Enable static properties (py::metaclass) by default

Now that only one shared metaclass is ever allocated, it's extremely
cheap to enable it for all pybind11 types.

* Deprecate the default py::metaclass() since it's not needed anymore.
* Allow users to specify a custom metaclass via py::metaclass(handle).

Author: dean0x7d

docs/advanced/classes.rst

@@ -437,24 +437,15 @@ The section on :ref:`properties` discussed the creation of instance properties
 that are implemented in terms of C++ getters and setters.
 
 Static properties can also be created in a similar way to expose getters and
-setters of static class attributes. Two things are important to note:
-
-1. Static properties are implemented by instrumenting the *metaclass* of the
-   class in question -- however, this requires the class to have a modifiable
-   metaclass in the first place. pybind11 provides a ``py::metaclass()``
-   annotation that must be specified in the ``class_`` constructor, or any
-   later method calls to ``def_{property_,∅}_{readwrite,readonly}_static`` will
-   fail (see the example below).
-
-2. For static properties defined in terms of setter and getter functions, note
-   that the implicit ``self`` argument also exists in this case and is used to
-   pass the Python ``type`` subclass instance. This parameter will often not be
-   needed by the C++ side, and the following example illustrates how to
-   instantiate a lambda getter function that ignores it:
+setters of static class attributes. Note that the implicit ``self`` argument
+also exists in this case and is used to pass the Python ``type`` subclass
+instance. This parameter will often not be needed by the C++ side, and the
+following example illustrates how to instantiate a lambda getter function
+that ignores it:
 
 .. code-block:: cpp
 
-    py::class_<Foo>(m, "Foo", py::metaclass())
+    py::class_<Foo>(m, "Foo")
         .def_property_readonly_static("foo", [](py::object /* self */) { return Foo(); });
 
 Operator overloading

include/pybind11/attr.h

@@ -54,7 +54,15 @@ struct dynamic_attr { };
 struct buffer_protocol { };
 
 /// Annotation which requests that a special metaclass is created for a type
-struct metaclass { };
+struct metaclass {
+    handle value;
+
+    PYBIND11_DEPRECATED("py::metaclass() is no longer required. It's turned on by default now.")
+    metaclass() = default;
+
+    /// Override pybind11's default metaclass
+    explicit metaclass(handle value) : value(value) { }
+};
 
 /// Annotation to mark enums as an arithmetic type
 struct arithmetic { };
@@ -149,8 +157,7 @@ struct function_record {
 /// Special data structure which (temporarily) holds metadata about a bound class
 struct type_record {
     PYBIND11_NOINLINE type_record()
-        : multiple_inheritance(false), dynamic_attr(false),
-          buffer_protocol(false), metaclass(false) { }
+        : multiple_inheritance(false), dynamic_attr(false), buffer_protocol(false) { }
 
     /// Handle to the parent scope
     handle scope;
@@ -179,6 +186,9 @@ struct type_record {
     /// Optional docstring
     const char *doc = nullptr;
 
+    /// Custom metaclass (optional)
+    handle metaclass;
+
     /// Multiple inheritance marker
     bool multiple_inheritance : 1;
 
@@ -188,9 +198,6 @@ struct type_record {
     /// Does the class implement the buffer protocol?
     bool buffer_protocol : 1;
 
-    /// Does the class require its own metaclass?
-    bool metaclass : 1;
-
     /// Is the default (unique_ptr) holder type used?
     bool default_holder : 1;
 
@@ -356,7 +363,7 @@ struct process_attribute<buffer_protocol> : process_attribute_default<buffer_pro
 
 template <>
 struct process_attribute<metaclass> : process_attribute_default<metaclass> {
-    static void init(const metaclass &, type_record *r) { r->metaclass = true; }
+    static void init(const metaclass &m, type_record *r) { r->metaclass = m.value; }
 };
 
 

include/pybind11/class_support.h

@@ -244,7 +244,8 @@ inline PyObject *make_object_base_type(size_t instance_size) {
        issue no Python C API calls which could potentially invoke the
        garbage collector (the GC will call type_traverse(), which will in
        turn find the newly constructed type in an invalid state) */
-    auto heap_type = (PyHeapTypeObject *) PyType_Type.tp_alloc(&PyType_Type, 0);
+    auto metaclass = get_internals().default_metaclass;
+    auto heap_type = (PyHeapTypeObject *) metaclass->tp_alloc(metaclass, 0);
     if (!heap_type)
         pybind11_fail("make_object_base_type(): error allocating type!");
 
@@ -437,7 +438,10 @@ inline PyObject* make_new_python_type(const type_record &rec) {
        issue no Python C API calls which could potentially invoke the
        garbage collector (the GC will call type_traverse(), which will in
        turn find the newly constructed type in an invalid state) */
-    auto heap_type = (PyHeapTypeObject *) PyType_Type.tp_alloc(&PyType_Type, 0);
+    auto metaclass = rec.metaclass.ptr() ? (PyTypeObject *) rec.metaclass.ptr()
+                                         : internals.default_metaclass;
+
+    auto heap_type = (PyHeapTypeObject *) metaclass->tp_alloc(metaclass, 0);
     if (!heap_type)
         pybind11_fail(std::string(rec.name) + ": Unable to create type object!");
 
@@ -457,12 +461,6 @@ inline PyObject* make_new_python_type(const type_record &rec) {
     /* Don't inherit base __init__ */
     type->tp_init = pybind11_object_init;
 
-    /* Custom metaclass if requested (used for static properties) */
-    if (rec.metaclass) {
-        Py_INCREF(internals.default_metaclass);
-        Py_TYPE(type) = (PyTypeObject *) internals.default_metaclass;
-    }
-
     /* Supported protocols */
     type->tp_as_number = &heap_type->as_number;
     type->tp_as_sequence = &heap_type->as_sequence;

include/pybind11/pybind11.h

@@ -829,18 +829,6 @@ class generic_type : public object {
         const auto is_static = !(rec_fget->is_method && rec_fget->scope);
         const auto has_doc = rec_fget->doc && pybind11::options::show_user_defined_docstrings();
 
-        if (is_static) {
-            auto mclass = handle((PyObject *) Py_TYPE(m_ptr));
-
-            if ((PyTypeObject *) mclass.ptr() == &PyType_Type)
-                pybind11_fail(
-                    "Adding static properties to the type '" +
-                    std::string(((PyTypeObject *) m_ptr)->tp_name) +
-                    "' requires the type to have a custom metaclass. Please "
-                    "ensure that one is created by supplying the pybind11::metaclass() "
-                    "annotation to the associated class_<>(..) invocation.");
-        }
-
         auto property = handle((PyObject *) (is_static ? get_internals().static_property_type
                                                        : &PyProperty_Type));
         attr(name) = property(fget.ptr() ? fget : none(),

tests/test_methods_and_attributes.cpp

@@ -202,7 +202,7 @@ test_initializer methods_and_attributes([](py::module &m) {
         .def("__str__", &ExampleMandA::toString)
         .def_readwrite("value", &ExampleMandA::value);
 
-    py::class_<TestProperties>(m, "TestProperties", py::metaclass())
+    py::class_<TestProperties>(m, "TestProperties")
         .def(py::init<>())
         .def_readonly("def_readonly", &TestProperties::value)
         .def_readwrite("def_readwrite", &TestProperties::value)
@@ -228,7 +228,7 @@ test_initializer methods_and_attributes([](py::module &m) {
     auto static_set2 = [](py::object, int v) { TestPropRVP::sv2.value = v; };
     auto rvp_copy = py::return_value_policy::copy;
 
-    py::class_<TestPropRVP>(m, "TestPropRVP", py::metaclass())
+    py::class_<TestPropRVP>(m, "TestPropRVP")
         .def(py::init<>())
         .def_property_readonly("ro_ref", &TestPropRVP::get1)
         .def_property_readonly("ro_copy", &TestPropRVP::get2, rvp_copy)
@@ -245,6 +245,10 @@ test_initializer methods_and_attributes([](py::module &m) {
         .def_property_readonly("rvalue", &TestPropRVP::get_rvalue)
         .def_property_readonly_static("static_rvalue", [](py::object) { return SimpleValue(); });
 
+    struct MetaclassOverride { };
+    py::class_<MetaclassOverride>(m, "MetaclassOverride", py::metaclass((PyObject *) &PyType_Type))
+        .def_property_readonly_static("readonly", [](py::object) { return 1; });
+
 #if !defined(PYPY_VERSION)
     py::class_<DynamicClass>(m, "DynamicClass", py::dynamic_attr())
         .def(py::init());

tests/test_methods_and_attributes.py

@@ -126,6 +126,22 @@ def check_self(self):
     instance.static_cls = check_self
 
 
+def test_metaclass_override():
+    """Overriding pybind11's default metaclass changes the behavior of `static_property`"""
+    from pybind11_tests import MetaclassOverride
+
+    assert type(ExampleMandA).__name__ == "pybind11_type"
+    assert type(MetaclassOverride).__name__ == "type"
+
+    assert MetaclassOverride.readonly == 1
+    assert type(MetaclassOverride.__dict__["readonly"]).__name__ == "pybind11_static_property"
+
+    # Regular `type` replaces the property instead of calling `__set__()`
+    MetaclassOverride.readonly = 2
+    assert MetaclassOverride.readonly == 2
+    assert isinstance(MetaclassOverride.__dict__["readonly"], int)
+
+
 @pytest.mark.parametrize("access", ["ro", "rw", "static_ro", "static_rw"])
 def test_property_return_value_policies(access):
     from pybind11_tests import TestPropRVP

tests/test_multiple_inheritance.cpp

@@ -123,24 +123,24 @@ test_initializer mi_static_properties([](py::module &pm) {
         .def(py::init<>())
         .def("vanilla", &Vanilla::vanilla);
 
-    py::class_<WithStatic1>(m, "WithStatic1", py::metaclass())
+    py::class_<WithStatic1>(m, "WithStatic1")
         .def(py::init<>())
         .def_static("static_func1", &WithStatic1::static_func1)
         .def_readwrite_static("static_value1", &WithStatic1::static_value1);
 
-    py::class_<WithStatic2>(m, "WithStatic2", py::metaclass())
+    py::class_<WithStatic2>(m, "WithStatic2")
         .def(py::init<>())
         .def_static("static_func2", &WithStatic2::static_func2)
         .def_readwrite_static("static_value2", &WithStatic2::static_value2);
 
     py::class_<VanillaStaticMix1, Vanilla, WithStatic1, WithStatic2>(
-        m, "VanillaStaticMix1", py::metaclass())
+        m, "VanillaStaticMix1")
         .def(py::init<>())
         .def_static("static_func", &VanillaStaticMix1::static_func)
         .def_readwrite_static("static_value", &VanillaStaticMix1::static_value);
 
     py::class_<VanillaStaticMix2, WithStatic1, Vanilla, WithStatic2>(
-        m, "VanillaStaticMix2", py::metaclass())
+        m, "VanillaStaticMix2")
         .def(py::init<>())
         .def_static("static_func", &VanillaStaticMix2::static_func)
         .def_readwrite_static("static_value", &VanillaStaticMix2::static_value);

tests/test_python_types.cpp

@@ -190,7 +190,7 @@ struct MoveOutContainer {
 test_initializer python_types([](py::module &m) {
     /* No constructor is explicitly defined below. An exception is raised when
        trying to construct it directly from Python */
-    py::class_<ExamplePythonTypes>(m, "ExamplePythonTypes", "Example 2 documentation", py::metaclass())
+    py::class_<ExamplePythonTypes>(m, "ExamplePythonTypes", "Example 2 documentation")
         .def("get_dict", &ExamplePythonTypes::get_dict, "Return a Python dictionary")
         .def("get_dict_2", &ExamplePythonTypes::get_dict_2, "Return a C++ dictionary")
         .def("get_list", &ExamplePythonTypes::get_list, "Return a Python list")