Add .keys and .values to bind_map

Both of these implement views (rather than just iterators), and `.items`
is also upgraded to a view. In practical terms, this allows a view to be
iterated multiple times and have its size taken, neither of which works
with an iterator.

The views implement `__len__`, `__iter__`, and the keys view implements
`__contains__`. Testing membership also works in item and value views
because Python falls back to iteration. This won't be optimal
for item values since it's linear rather than O(log n) or O(1), but I
didn't fancy trying to get all the corner cases to match Python
behaviour (tuple of wrong types, wrong length tuple, not a tuple etc).

Missing relative to Python dictionary views is `__reversed__` (only
added to Python in 3.8). Implementing that could break code that binds
custom map classes which don't provide `rbegin`/`rend` (at least without
doing clever things with SFINAE), so I've not tried.

The size increase on my system is 131072 bytes, which is rather large
(5%) but also suspiciously round (2^17) and makes me suspect some
quantisation effect.

Author: bmerry

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)...);
+    py::class_<KeysView> keys_view(
+        scope, ("KeysView[" + name + "]").c_str(), pybind11::module_local(local));
+    py::class_<ValuesView> values_view(
+        scope, ("ValuesView[" + name + "]").c_str(), pybind11::module_local(local));
+    py::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__",
@@ -669,6 +705,38 @@ 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;
+        }
+    );
+
+    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 +160,39 @@ 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}"
 
+    # 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 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)