nb_bound_method: add introspection attributes and fix surprising beha…

…vior around wrapper descriptors (#216)

* nb_bound_method: add introspection attributes and fix surprising behavior around wrapper descriptors

docs/changelog.rst

@@ -113,6 +113,12 @@ Version 1.3.0 (TBD)
   on object ownership <enable_shared_from_this>` for more details.
   (PR `#212 <https://github.com/wjakob/nanobind/pull/212>`__).
 
+* Added introspection attributes ``__self__`` and ``__func__`` to nanobind
+  bound methods, to make them more like regular Python bound methods.
+  Fixed a bug where ``some_obj.method.__call__()`` would behave differently
+  than ``some_obj.method()``.
+  (PR `#216 <https://github.com/wjakob/nanobind/pull/216>`__).
+
 * ABI version 8.
 
 Version 1.2.0 (April 24, 2023)

src/nb_func.cpp

@@ -1128,9 +1128,22 @@ PyObject *nb_func_getattro(PyObject *self, PyObject *name_) {
         return PyObject_GenericGetAttr(self, name_);
 }
 
-PyObject *nb_bound_method_getattro(PyObject *self, PyObject *name) {
+PyObject *nb_bound_method_getattro(PyObject *self, PyObject *name_) {
+    bool passthrough = false;
+    if (const char *name = PyUnicode_AsUTF8AndSize(name_, nullptr)) {
+        // These attributes do exist on nb_bound_method (because they
+        // exist on every type) but we want to take their special handling
+        // from nb_func_getattro instead.
+        passthrough = (strcmp(name, "__doc__") == 0 ||
+                       strcmp(name, "__module__") == 0);
+    }
+    if (!passthrough) {
+        if (PyObject* res = PyObject_GenericGetAttr(self, name_))
+            return res;
+        PyErr_Clear();
+    }
     nb_func *func = ((nb_bound_method *) self)->func;
-    return nb_func_getattro((PyObject *) func, name);
+    return nb_func_getattro((PyObject *) func, name_);
 }
 
 /// Excise a substring from 's'

src/nb_internals.cpp

@@ -163,6 +163,10 @@ static PyType_Spec nb_method_spec = {
 static PyMemberDef nb_bound_method_members[] = {
     { "__vectorcalloffset__", T_PYSSIZET,
       (Py_ssize_t) offsetof(nb_bound_method, vectorcall), READONLY, nullptr },
+    { "__func__", T_OBJECT_EX,
+      (Py_ssize_t) offsetof(nb_bound_method, func), READONLY, nullptr },
+    { "__self__", T_OBJECT_EX,
+      (Py_ssize_t) offsetof(nb_bound_method, self), READONLY, nullptr },
     { nullptr, 0, 0, 0, nullptr }
 };
 

tests/test_classes.py

@@ -607,6 +607,7 @@ def test33_polymorphic_downcast():
     assert isinstance(t.polymorphic_factory(), t.PolymorphicSubclass)
     assert isinstance(t.polymorphic_factory_2(), t.PolymorphicBase)
 
+
 def test34_trampoline_optimization():
     class Rufus(t.Dog):
         def __init__(self):
@@ -631,3 +632,19 @@ def name(self):
             assert t.go(d2) == 'Rufus says woof'
         finally:
             t.Dog.name = old
+
+
+def test35_method_introspection():
+    obj = t.Struct(5)
+    m = obj.value
+    assert m() == m.__call__() == 5
+    assert hash(m) == m.__hash__()
+    assert repr(m) == m.__repr__()
+    assert "bound_method" in repr(m)
+    assert m.__self__ is obj
+    assert m.__func__ is t.Struct.value
+    # attributes not defined by nb_bound_method are forwarded to nb_method:
+    assert m.__name__ == "value"
+    assert m.__qualname__ == "Struct.value"
+    assert m.__module__ == t.__name__
+    assert m.__doc__ == t.Struct.value.__doc__ == "value(self) -> int"