDynamicSetAttr(PyObject *obj, PyObject *name, PyObject *value) {
...
descr = _PyType_Lookup(tp, name);
f = NULL;
if (descr != NULL)
f = descr->ob_type->tp_descr_set;
if (!f) {
if (PyString_Check(name)) {
encoded_name = name;
SWIG_Py_INCREF(name);
} else {
encoded_name = PyUnicode_AsUTF8String(name);
if (!encoded_name)
goto done;
}
PyErr_Format(PyExc_AttributeError, "'%.100s' object has no attribute '%.200s'", tp->tp_name, PyString_AsString(encoded_name
SWIG_Py_DECREF(encoded_name);
} else {
res = f(descr, obj, value);
}yappi:
// get name from type+name
mo = _PyType_Lookup((PyTypeObject *)obj_type, name);
Py_XINCREF(mo);
...
if (mo != NULL) {
pit->name = PyObject_Repr(mo);
Py_DECREF(mo);
return pit;
}vstinner commented on Feb 10, 2026 The C API Working Group decided to not add int PyType_Lookup(PyTypeObject *type, PyObject *name, PyObject **attr) API: capi-workgroup/decisions#83. So I close the issue. For now, the status quo is to keep the private _PyType_Lookup() function.
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