Function unpacking_collector

include/pybind11/cast.h:2215–2246

public:
    template <typename... Ts>
    explicit unpacking_collector(Ts &&...values)
        : m_names(reinterpret_steal<tuple>(
              handle())) // initialize to null to avoid useless allocation of 0-length tuple
    {
        /*
        Python can sometimes utilize an extra space before the arguments to prepend `self`.
        This is important enough that there is a special flag for it:
        PY_VECTORCALL_ARGUMENTS_OFFSET.
        All we have to do is allocate an extra space at the beginning of this array, and set the
        flag. Note that the extra space is not passed directly in to vectorcall.
        */
        m_args.reserve(sizeof...(values) + 1);
        m_args.push_back_null();

        if (args_has_keyword_or_ds<Ts...>()) {
            list names_list;

            // collect_arguments guarantees this can't be constructed with kwargs before the last
            // positional so we don't need to worry about Ts... being in anything but normal python
            // order.
            using expander = int[];
            (void) expander{0, (process(names_list, std::forward<Ts>(values)), 0)...};

            m_names = reinterpret_steal<tuple>(PyList_AsTuple(names_list.ptr()));
        } else {
            auto not_used
                = reinterpret_steal<list>(handle()); // initialize as null (to avoid an allocation)

            using expander = int[];
            (void) expander{0, (process(not_used, std::forward<Ts>(values)), 0)...};
        }
    }

    /// Call a Python function and pass the collected arguments
    object call(PyObject *ptr) const {