Add .keys and .values to bind_map

include/pybind11/stl_bind.h

@@ -595,13 +595,33 @@ template <typename Map, typename Class_> auto map_if_insertion_operator(Class_ &
     );
 }
 
+template<typename Map>
+struct keys_view
+{
+    Map &map;
+};
+
+template<typename Map>
+struct values_view
+{
+    Map &map;
+};
+
+template<typename Map>
+struct items_view
+{
+    Map &map;
+};
 
 PYBIND11_NAMESPACE_END(detail)
 
 template <typename Map, typename holder_type = std::unique_ptr<Map>, typename... Args>
 class_<Map, holder_type> bind_map(handle scope, const std::string &name, Args&&... args) {
     using KeyType = typename Map::key_type;
     using MappedType = typename Map::mapped_type;
+    using KeysView = detail::keys_view<Map>;
+    using ValuesView = detail::values_view<Map>;
+    using ItemsView = detail::items_view<Map>;
     using Class_ = class_<Map, holder_type>;
 
     // If either type is a non-module-local bound type then make the map binding non-local as well;
@@ -615,6 +635,12 @@ class_<Map, holder_type> bind_map(handle scope, const std::string &name, Args&&.
     }
 
     Class_ cl(scope, name.c_str(), pybind11::module_local(local), std::forward<Args>(args)...);
+    class_<KeysView> keys_view(
+        scope, ("KeysView[" + name + "]").c_str(), pybind11::module_local(local));
+    class_<ValuesView> values_view(
+        scope, ("ValuesView[" + name + "]").c_str(), pybind11::module_local(local));
+    class_<ItemsView> items_view(
+        scope, ("ItemsView[" + name + "]").c_str(), pybind11::module_local(local));
 
     cl.def(init<>());
 
@@ -628,12 +654,22 @@ class_<Map, holder_type> bind_map(handle scope, const std::string &name, Args&&.
 
     cl.def("__iter__",
            [](Map &m) { return make_key_iterator(m.begin(), m.end()); },
-           keep_alive<0, 1>() /* Essential: keep list alive while iterator exists */
+           keep_alive<0, 1>() /* Essential: keep map alive while iterator exists */
+    );
+
+    cl.def("keys",
+           [](Map &m) { return KeysView{m}; },
+           keep_alive<0, 1>() /* Essential: keep map alive while view exists */
+    );
+
+    cl.def("values",
+           [](Map &m) { return ValuesView{m}; },
+           keep_alive<0, 1>() /* Essential: keep map alive while view exists */
     );
 
     cl.def("items",
-           [](Map &m) { return make_iterator(m.begin(), m.end()); },
-           keep_alive<0, 1>() /* Essential: keep list alive while iterator exists */
+           [](Map &m) { return ItemsView{m}; },
+           keep_alive<0, 1>() /* Essential: keep map alive while view exists */
     );
 
     cl.def("__getitem__",
@@ -654,6 +690,8 @@ class_<Map, holder_type> bind_map(handle scope, const std::string &name, Args&&.
            return true;
         }
     );
+    // Fallback for when the object is not of the key type
+    cl.def("__contains__", [](Map &, const object &) -> bool { return false; });
 
     // Assignment provided only if the type is copyable
     detail::map_assignment<Map, Class_>(cl);
@@ -669,6 +707,40 @@ class_<Map, holder_type> bind_map(handle scope, const std::string &name, Args&&.
 
     cl.def("__len__", &Map::size);
 
+    keys_view.def("__len__", [](KeysView &view) { return view.map.size(); });
+    keys_view.def("__iter__",
+        [](KeysView &view) {
+            return make_key_iterator(view.map.begin(), view.map.end());
+        },
+        keep_alive<0, 1>() /* Essential: keep view alive while iterator exists */
+    );
+    keys_view.def("__contains__",
+        [](KeysView &view, const KeyType &k) -> bool {
+            auto it = view.map.find(k);
+            if (it == view.map.end())
+                return false;
+            return true;
+        }
+    );
+    // Fallback for when the object is not of the key type
+    keys_view.def("__contains__", [](KeysView &, const object &) -> bool { return false; });
+
+    values_view.def("__len__", [](ValuesView &view) { return view.map.size(); });
+    values_view.def("__iter__",
+        [](ValuesView &view) {
+            return make_value_iterator(view.map.begin(), view.map.end());
+        },
+        keep_alive<0, 1>() /* Essential: keep view alive while iterator exists */
+    );
+
+    items_view.def("__len__", [](ItemsView &view) { return view.map.size(); });
+    items_view.def("__iter__",
+        [](ItemsView &view) {
+            return make_iterator(view.map.begin(), view.map.end());
+        },
+        keep_alive<0, 1>() /* Essential: keep view alive while iterator exists */
+    );
+
     return cl;
 }
 

tests/test_stl_binders.py

@@ -160,15 +160,43 @@ def test_map_string_double():
     mm["b"] = 2.5
 
     assert list(mm) == ["a", "b"]
-    assert list(mm.items()) == [("a", 1), ("b", 2.5)]
     assert str(mm) == "MapStringDouble{a: 1, b: 2.5}"
+    assert "b" in mm
+    assert "c" not in mm
+    assert 123 not in mm
+
+    # Check that keys, values, items are views, not merely iterable
+    keys = mm.keys()
+    values = mm.values()
+    items = mm.items()
+    assert list(keys) == ["a", "b"]
+    assert len(keys) == 2
+    assert "a" in keys
+    assert "c" not in keys
+    assert 123 not in keys
+    assert list(items) == [("a", 1), ("b", 2.5)]
+    assert len(items) == 2
+    assert ("b", 2.5) in items
+    assert "hello" not in items
+    assert ("b", 2.5, None) not in items
+    assert list(values) == [1, 2.5]
+    assert len(values) == 2
+    assert 1 in values
+    assert 2 not in values
+    # Check that views update when the map is updated
+    mm["c"] = -1
+    assert list(keys) == ["a", "b", "c"]
+    assert list(values) == [1, 2.5, -1]
+    assert list(items) == [("a", 1), ("b", 2.5), ("c", -1)]
 
     um = m.UnorderedMapStringDouble()
     um["ua"] = 1.1
     um["ub"] = 2.6
 
     assert sorted(list(um)) == ["ua", "ub"]
+    assert list(um.keys()) == list(um)
     assert sorted(list(um.items())) == [("ua", 1.1), ("ub", 2.6)]
+    assert list(zip(um.keys(), um.values())) == list(um.items())
     assert "UnorderedMapStringDouble" in str(um)