git merge smart_holder (after pybind/pybind11#4601 was merged)

include/pybind11/cast.h

@@ -1236,14 +1236,32 @@ T cast(const handle &handle) {
 }
 
 // Note that `cast<PyObject *>(obj)` increments the reference count of `obj`.
-// This is necessary for the case that `obj` is a temporary.
+// This is necessary for the case that `obj` is a temporary, and could
+// not possibly be different, given
+// 1. the established convention that the passed `handle` is borrowed, and
+// 2. we don't want to force all generic code using `cast<T>()` to special-case
+//    handling of `T` = `PyObject *` (to increment the reference count there).
 // It is the responsibility of the caller to ensure that the reference count
 // is decremented.
 template <typename T,
-          detail::enable_if_t<detail::is_same_ignoring_cvref<T, PyObject *>::value, int> = 0>
-T cast(const handle &handle) {
+          typename Handle,
+          detail::enable_if_t<detail::is_same_ignoring_cvref<T, PyObject *>::value
+                                  && detail::is_same_ignoring_cvref<Handle, handle>::value,
+                              int>
+          = 0>
+T cast(Handle &&handle) {
     return handle.inc_ref().ptr();
 }
+// To optimize way an inc_ref/dec_ref cycle:
+template <typename T,
+          typename Object,
+          detail::enable_if_t<detail::is_same_ignoring_cvref<T, PyObject *>::value
+                                  && detail::is_same_ignoring_cvref<Object, object>::value,
+                              int>
+          = 0>
+T cast(Object &&obj) {
+    return obj.release().ptr();
+}
 
 // C++ type -> py::object
 template <typename T, detail::enable_if_t<!detail::is_pyobject<T>::value, int> = 0>

include/pybind11/type_caster_pyobject_ptr.h

@@ -13,7 +13,7 @@ PYBIND11_NAMESPACE_BEGIN(detail)
 template <>
 class type_caster<PyObject> {
 public:
-    static constexpr auto name = const_name("PyObject *");
+    static constexpr auto name = const_name("object"); // See discussion under PR #4601.
 
     // This overload is purely to guard against accidents.
     template <typename T,

tests/test_type_caster_pyobject_ptr.cpp

@@ -25,7 +25,7 @@ TEST_SUBMODULE(type_caster_pyobject_ptr, m) {
         PyObject *ptr = PyLong_FromLongLong(6758L);
         return py::cast(ptr, py::return_value_policy::take_ownership);
     });
-    m.def("cast_to_pyobject_ptr", [](py::handle obj) {
+    m.def("cast_handle_to_pyobject_ptr", [](py::handle obj) {
         auto rc1 = obj.ref_count();
         auto *ptr = py::cast<PyObject *>(obj);
         auto rc2 = obj.ref_count();
@@ -34,6 +34,27 @@ TEST_SUBMODULE(type_caster_pyobject_ptr, m) {
         }
         return 100 - py::reinterpret_steal<py::object>(ptr).attr("value").cast<int>();
     });
+    m.def("cast_object_to_pyobject_ptr", [](py::object obj) {
+        py::handle hdl = obj;
+        auto rc1 = hdl.ref_count();
+        auto *ptr = py::cast<PyObject *>(std::move(obj));
+        auto rc2 = hdl.ref_count();
+        if (rc2 != rc1) {
+            return -1;
+        }
+        return 300 - py::reinterpret_steal<py::object>(ptr).attr("value").cast<int>();
+    });
+    m.def("cast_list_to_pyobject_ptr", [](py::list lst) {
+        // This is to cover types implicitly convertible to object.
+        py::handle hdl = lst;
+        auto rc1 = hdl.ref_count();
+        auto *ptr = py::cast<PyObject *>(std::move(lst));
+        auto rc2 = hdl.ref_count();
+        if (rc2 != rc1) {
+            return -1;
+        }
+        return 400 - static_cast<int>(py::len(py::reinterpret_steal<py::list>(ptr)));
+    });
 
     m.def(
         "return_pyobject_ptr",
@@ -98,6 +119,8 @@ TEST_SUBMODULE(type_caster_pyobject_ptr, m) {
         return i;
     });
 
+    m.def("pass_pyobject_ptr_and_int", [](PyObject *, int) {});
+
 #ifdef PYBIND11_NO_COMPILE_SECTION // Change to ifndef for manual testing.
     {
         PyObject *ptr = nullptr;

tests/test_type_caster_pyobject_ptr.py

@@ -13,8 +13,16 @@ def test_cast_from_pyobject_ptr():
     assert m.cast_from_pyobject_ptr() == 6758
 
 
-def test_cast_to_pyobject_ptr():
-    assert m.cast_to_pyobject_ptr(ValueHolder(24)) == 76
+def test_cast_handle_to_pyobject_ptr():
+    assert m.cast_handle_to_pyobject_ptr(ValueHolder(24)) == 76
+
+
+def test_cast_object_to_pyobject_ptr():
+    assert m.cast_object_to_pyobject_ptr(ValueHolder(43)) == 257
+
+
+def test_cast_list_to_pyobject_ptr():
+    assert m.cast_list_to_pyobject_ptr([1, 2, 3, 4, 5]) == 395
 
 
 def test_return_pyobject_ptr():
@@ -89,3 +97,8 @@ def test_return_list_pyobject_ptr_reference():
     # Insert `while True:` as the first line of this function and monitor the
     # process RES (Resident Memory Size) with the Unix top command.
     assert m.dec_ref_each_pyobject_ptr(vec_obj) == 2
+
+
+def test_type_caster_name_via_incompatible_function_arguments_type_error():
+    with pytest.raises(TypeError, match=r"1\. \(arg0: object, arg1: int\) -> None"):
+        m.pass_pyobject_ptr_and_int(ValueHolder(101), ValueHolder(202))