gh-144989: Expose special method lookup at Python level

Doc/library/types.rst

@@ -521,6 +521,44 @@ Additional Utility Classes and Functions
 
    .. versionadded:: 3.4
 
+.. function:: lookup_special_method(obj, attr, /)
+
+     Lookup special method name ``attr`` on ``obj``.
+
+     Lookup method ``attr`` on ``obj`` without looking in the instance
+     dictionary. For methods defined in class ``__dict__`` or ``__slots__``, it
+     returns the unbound function (descriptor), not a bound method. The
+     caller is responsible for passing the object as the first argument when
+     calling it:
+
+     .. code-block:: python
+
+         >>> class A:
+         ...    def __enter__(self):
+         ...        return "A.__enter__"
+         ...
+         >>> class B:
+         ...    __slots__ = ("__enter__",)
+         ...    def __init__(self):
+         ...        def __enter__(self):
+         ...            return "B.__enter__"
+         ...        self.__enter__ = __enter__
+         ...
+         >>> a = A()
+         >>> b = B()
+         >>> enter_a = types.lookup_special_method(a, "__enter__")
+         >>> enter_b = types.lookup_special_method(b, "__enter__")
+         >>> enter_a(a)
+         'A.__enter__'
+         >>> enter_b(b)
+         'B.__enter__'
+
+     For other descriptors (property, etc.), it returns the result of the
+     descriptor's ``__get__`` method. Returns ``None`` if the method is not
+     found.
+
+   .. versionadded:: next
+
 
 Coroutine Utility Functions
 ---------------------------

Doc/whatsnew/3.15.rst

@@ -1064,6 +1064,8 @@ types
   This represents the type of the :attr:`frame.f_locals` attribute,
   as described in :pep:`667`.
 
+* Expose ``_PyObject_LookupSpecialMethod`` as
+  :func:`types.lookup_special_method`.
 
 unicodedata
 -----------

Lib/test/test_types.py

@@ -43,7 +43,8 @@ class TypesTests(unittest.TestCase):
     def test_names(self):
         c_only_names = {'CapsuleType', 'LazyImportType'}
         ignored = {'new_class', 'resolve_bases', 'prepare_class',
-                   'get_original_bases', 'DynamicClassAttribute', 'coroutine'}
+                   'get_original_bases', 'DynamicClassAttribute', 'coroutine',
+                   'lookup_special_method'}
 
         for name in c_types.__all__:
             if name not in c_only_names | ignored:
@@ -59,7 +60,7 @@ def test_names(self):
             'MemberDescriptorType', 'MethodDescriptorType', 'MethodType',
             'MethodWrapperType', 'ModuleType', 'NoneType',
             'NotImplementedType', 'SimpleNamespace', 'TracebackType',
-            'UnionType', 'WrapperDescriptorType',
+            'UnionType', 'WrapperDescriptorType', 'lookup_special_method',
         }
         self.assertEqual(all_names, set(c_types.__all__))
         self.assertEqual(all_names - c_only_names, set(py_types.__all__))
@@ -726,6 +727,53 @@ def test_frame_locals_proxy_type(self):
         self.assertIsNotNone(frame)
         self.assertIsInstance(frame.f_locals, types.FrameLocalsProxyType)
 
+    def _test_lookup_special_method(self, lookup):
+        class CM1:
+            def __enter__(self):
+                return "__enter__ from class __dict__"
+
+        class CM2:
+            def __init__(self):
+                def __enter__(self):
+                    return "__enter__ from instance __dict__"
+                self.__enter__ = __enter__
+
+        class CM3:
+            __slots__ = ("__enter__",)
+            def __init__(self):
+                def __enter__(self):
+                    return "__enter__ from __slots__"
+                self.__enter__ = __enter__
+        cm1 = CM1()
+        meth = lookup(cm1, "__enter__")
+        self.assertIsNotNone(meth)
+        self.assertEqual(meth(cm1), "__enter__ from class __dict__")
+
+        meth = lookup(cm1, "__missing__")
+        self.assertIsNone(meth)
+
+        with self.assertRaises(TypeError):
+            lookup(cm1, 123)
+
+        cm2 = CM2()
+        meth = lookup(cm2, "__enter__")
+        self.assertIsNone(meth)
+
+        cm3 = CM3()
+        meth = lookup(cm3, "__enter__")
+        self.assertIsNotNone(meth)
+        self.assertEqual(meth(cm3), "__enter__ from __slots__")
+
+        meth = lookup([], "__len__")
+        self.assertIsNotNone(meth)
+        self.assertEqual(meth([]), 0)
+
+    def test_lookup_special_method(self):
+        c_lookup = getattr(c_types, "lookup_special_method")
+        py_lookup = getattr(py_types, "lookup_special_method")
+        self._test_lookup_special_method(c_lookup)
+        self._test_lookup_special_method(py_lookup)
+
 
 class UnionTests(unittest.TestCase):
 

Lib/types.py

@@ -81,6 +81,58 @@ def _m(self): pass
 
     del sys, _f, _g, _C, _c, _ag, _cell_factory  # Not for export
 
+    def lookup_special_method(obj, attr, /):
+        """Lookup special method name `attr` on `obj`.
+
+        Lookup method `attr` on `obj` without looking in the instance
+        dictionary. For methods defined in class `__dict__` or `__slots__`, it
+        returns the unbound function (descriptor), not a bound method. The
+        caller is responsible for passing the object as the first argument when
+        calling it:
+
+        >>> class A:
+        ...    def __enter__(self):
+        ...        return "A.__enter__"
+        ...
+        >>> class B:
+        ...    __slots__ = ("__enter__",)
+        ...    def __init__(self):
+        ...        def __enter__(self):
+        ...            return "B.__enter__"
+        ...        self.__enter__ = __enter__
+        ...
+        >>> a = A()
+        >>> b = B()
+        >>> enter_a = types.lookup_special_method(a, "__enter__")
+        >>> enter_b = types.lookup_special_method(b, "__enter__")
+        >>> enter_a(a)
+        'A.__enter__'
+        >>> enter_b(b)
+        'B.__enter__'
+
+        For other descriptors (property, etc.), it returns the result of the
+        descriptor's `__get__` method. Returns `None` if the method is not
+        found.
+        """
+        from inspect import getattr_static, isfunction, ismethoddescriptor
+        cls = type(obj)
+        if not isinstance(attr, str):
+            raise TypeError(
+                f"attribute name must be string, not '{type(attr).__name__}'"
+            )
+        try:
+            descr = getattr_static(cls, attr)
+        except AttributeError:
+            return None
+        if hasattr(descr, "__get__"):
+            if isfunction(descr) or ismethoddescriptor(descr):
+                # do not create bound method to mimic the behavior of
+                # _PyObject_LookupSpecialMethod
+                return descr
+            else:
+                return descr.__get__(obj, cls)
+        return descr
+
 
 # Provide a PEP 3115 compliant mechanism for class creation
 def new_class(name, bases=(), kwds=None, exec_body=None):

Misc/NEWS.d/next/Library/2026-02-19-17-46-59.gh-issue-144989.JAuJyG.rst

@@ -0,0 +1,2 @@
+Expose ``_PyObject_LookupSpecialMethod`` as
+:func:`types.lookup_special_method`.

Modules/_typesmodule.c

@@ -6,6 +6,79 @@
 #include "pycore_namespace.h"     // _PyNamespace_Type
 #include "pycore_object.h"        // _PyNone_Type, _PyNotImplemented_Type
 #include "pycore_unionobject.h"   // _PyUnion_Type
+#include "pycore_typeobject.h"    // _PyObject_LookupSpecialMethod
+#include "pycore_stackref.h"      // _PyStackRef
+#include "clinic/_typesmodule.c.h"
+
+/*[clinic input]
+module _types
+[clinic start generated code]*/
+/*[clinic end generated code: output=da39a3ee5e6b4b0d input=530308b1011b659d]*/
+
+/*[clinic input]
+_types.lookup_special_method
+
+    obj: 'O'
+    attr: 'O'
+    /
+
+Lookup special method name `attr` on `obj`.
+
+Lookup method `attr` on `obj` without looking in the instance
+dictionary. For methods defined in class `__dict__` or `__slots__`, it
+returns the unbound function (descriptor), not a bound method. The
+caller is responsible for passing the object as the first argument when
+calling it:
+
+>>> class A:
+...    def __enter__(self):
+...        return "A.__enter__"
+...
+>>> class B:
+...    __slots__ = ("__enter__",)
+...    def __init__(self):
+...        def __enter__(self):
+...            return "B.__enter__"
+...        self.__enter__ = __enter__
+...
+>>> a = A()
+>>> b = B()
+>>> enter_a = types.lookup_special_method(a, "__enter__")
+>>> enter_b = types.lookup_special_method(b, "__enter__")
+>>> enter_a(a)
+'A.__enter__'
+>>> enter_b(b)
+'B.__enter__'
+
+For other descriptors (property, etc.), it returns the result of the
+descriptor's `__get__` method. Returns `None` if the method is not
+found.
+[clinic start generated code]*/
+
+static PyObject *
+_types_lookup_special_method_impl(PyObject *module, PyObject *obj,
+                                  PyObject *attr)
+/*[clinic end generated code: output=890e22cc0b8e0d34 input=e317288370125cd5]*/
+{
+    if (!PyUnicode_Check(attr)) {
+        PyErr_Format(PyExc_TypeError,
+                     "attribute name must be string, not '%.200s'",
+                     Py_TYPE(attr)->tp_name);
+        return NULL;
+    }
+    _PyStackRef method_and_self[2];
+    method_and_self[0] = PyStackRef_NULL;
+    method_and_self[1] = PyStackRef_FromPyObjectBorrow(obj);
+    int result = _PyObject_LookupSpecialMethod(attr, method_and_self);
+    if (result == -1) {
+        return NULL;
+    }
+    if (result == 0) {
+        Py_RETURN_NONE;
+    }
+    PyObject *method = PyStackRef_AsPyObjectSteal(method_and_self[0]);
+    return method;
+}
 
 static int
 _types_exec(PyObject *m)
@@ -60,12 +133,18 @@ static struct PyModuleDef_Slot _typesmodule_slots[] = {
     {0, NULL}
 };
 
+static PyMethodDef _typesmodule_methods[] = {
+    _TYPES_LOOKUP_SPECIAL_METHOD_METHODDEF
+    {NULL, NULL, 0, NULL}
+};
+
 static struct PyModuleDef typesmodule = {
     .m_base = PyModuleDef_HEAD_INIT,
     .m_name = "_types",
     .m_doc = "Define names for built-in types.",
     .m_size = 0,
     .m_slots = _typesmodule_slots,
+    .m_methods = _typesmodule_methods,
 };
 
 PyMODINIT_FUNC