bpo-44356: [Enum] allow multiple data-type mixins if they are all the same (GH-26649)

This enables, for example, two base Enums to both inherit from `str`, and then both be mixed into the same final Enum:

    class Str1Enum(str, Enum):
        # some behavior here

    class Str2Enum(str, Enum):
        # some more behavior here

    class FinalStrEnum(Str1Enum, Str2Enum):
        # this now works

3 files changed, 52 insertions, 4 deletions

diff --git a/Lib/enum.py b/Lib/enum.py
index f74cc8c..54633d8 100644
--- a/Lib/enum.py
+++ b/Lib/enum.py
@@ -819,7 +819,7 @@ class EnumType(type):
             return object, Enum
 
         def _find_data_type(bases):
-            data_types = []
+            data_types = set()
             for chain in bases:
                 candidate = None
                 for base in chain.__mro__:
@@ -827,19 +827,19 @@ class EnumType(type):
                         continue
                     elif issubclass(base, Enum):
                         if base._member_type_ is not object:
-                            data_types.append(base._member_type_)
+                            data_types.add(base._member_type_)
                             break
                     elif '__new__' in base.__dict__:
                         if issubclass(base, Enum):
                             continue
-                        data_types.append(candidate or base)
+                        data_types.add(candidate or base)
                         break
                     else:
                         candidate = base
             if len(data_types) > 1:
                 raise TypeError('%r: too many data types: %r' % (class_name, data_types))
             elif data_types:
-                return data_types[0]
+                return data_types.pop()
             else:
                 return None
 
diff --git a/Lib/test/test_enum.py b/Lib/test/test_enum.py
index 34b190b..40794e3 100644
--- a/Lib/test/test_enum.py
+++ b/Lib/test/test_enum.py
@@ -2144,6 +2144,53 @@ class TestEnum(unittest.TestCase):
                 return member
         self.assertEqual(Fee.TEST, 2)
 
+    def test_miltuple_mixin_with_common_data_type(self):
+        class CaseInsensitiveStrEnum(str, Enum):
+            @classmethod
+            def _missing_(cls, value):
+                for member in cls._member_map_.values():
+                    if member._value_.lower() == value.lower():
+                        return member
+                return super()._missing_(value)
+        #
+        class LenientStrEnum(str, Enum):
+            def __init__(self, *args):
+                self._valid = True
+            @classmethod
+            def _missing_(cls, value):
+                # encountered an unknown value!
+                # Luckily I'm a LenientStrEnum, so I won't crash just yet.
+                # You might want to add a new case though.
+                unknown = cls._member_type_.__new__(cls, value)
+                unknown._valid = False
+                unknown._name_ = value.upper()
+                unknown._value_ = value
+                cls._member_map_[value] = unknown
+                return unknown
+            @property
+            def valid(self):
+                return self._valid
+        #
+        class JobStatus(CaseInsensitiveStrEnum, LenientStrEnum):
+            ACTIVE = "active"
+            PENDING = "pending"
+            TERMINATED = "terminated"
+        #
+        JS = JobStatus
+        self.assertEqual(list(JobStatus), [JS.ACTIVE, JS.PENDING, JS.TERMINATED])
+        self.assertEqual(JS.ACTIVE, 'active')
+        self.assertEqual(JS.ACTIVE.value, 'active')
+        self.assertIs(JS('Active'), JS.ACTIVE)
+        self.assertTrue(JS.ACTIVE.valid)
+        missing = JS('missing')
+        self.assertEqual(list(JobStatus), [JS.ACTIVE, JS.PENDING, JS.TERMINATED])
+        self.assertEqual(JS.ACTIVE, 'active')
+        self.assertEqual(JS.ACTIVE.value, 'active')
+        self.assertIs(JS('Active'), JS.ACTIVE)
+        self.assertTrue(JS.ACTIVE.valid)
+        self.assertTrue(isinstance(missing, JS))
+        self.assertFalse(missing.valid)
+
     def test_empty_globals(self):
         # bpo-35717: sys._getframe(2).f_globals['__name__'] fails with KeyError
         # when using compile and exec because f_globals is empty
diff --git a/Misc/NEWS.d/next/Library/2021-06-10-08-35-38.bpo-44356.6oDFhO.rst b/Misc/NEWS.d/next/Library/2021-06-10-08-35-38.bpo-44356.6oDFhO.rst
new file mode 100644
index 0000000..954a803
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2021-06-10-08-35-38.bpo-44356.6oDFhO.rst
@@ -0,0 +1 @@
+[Enum] Allow multiple data-type mixins if they are all the same.