[Enum] improve pickle support (#26666)

search all bases for a __reduce__ style method; if a __new__ method is
found first the enum will be made unpicklable

Lib/enum.py

@@ -242,8 +242,32 @@ def __new__(metacls, cls, bases, classdict, **kwds):
                 methods = ('__getnewargs_ex__', '__getnewargs__',
                         '__reduce_ex__', '__reduce__')
                 if not any(m in member_type.__dict__ for m in methods):
-                    _make_class_unpicklable(enum_class)
-
+                    if '__new__' in classdict:
+                        # too late, sabotage
+                        _make_class_unpicklable(enum_class)
+                    else:
+                        # final attempt to verify that pickling would work:
+                        # travel mro until __new__ is found, checking for
+                        # __reduce__ and friends along the way -- if any of them
+                        # are found before/when __new__ is found, pickling should
+                        # work
+                        sabotage = None
+                        for chain in bases:
+                            for base in chain.__mro__:
+                                if base is object:
+                                    continue
+                                elif any(m in base.__dict__ for m in methods):
+                                    # found one, we're good
+                                    sabotage = False
+                                    break
+                                elif '__new__' in base.__dict__:
+                                    # not good
+                                    sabotage = True
+                                    break
+                            if sabotage is not None:
+                                break
+                        if sabotage:
+                            _make_class_unpicklable(enum_class)
         # instantiate them, checking for duplicates as we go
         # we instantiate first instead of checking for duplicates first in case
         # a custom __new__ is doing something funky with the values -- such as
@@ -572,7 +596,7 @@ def _find_data_type(bases):
                         data_types.add(candidate or base)
                         break
                     else:
-                        candidate = base
+                        candidate = candidate or base
             if len(data_types) > 1:
                 raise TypeError('%r: too many data types: %r' % (class_name, data_types))
             elif data_types:

Lib/test/test_enum.py

@@ -594,13 +594,49 @@ class Test2Enum(MyStrEnum, MyMethodEnum):
 
     def test_inherited_data_type(self):
         class HexInt(int):
+            __qualname__ = 'HexInt'
             def __repr__(self):
                 return hex(self)
         class MyEnum(HexInt, enum.Enum):
+            __qualname__ = 'MyEnum'
             A = 1
             B = 2
             C = 3
         self.assertEqual(repr(MyEnum.A), '<MyEnum.A: 0x1>')
+        globals()['HexInt'] = HexInt
+        globals()['MyEnum'] = MyEnum
+        test_pickle_dump_load(self.assertIs, MyEnum.A)
+        test_pickle_dump_load(self.assertIs, MyEnum)
+        #
+        class SillyInt(HexInt):
+            __qualname__ = 'SillyInt'
+            pass
+        class MyOtherEnum(SillyInt, enum.Enum):
+            __qualname__ = 'MyOtherEnum'
+            D = 4
+            E = 5
+            F = 6
+        self.assertIs(MyOtherEnum._member_type_, SillyInt)
+        globals()['SillyInt'] = SillyInt
+        globals()['MyOtherEnum'] = MyOtherEnum
+        test_pickle_dump_load(self.assertIs, MyOtherEnum.E)
+        test_pickle_dump_load(self.assertIs, MyOtherEnum)
+        #
+        class BrokenInt(int):
+            __qualname__ = 'BrokenInt'
+            def __new__(cls, value):
+                return int.__new__(cls, value)
+        class MyBrokenEnum(BrokenInt, Enum):
+            __qualname__ = 'MyBrokenEnum'
+            G = 7
+            H = 8
+            I = 9
+        self.assertIs(MyBrokenEnum._member_type_, BrokenInt)
+        self.assertIs(MyBrokenEnum(7), MyBrokenEnum.G)
+        globals()['BrokenInt'] = BrokenInt
+        globals()['MyBrokenEnum'] = MyBrokenEnum
+        test_pickle_exception(self.assertRaises, TypeError, MyBrokenEnum.G)
+        test_pickle_exception(self.assertRaises, PicklingError, MyBrokenEnum)
 
     def test_too_many_data_types(self):
         with self.assertRaisesRegex(TypeError, 'too many data types'):

Misc/NEWS.d/next/Library/2021-06-10-21-04-14.bpo-44342.nNH5jA.rst

@@ -0,0 +1,2 @@
+[Enum] Be more robust in searching for pickle support before making an enum
+class unpicklable.