clarification regarding issue #750

tests/test_classes.cpp

@@ -657,4 +657,34 @@ NB_MODULE(test_classes_ext, m) {
         }), "s"_a)
         .def("value", &UniqueInt::value)
         .def("lookups", &UniqueInt::lookups);
+
+    // issue #750
+    PyCFunctionWithKeywords dummy_init = [](PyObject *, PyObject *,
+                                            PyObject *) -> PyObject * {
+        PyErr_SetString(PyExc_RuntimeError, "This should never be called!");
+        return nullptr;
+    };
+
+    PyType_Slot init_slots[] {
+        // the presence of this slot enables normal object construction via __init__ and __new__
+        // instead of an optimized codepath within nanobind that skips these. That in turn
+        // makes it possible to intercept calls and implement custom logic.
+        { Py_tp_init, (void *) dummy_init },
+        { 0, nullptr }
+    };
+
+    struct MonkeyPatchable {
+        int value = 123;
+    };
+
+    nb::class_<MonkeyPatchable>(m, "MonkeyPatchable", nb::type_slots(init_slots))
+        .def(nb::init<>())
+        .def_static("custom_init", [](nb::handle_t<MonkeyPatchable> h) {
+            if (nb::inst_ready(h))
+                nb::raise_type_error("Input is already initialized!");
+            MonkeyPatchable *p = nb::inst_ptr<MonkeyPatchable>(h);
+            new (p) MonkeyPatchable{456};
+            nb::inst_mark_ready(h);
+        })
+        .def_rw("value", &MonkeyPatchable::value);
 }

tests/test_classes.py

@@ -891,3 +891,15 @@ def test46_custom_new():
 def test47_inconstructible():
     with pytest.raises(TypeError, match="no constructor defined"):
         t.Foo()
+
+def test48_monekypatchable():
+    # issue 750: how to monkeypatch __init__
+    q = t.MonkeyPatchable()
+    assert q.value == 123
+
+    def my_init(self):
+        t.MonkeyPatchable.custom_init(self)
+
+    t.MonkeyPatchable.__init__ = my_init
+    q = t.MonkeyPatchable()
+    assert q.value == 456

tests/test_classes_ext.pyi.ref

@@ -110,6 +110,18 @@ class Int:
 
     def __repr__(self) -> str: ...
 
+class MonkeyPatchable:
+    def __init__(self) -> None: ...
+
+    @staticmethod
+    def custom_init(arg: MonkeyPatchable, /) -> None: ...
+
+    @property
+    def value(self) -> int: ...
+
+    @value.setter
+    def value(self, arg: int, /) -> None: ...
+
 class MyClass:
     def __init__(self) -> None: ...
 

tests/test_ndarray.py

@@ -20,6 +20,7 @@ def needs_torch(x):
 
 try:
     import tensorflow as tf
+    import tensorflow.config
     def needs_tensorflow(x):
         return x
 except: