I made this patch to declared a python list without converting all elements in python object (too slow) or use a CListValue which required CValue items (too expensive in memory). In the case of a big list of points like a collision contacts points list, to use a CListValue we must implement a new class based on CValue for 3D vector to create a python proxy even if mathutils do it perfectly, we must also convert all points (frequently ~100 points) when fill the CListValue even if the list is not used (in the case of the collision callback). The easy way is to use callback (it doesn't worth to do an inheritance) which convert the item in PyObject only during an acces.
5 callbacks are used :

• Check if the list is valid = allow acces (like PyObjectPlus.invalid)
• Get the list size
• Get an item in the list by index.
• Get an item name in the list by index (used for operator list["name"])
• Set an item in the list at the index position.

All of these callback take as first argument the client instance.
Why do we use a void * for the client instance ? : In KX_PythonInitTypes.cpp we have to initialize each python inherited class, if we use a template (the only other way) we must add this class each time we use a new type with in KX_PythonInitTypes.cpp

To check if the list can be accessed from python by the user, we check if the python proxy, which is the m_base member, is still a valid proxy like in PyObjectPlus. But we can use a callback for more control of user access (e.g a list of collision point invalidate a frame later, in this case no real python owner).

This python list is easily defined with :

CPythonCallBackList(
void *client, // The client instance
PyObject *base, // The python instance which owned this list, used to know if the list is valid (like in KX_PythonSeq)
bool (*checkValid)(void *), // A callback to check if this list is till valid (optional)
int (*getSize)(void *), // A callback to get size
PyObject *(*getItem)(void *, int), // A callback to get an item
const char *(*getItemName)(void *, int), // A callback to get an item name (optional) use for acces by string key
bool (*setItem)(void *, int, PyObject *) // A callback to set an item (optional)
)

To show its usecase i replaced the odd KX_PythonSeq, it modify KX_Gameobject.sensors/controllers/actuators, SCA_IController.sensors/actuators and BL_ArmatureObject.constraints/channels.

Example :