Fix overriding static properties in derived classes

Fixes #775.

Assignments of the form `Type.static_prop = value` should be translated to
`Type.static_prop.__set__(value)` except when `isinstance(value, static_prop)`.

include/pybind11/class_support.h

@@ -124,8 +124,15 @@ extern "C" inline int pybind11_meta_setattro(PyObject* obj, PyObject* name, PyOb
     // descriptor (`property`) instead of calling `tp_descr_get` (`property.__get__()`).
     PyObject *descr = _PyType_Lookup((PyTypeObject *) obj, name);
 
-    // Call `static_property.__set__()` instead of replacing the `static_property`.
-    if (descr && PyObject_IsInstance(descr, (PyObject *) get_internals().static_property_type)) {
+    // The following assignment combinations are possible:
+    //   1. `Type.static_prop = value`             --> descr_set: `Type.static_prop.__set__(value)`
+    //   2. `Type.static_prop = other_static_prop` --> setattro:  replace existing `static_prop`
+    //   3. `Type.regular_attribute = value`       --> setattro:  regular attribute assignment
+    const auto static_prop = (PyObject *) get_internals().static_property_type;
+    const auto call_descr_set = descr && PyObject_IsInstance(descr, static_prop)
+                                && !PyObject_IsInstance(value, static_prop);
+    if (call_descr_set) {
+        // Call `static_property.__set__()` instead of replacing the `static_property`.
 #if !defined(PYPY_VERSION)
         return Py_TYPE(descr)->tp_descr_set(descr, obj, value);
 #else
@@ -137,6 +144,7 @@ extern "C" inline int pybind11_meta_setattro(PyObject* obj, PyObject* name, PyOb
         }
 #endif
     } else {
+        // Replace existing attribute.
         return PyType_Type.tp_setattro(obj, name, value);
     }
 }

tests/test_methods_and_attributes.cpp

@@ -75,6 +75,13 @@ struct TestProperties {
 
 int TestProperties::static_value = 1;
 
+struct TestPropertiesOverride : TestProperties {
+    int value = 99;
+    static int static_value;
+};
+
+int TestPropertiesOverride::static_value = 99;
+
 struct SimpleValue { int value = 1; };
 
 struct TestPropRVP {
@@ -219,6 +226,11 @@ test_initializer methods_and_attributes([](py::module &m) {
                              [](py::object cls) { return cls; },
                              [](py::object cls, py::function f) { f(cls); });
 
+    py::class_<TestPropertiesOverride, TestProperties>(m, "TestPropertiesOverride")
+        .def(py::init<>())
+        .def_readonly("def_readonly", &TestPropertiesOverride::value)
+        .def_readonly_static("def_readonly_static", &TestPropertiesOverride::static_value);
+
     py::class_<SimpleValue>(m, "SimpleValue")
         .def_readwrite("value", &SimpleValue::value);
 

tests/test_methods_and_attributes.py

@@ -110,6 +110,12 @@ def test_static_properties():
     assert Type.def_readwrite_static == 2
     assert instance.def_readwrite_static == 2
 
+    # It should be possible to override properties in derived classes
+    from pybind11_tests import TestPropertiesOverride as TypeOverride
+
+    assert TypeOverride().def_readonly == 99
+    assert TypeOverride.def_readonly_static == 99
+
 
 def test_static_cls():
     """Static property getter and setters expect the type object as the their only argument"""