[3.8] bpo-42248: [Enum] ensure exceptions raised in _missing_ are released (GH-25350). (GH-25369)

(cherry picked from commit 8c14f5a)

Co-authored-by: Ethan Furman <ethan@stoneleaf.us>

Author: ethanfurman

Lib/enum.py

@@ -654,19 +654,24 @@ def __new__(cls, value):
         except Exception as e:
             exc = e
             result = None
-        if isinstance(result, cls):
-            return result
-        else:
-            ve_exc = ValueError("%r is not a valid %s" % (value, cls.__name__))
-            if result is None and exc is None:
-                raise ve_exc
-            elif exc is None:
-                exc = TypeError(
-                        'error in %s._missing_: returned %r instead of None or a valid member'
-                        % (cls.__name__, result)
-                        )
-            exc.__context__ = ve_exc
-            raise exc
+        try:
+            if isinstance(result, cls):
+                return result
+            else:
+                ve_exc = ValueError("%r is not a valid %s" % (value, cls.__name__))
+                if result is None and exc is None:
+                    raise ve_exc
+                elif exc is None:
+                    exc = TypeError(
+                            'error in %s._missing_: returned %r instead of None or a valid member'
+                            % (cls.__name__, result)
+                            )
+                exc.__context__ = ve_exc
+                raise exc
+        finally:
+            # ensure all variables that could hold an exception are destroyed
+            exc = None
+            ve_exc = None
 
     def _generate_next_value_(name, start, count, last_values):
         """

Lib/test/test_enum.py

@@ -1897,6 +1897,38 @@ def _missing_(cls, item):
         else:
             raise Exception('Exception not raised.')
 
+    def test_missing_exceptions_reset(self):
+        import weakref
+        #
+        class TestEnum(enum.Enum):
+            VAL1 = 'val1'
+            VAL2 = 'val2'
+        #
+        class Class1:
+            def __init__(self):
+                # Gracefully handle an exception of our own making
+                try:
+                    raise ValueError()
+                except ValueError:
+                    pass
+        #
+        class Class2:
+            def __init__(self):
+                # Gracefully handle an exception of Enum's making
+                try:
+                    TestEnum('invalid_value')
+                except ValueError:
+                    pass
+        # No strong refs here so these are free to die.
+        class_1_ref = weakref.ref(Class1())
+        class_2_ref = weakref.ref(Class2())
+        #
+        # The exception raised by Enum creates a reference loop and thus
+        # Class2 instances will stick around until the next gargage collection
+        # cycle, unlike Class1.
+        self.assertIs(class_1_ref(), None)
+        self.assertIs(class_2_ref(), None)
+
     def test_multiple_mixin(self):
         class MaxMixin:
             @classproperty

Misc/NEWS.d/next/Library/2021-04-11-21-10-57.bpo-42248.pedB1E.rst

@@ -0,0 +1 @@
+[Enum] ensure exceptions raised in ``_missing__`` are released