Accept any sequence type as std::vector (or std::list)

include/pybind11/pytypes.h

@@ -29,12 +29,14 @@ namespace accessor_policies {
     struct obj_attr;
     struct str_attr;
     struct generic_item;
+    struct sequence_item;
     struct list_item;
     struct tuple_item;
 }
 using obj_attr_accessor = accessor<accessor_policies::obj_attr>;
 using str_attr_accessor = accessor<accessor_policies::str_attr>;
 using item_accessor = accessor<accessor_policies::generic_item>;
+using sequence_accessor = accessor<accessor_policies::sequence_item>;
 using list_accessor = accessor<accessor_policies::list_item>;
 using tuple_accessor = accessor<accessor_policies::tuple_item>;
 
@@ -261,6 +263,23 @@ struct generic_item {
     }
 };
 
+struct sequence_item {
+    using key_type = size_t;
+
+    static object get(handle obj, size_t index) {
+        PyObject *result = PySequence_GetItem(obj.ptr(), static_cast<ssize_t>(index));
+        if (!result) { throw error_already_set(); }
+        return {result, true};
+    }
+
+    static void set(handle obj, size_t index, handle val) {
+        // PySequence_SetItem does not steal a reference to 'val'
+        if (PySequence_SetItem(obj.ptr(), static_cast<ssize_t>(index), val.ptr()) != 0) {
+            throw error_already_set();
+        }
+    }
+};
+
 struct list_item {
     using key_type = size_t;
 
@@ -673,6 +692,13 @@ class dict : public object {
     bool contains(const char *key) const { return PyDict_Contains(ptr(), pybind11::str(key).ptr()) == 1; }
 };
 
+class sequence : public object {
+public:
+    PYBIND11_OBJECT(sequence, object, PySequence_Check)
+    size_t size() const { return (size_t) PySequence_Size(m_ptr); }
+    detail::sequence_accessor operator[](size_t index) const { return {*this, index}; }
+};
+
 class list : public object {
 public:
     PYBIND11_OBJECT(list, object, PyList_Check)

include/pybind11/stl.h

@@ -97,13 +97,13 @@ template <typename Type, typename Value> struct list_caster {
     using value_conv = make_caster<Value>;
 
     bool load(handle src, bool convert) {
-        list l(src, true);
-        if (!l.check())
+        sequence s(src, true);
+        if (!s.check())
             return false;
         value_conv conv;
         value.clear();
-        reserve_maybe(l, &value);
-        for (auto it : l) {
+        reserve_maybe(s, &value);
+        for (auto it : s) {
             if (!conv.load(it, convert))
                 return false;
             value.push_back((Value) conv);
@@ -113,8 +113,8 @@ template <typename Type, typename Value> struct list_caster {
 
     template <typename T = Type,
               enable_if_t<std::is_same<decltype(std::declval<T>().reserve(0)), void>::value, int> = 0>
-    void reserve_maybe(list l, Type *) { value.reserve(l.size()); }
-    void reserve_maybe(list, void *) { }
+    void reserve_maybe(sequence s, Type *) { value.reserve(s.size()); }
+    void reserve_maybe(sequence, void *) { }
 
     static handle cast(const Type &src, return_value_policy policy, handle parent) {
         list l(src.size());

tests/test_python_types.py

@@ -71,6 +71,14 @@ def test_instance(capture):
         list item 0: value
         list item 1: value2
     """
+    with capture:
+        list_result = instance.get_list_2()
+        list_result.append('value2')
+        instance.print_list_2(tuple(list_result))
+    assert capture.unordered == """
+        list item 0: value
+        list item 1: value2
+    """
     array_result = instance.get_array()
     assert array_result == ['array entry 1', 'array entry 2']
     with capture: