Allow extending py3 enums

Author: aldanor

include/pybind11/pybind11.h

@@ -1282,22 +1282,36 @@ class py3_enum {
         update();
     }
 
-    py3_enum& value(const char* name, T value) {
-        entries[name] = cast(static_cast<underlying_type>(value));
-        update();
+    py3_enum& value(const char* name, T value) & {
+        add_entry(name, value);
         return *this;
     }
 
+    py3_enum&& value(const char* name, T value) && {
+        add_entry(name, value);
+        return std::move(*this);
+    }
+
+    class_<T> extend() && {
+        return cast<class_<T>>(type);
+    }
+
 private:
     const char *name;
     handle scope;
     dict entries;
     object ctor;
     object unique;
     dict kwargs;
+    object type;
+
+    void add_entry(const char *name, T value) {
+        entries[name] = cast(static_cast<underlying_type>(value));
+        update();
+    }
 
     void update() {
-        object type = unique(ctor(**kwargs));
+        type = unique(ctor(**kwargs));
         setattr(scope, name, type);
         detail::type_caster<T>::bind(type, entries);
     }

tests/test_enum.cpp

@@ -82,10 +82,14 @@ test_initializer enums([](py::module &m) {
     auto scope = py::class_<DummyScope>(m, "DummyScope");
     py::py3_enum<Py3EnumEmpty>(scope, "Py3EnumEmpty");
 
-    py::py3_enum<Py3Enum>(m, "Py3Enum")
+    auto e = py::py3_enum<Py3Enum>(m, "Py3Enum")
         .value("A", Py3Enum::A)
         .value("B", Py3Enum::B)
-        .value("C", Py3Enum::C);
+        .value("C", Py3Enum::C)
+        .extend()
+        .def("add", [](Py3Enum x, int y) { return static_cast<int>(x) + y; })
+        .def_property_readonly("is_b", [](Py3Enum e) { return e == Py3Enum::B; })
+        .def_property_readonly_static("ultimate_answer", [](py::object) { return 42; });
 
     py::py3_enum<Py3EnumScoped>(m, "Py3EnumScoped")
         .value("X", Py3EnumScoped::X)

tests/test_enum.py

@@ -164,3 +164,7 @@ def test_py3_enum():
     with pytest.raises(ValueError) as excinfo:
         non_unique_py3_enum()
     assert 'duplicate values found' in str(excinfo.value)
+
+    assert Py3Enum.ultimate_answer == 42
+    assert not Py3Enum.A.is_b and Py3Enum.B.is_b and not Py3Enum.C.is_b
+    assert Py3Enum.A.add(10) == -32 and Py3Enum.C.add(-1) == 41