improved constructors for nb::tuple and nb::list wrappers

docs/api_core.rst

@@ -620,6 +620,15 @@ Wrapper classes
 
    Wrapper class representing Python ``tuple`` instances.
 
+   .. cpp:function:: tuple()
+
+      Create an empty tuple
+
+   .. cpp:function:: tuple(handle h)
+
+      Attempt to convert a given Python object into a tuple. Analogous to the
+      expression ``tuple(h)`` in Python.
+
    .. cpp:function:: size_t size() const
 
       Return the number of tuple elements.
@@ -648,6 +657,15 @@ Wrapper classes
 
    Wrapper class representing Python ``list`` instances.
 
+   .. cpp:function:: list()
+
+      Create an empty list
+
+   .. cpp:function:: list(handle h)
+
+      Attempt to convert a given Python object into a list. Analogous to the
+      expression ``list(h)`` in Python.
+
    .. cpp:function:: size_t size() const
 
       Return the number of list elements.

include/nanobind/nb_lib.h

@@ -135,6 +135,14 @@ NB_CORE PyObject *float_from_obj(PyObject *o);
 
 // ========================================================================
 
+/// Convert a Python object into a Python list
+NB_CORE PyObject *list_from_obj(PyObject *o);
+
+/// Convert a Python object into a Python tuple
+NB_CORE PyObject *tuple_from_obj(PyObject *o);
+
+// ========================================================================
+
 /// Get an object attribute or raise an exception
 NB_CORE PyObject *getattr(PyObject *obj, const char *key);
 NB_CORE PyObject *getattr(PyObject *obj, PyObject *key);

include/nanobind/nb_types.h

@@ -415,7 +415,10 @@ class bytes : public object {
 };
 
 class tuple : public object {
-    NB_OBJECT_DEFAULT(tuple, object, "tuple", PyTuple_Check)
+    NB_OBJECT(tuple, object, "tuple", PyTuple_Check)
+    tuple() : object(PyTuple_New(0), detail::steal_t()) { }
+    explicit tuple(handle h)
+        : object(detail::tuple_from_obj(h.ptr()), detail::steal_t{}) { }
     size_t size() const { return (size_t) NB_TUPLE_GET_SIZE(m_ptr); }
     template <typename T, detail::enable_if_t<std::is_arithmetic_v<T>> = 1>
     detail::accessor<detail::num_item_tuple> operator[](T key) const;
@@ -433,6 +436,8 @@ class type_object : public object {
 class list : public object {
     NB_OBJECT(list, object, "list", PyList_Check)
     list() : object(PyList_New(0), detail::steal_t()) { }
+    explicit list(handle h)
+        : object(detail::list_from_obj(h.ptr()), detail::steal_t{}) { }
     size_t size() const { return (size_t) NB_LIST_GET_SIZE(m_ptr); }
 
     template <typename T> void append(T &&value);

src/common.cpp

@@ -552,6 +552,22 @@ PyObject *float_from_obj(PyObject *o) {
 
 // ========================================================================
 
+PyObject *tuple_from_obj(PyObject *o) {
+    PyObject *result = PySequence_Tuple(o);
+    if (!result)
+        raise_python_error();
+    return result;
+}
+
+PyObject *list_from_obj(PyObject *o) {
+    PyObject *result = PySequence_List(o);
+    if (!result)
+        raise_python_error();
+    return result;
+}
+
+// ========================================================================
+
 PyObject **seq_get(PyObject *seq, size_t *size_out, PyObject **temp_out) noexcept {
     PyObject *temp = nullptr;
     size_t size = 0;