[3.12] gh-116040: [Enum] fix by-value calls when second value is falsey (GH-116072) (GH-116476)

e.g. Cardinal(1, 0)

(cherry picked from commit 13ffd4bd9f529b6a5fe33741fbd57f14b4b80137)

2 files changed, 45 insertions, 4 deletions

diff --git a/Lib/enum.py b/Lib/enum.py
index 6d3bbdc..110fbda 100644
--- a/Lib/enum.py
+++ b/Lib/enum.py
@@ -166,6 +166,13 @@ def _dedent(text):
         lines[j] = l[i:]
     return '\n'.join(lines)
 
+class _not_given:
+    def __repr__(self):
+        return('<not given>')
+    def __bool__(self):
+        return False
+_not_given = _not_given()
+
 class _auto_null:
     def __repr__(self):
         return '_auto_null'
@@ -718,7 +725,7 @@ class EnumType(type):
         """
         return True
 
-    def __call__(cls, value, names=None, *values, module=None, qualname=None, type=None, start=1, boundary=None):
+    def __call__(cls, value, names=_not_given, *values, module=None, qualname=None, type=None, start=1, boundary=None):
         """
         Either returns an existing member, or creates a new enum class.
 
@@ -747,18 +754,18 @@ class EnumType(type):
         """
         if cls._member_map_:
             # simple value lookup if members exist
-            if names:
+            if names is not _not_given:
                 value = (value, names) + values
             return cls.__new__(cls, value)
         # otherwise, functional API: we're creating a new Enum type
-        if names is None and type is None:
+        if names is _not_given and type is None:
             # no body? no data-type? possibly wrong usage
             raise TypeError(
                     f"{cls} has no members; specify `names=()` if you meant to create a new, empty, enum"
                     )
         return cls._create_(
                 class_name=value,
-                names=names,
+                names=names or None,
                 module=module,
                 qualname=qualname,
                 type=type,
diff --git a/Lib/test/test_enum.py b/Lib/test/test_enum.py
index c58dc36..d5e9444 100644
--- a/Lib/test/test_enum.py
+++ b/Lib/test/test_enum.py
@@ -3312,6 +3312,40 @@ class TestSpecial(unittest.TestCase):
                     member._value_ = Base(value)
                     return member
 
+    def test_second_tuple_item_is_falsey(self):
+        class Cardinal(Enum):
+            RIGHT = (1, 0)
+            UP = (0, 1)
+            LEFT = (-1, 0)
+            DOWN = (0, -1)
+        self.assertIs(Cardinal(1, 0), Cardinal.RIGHT)
+        self.assertIs(Cardinal(-1, 0), Cardinal.LEFT)
+
+    def test_no_members(self):
+        with self.assertRaisesRegex(
+                TypeError,
+                'has no members',
+            ):
+            Enum(7)
+        with self.assertRaisesRegex(
+                TypeError,
+                'has no members',
+            ):
+            Flag(7)
+
+    def test_empty_names(self):
+        for nothing, e_type in (
+                ('', None),
+                ('', int),
+                ([], None),
+                ([], int),
+                ({}, None),
+                ({}, int),
+            ):
+            empty_enum = Enum('empty_enum', nothing, type=e_type)
+            self.assertEqual(len(empty_enum), 0)
+            self.assertRaises(TypeError, 'has no members', empty_enum, 0)
+
 
 class TestOrder(unittest.TestCase):
     "test usage of the `_order_` attribute"