enum: add missing Enum.value property (#2739)

* enum: Add Enum.value property

* simplify

* address review

Author: EricCousineau-TRI

include/pybind11/pybind11.h

@@ -1731,6 +1731,7 @@ template <typename Type> class enum_ : public class_<Type> {
         m_base.init(is_arithmetic, is_convertible);
 
         def(init([](Scalar i) { return static_cast<Type>(i); }), arg("value"));
+        def_property_readonly("value", [](Type value) { return (Scalar) value; });
         def("__int__", [](Type value) { return (Scalar) value; });
         #if PY_MAJOR_VERSION < 3
             def("__long__", [](Type value) { return (Scalar) value; });

tests/test_enum.py

@@ -13,15 +13,24 @@ def test_unscoped_enum():
 
     # name property
     assert m.UnscopedEnum.EOne.name == "EOne"
+    assert m.UnscopedEnum.EOne.value == 1
     assert m.UnscopedEnum.ETwo.name == "ETwo"
-    assert m.EOne.name == "EOne"
-    # name readonly
+    assert m.UnscopedEnum.ETwo.value == 2
+    assert m.EOne is m.UnscopedEnum.EOne
+    # name, value readonly
     with pytest.raises(AttributeError):
         m.UnscopedEnum.EOne.name = ""
-    # name returns a copy
-    foo = m.UnscopedEnum.EOne.name
-    foo = "bar"
+    with pytest.raises(AttributeError):
+        m.UnscopedEnum.EOne.value = 10
+    # name, value returns a copy
+    # TODO: Neither the name nor value tests actually check against aliasing.
+    # Use a mutable type that has reference semantics.
+    nonaliased_name = m.UnscopedEnum.EOne.name
+    nonaliased_name = "bar"  # noqa: F841
     assert m.UnscopedEnum.EOne.name == "EOne"
+    nonaliased_value = m.UnscopedEnum.EOne.value
+    nonaliased_value = 10  # noqa: F841
+    assert m.UnscopedEnum.EOne.value == 1
 
     # __members__ property
     assert m.UnscopedEnum.__members__ == {
@@ -33,8 +42,8 @@ def test_unscoped_enum():
     with pytest.raises(AttributeError):
         m.UnscopedEnum.__members__ = {}
     # __members__ returns a copy
-    foo = m.UnscopedEnum.__members__
-    foo["bar"] = "baz"
+    nonaliased_members = m.UnscopedEnum.__members__
+    nonaliased_members["bar"] = "baz"
     assert m.UnscopedEnum.__members__ == {
         "EOne": m.UnscopedEnum.EOne,
         "ETwo": m.UnscopedEnum.ETwo,