Implement classmethod pytype

include/pybind11/pybind11.h

@@ -1578,6 +1578,18 @@ public:
         return *this;
     }
 
+    template <typename Func, typename... Extra>
+    class_ &def_classmethod(const char *name_, Func &&f, const Extra &...extra) {
+        cpp_function cf(std::forward<Func>(f),
+                        name(name_),
+                        is_method(*this),
+                        sibling(getattr(*this, name_, none())),
+                        extra...);
+        auto cf_name = cf.name();
+        attr(std::move(cf_name)) = classmethod(std::move(cf));
+        return *this;
+    }
+
     template <detail::op_id id, detail::op_type ot, typename L, typename R, typename... Extra>
     class_ &def(const detail::op_<id, ot, L, R> &op, const Extra &...extra) {
         op.execute(*this, extra...);

include/pybind11/pytypes.h

@@ -1224,6 +1224,7 @@ inline bool PyUnicode_Check_Permissive(PyObject *o) {
 #endif
 
 inline bool PyStaticMethod_Check(PyObject *o) { return o->ob_type == &PyStaticMethod_Type; }
+inline bool PyClassMethod_Check(PyObject *o) { return o->ob_type == &PyClassMethod_Type; }
 
 class kwargs_proxy : public handle {
 public:
@@ -2089,6 +2090,11 @@ public:
     PYBIND11_OBJECT_CVT(staticmethod, object, detail::PyStaticMethod_Check, PyStaticMethod_New)
 };
 
+class classmethod : public object {
+public:
+    PYBIND11_OBJECT_CVT(classmethod, object, detail::PyClassMethod_Check, PyClassMethod_New)
+};
+
 class buffer : public object {
 public:
     PYBIND11_OBJECT_DEFAULT(buffer, object, PyObject_CheckBuffer)

tests/test_class.cpp

@@ -62,7 +62,15 @@ TEST_SUBMODULE(class_, m) {
     };
 
     py::class_<NoConstructor>(m, "NoConstructor")
-        .def_static("new_instance", &NoConstructor::new_instance, "Return an instance");
+        .def_static("new_instance", &NoConstructor::new_instance, "Return an instance")
+        .def_classmethod(
+            "new_instance_uuid",
+            [](py::object &cls) {
+                py::int_ uuid = getattr(cls, "uuid", py::int_(0));
+                cls.attr("uuid") = uuid + py::int_(1);
+                return NoConstructorNew::new_instance();
+            },
+            "Returns a new instance and then increment the uuid");
 
     py::class_<NoConstructorNew>(m, "NoConstructorNew")
         .def(py::init([](const NoConstructorNew &self) { return self; })) // Need a NOOP __init__

tests/test_class.py

@@ -31,6 +31,12 @@ def test_instance_new(msg):
     assert cstats.alive() == 0
 
 
+def test_classmethod(num_instances=10):
+    for i in range(num_instances):
+        assert getattr(m.NoConstructor, "uuid", 0) == i
+        m.NoConstructor.new_instance_uuid()
+
+
 def test_type():
     assert m.check_type(1) == m.DerivedClass1
     with pytest.raises(RuntimeError) as execinfo: