From e5f866626ebb40f905464e5a3fd887c310f68249 Mon Sep 17 00:00:00 2001
From: Aaron Gokaslan <aaronGokaslan@gmail.com>
Date: Wed, 24 Aug 2022 10:07:25 -0400
Subject: [PATCH 1/2] Implement classmethod pytype

---
 include/pybind11/pybind11.h | 12 ++++++++++++
 include/pybind11/pytypes.h  |  6 ++++++
 tests/test_class.cpp        | 10 +++++++++-
 tests/test_class.py         |  6 ++++++
 4 files changed, 33 insertions(+), 1 deletion(-)

diff --git a/include/pybind11/pybind11.h b/include/pybind11/pybind11.h
index c889dc416..68db345ea 100644
--- a/include/pybind11/pybind11.h
+++ b/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...);
diff --git a/include/pybind11/pytypes.h b/include/pybind11/pytypes.h
index 6ba1f5f20..b96b3967c 100644
--- a/include/pybind11/pytypes.h
+++ b/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)
diff --git a/tests/test_class.cpp b/tests/test_class.cpp
index c8b8071dc..768cd072a 100644
--- a/tests/test_class.cpp
+++ b/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__
diff --git a/tests/test_class.py b/tests/test_class.py
index ff9196f0f..e06f92bfe 100644
--- a/tests/test_class.py
+++ b/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:

From 2e3d29de45843c028d25d2400edbcacf8c3de4fd Mon Sep 17 00:00:00 2001
From: Aaron Gokaslan <aaronGokaslan@gmail.com>
Date: Tue, 11 Oct 2022 11:30:09 -0700
Subject: [PATCH 2/2] Update classmethod with reviewer comments

---
 tests/test_class.cpp | 10 +++++-----
 tests/test_class.py  |  5 +++--
 2 files changed, 8 insertions(+), 7 deletions(-)

diff --git a/tests/test_class.cpp b/tests/test_class.cpp
index 768cd072a..0ccb2c051 100644
--- a/tests/test_class.cpp
+++ b/tests/test_class.cpp
@@ -64,13 +64,13 @@ TEST_SUBMODULE(class_, m) {
     py::class_<NoConstructor>(m, "NoConstructor")
         .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);
+            "new_instance_seq_id",
+            [](py::type &cls) {
+                py::int_ seq_id = getattr(cls, "seq_id", py::int_(0));
+                cls.attr("seq_id") = seq_id + py::int_(1);
                 return NoConstructorNew::new_instance();
             },
-            "Returns a new instance and then increment the uuid");
+            "Returns a new instance and then increment the seq_id");
 
     py::class_<NoConstructorNew>(m, "NoConstructorNew")
         .def(py::init([](const NoConstructorNew &self) { return self; })) // Need a NOOP __init__
diff --git a/tests/test_class.py b/tests/test_class.py
index e06f92bfe..dd18f66a0 100644
--- a/tests/test_class.py
+++ b/tests/test_class.py
@@ -32,9 +32,10 @@ def test_instance_new(msg):
 
 
 def test_classmethod(num_instances=10):
+    assert not hasattr(m.NoConstructor, "seq_id")
     for i in range(num_instances):
-        assert getattr(m.NoConstructor, "uuid", 0) == i
-        m.NoConstructor.new_instance_uuid()
+        m.NoConstructor.new_instance_seq_id()
+        assert m.NoConstructor.seq_id == i + 1
 
 
 def test_type():