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1. vstinner

   added

   type-featureA feature request or enhancementA feature request or enhancement

   interpreter-core(Objects, Python, Grammar, and Parser dirs)(Objects, Python, Grammar, and Parser dirs)

   topic-C-API

   on Oct 9, 2025
2. vstinner

   added 2 commits that reference this issue on Oct 9, 2025

   pythongh-139847: Add PyType_Lookup() function

   fb1523b

   pythongh-139847: Add PyType_Lookup() function

   480faa6
3. markshannon commented on Oct 10, 2025

   

   markshannon

   on Oct 10, 2025

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   This does seem to be filling a gap in the C API.

   Reacted by msmojtabafar
4. encukou commented on Oct 10, 2025

   

   encukou

   on Oct 10, 2025

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   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).
5. encukou commented on Oct 10, 2025

   

   encukou

   on Oct 10, 2025

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   I had some notes lying around; so I collected them here: https://discuss.python.org/t/api-for-special-attribute-lookup/104288
6. encukou commented on Oct 20, 2025

   

   encukou

   on Oct 20, 2025

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   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?
7. vstinner commented on Dec 3, 2025

   

   vstinner

   on Dec 3, 2025

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   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;
                   }
8. vstinner commented on Feb 10, 2026

   

   vstinner

   on Feb 10, 2026

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   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.
9. vstinner

   closed this as completedon Feb 10, 2026