[3.7] bpo-29577: Enum: mixin classes don't mix well with already mixed Enums (GH-9328)backport-5bdab64-3.7

* bpo-29577: allow multiple mixin classes.
(cherry picked from commit 5bdab641da0afd8aa581dfbde4f82d88d337c4b5)

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

4 files changed, 229 insertions, 34 deletions

diff --git a/Doc/library/enum.rst b/Doc/library/enum.rst
index a6aa863a1c..81e9766e42 100644
--- a/Doc/library/enum.rst
+++ b/Doc/library/enum.rst
@@ -387,10 +387,17 @@ whatever value(s) were given to the enum member will be passed into those
 methods.  See `Planet`_ for an example.
 
 
-Restricted subclassing of enumerations
---------------------------------------
+Restricted Enum subclassing
+---------------------------
 
-Subclassing an enumeration is allowed only if the enumeration does not define
+A new :class:`Enum` class must have one base Enum class, up to one concrete
+data type, and as many :class:`object`-based mixin classes as needed.  The
+order of these base classes is::
+
+    def EnumName([mix-in, ...,] [data-type,] base-enum):
+        pass
+
+Also, subclassing an enumeration is allowed only if the enumeration does not define
 any members.  So this is forbidden::
 
     >>> class MoreColor(Color):
diff --git a/Lib/enum.py b/Lib/enum.py
index 69b41fe7cb..4e8a56818b 100644
--- a/Lib/enum.py
+++ b/Lib/enum.py
@@ -449,37 +449,25 @@ class EnumMeta(type):
         if not bases:
             return object, Enum
 
-        # double check that we are not subclassing a class with existing
-        # enumeration members; while we're at it, see if any other data
-        # type has been mixed in so we can use the correct __new__
-        member_type = first_enum = None
-        for base in bases:
-            if  (base is not Enum and
-                    issubclass(base, Enum) and
-                    base._member_names_):
-                raise TypeError("Cannot extend enumerations")
-        # base is now the last base in bases
-        if not issubclass(base, Enum):
-            raise TypeError("new enumerations must be created as "
-                    "`ClassName([mixin_type,] enum_type)`")
-
-        # get correct mix-in type (either mix-in type of Enum subclass, or
-        # first base if last base is Enum)
-        if not issubclass(bases[0], Enum):
-            member_type = bases[0]     # first data type
-            first_enum = bases[-1]  # enum type
-        else:
-            for base in bases[0].__mro__:
-                # most common: (IntEnum, int, Enum, object)
-                # possible:    (<Enum 'AutoIntEnum'>, <Enum 'IntEnum'>,
-                #               <class 'int'>, <Enum 'Enum'>,
-                #               <class 'object'>)
-                if issubclass(base, Enum):
-                    if first_enum is None:
-                        first_enum = base
-                else:
-                    if member_type is None:
-                        member_type = base
+        def _find_data_type(bases):
+            for chain in bases:
+                for base in chain.__mro__:
+                    if base is object:
+                        continue
+                    elif '__new__' in base.__dict__:
+                        if issubclass(base, Enum) and not hasattr(base, '__new_member__'):
+                            continue
+                        return base
+
+        # ensure final parent class is an Enum derivative, find any concrete
+        # data type, and check that Enum has no members
+        first_enum = bases[-1]
+        if not issubclass(first_enum, Enum):
+            raise TypeError("new enumerations should be created as "
+                    "`EnumName([mixin_type, ...] [data_type,] enum_type)`")
+        member_type = _find_data_type(bases) or object
+        if first_enum._member_names_:
+            raise TypeError("Cannot extend enumerations")
 
         return member_type, first_enum
 
diff --git a/Lib/test/test_enum.py b/Lib/test/test_enum.py
index 4b17228946..6c147d7ca6 100644
--- a/Lib/test/test_enum.py
+++ b/Lib/test/test_enum.py
@@ -120,6 +120,22 @@ class TestHelpers(unittest.TestCase):
                 '__', '___', '____', '_____',):
             self.assertFalse(enum._is_dunder(s))
 
+# for subclassing tests
+
+class classproperty:
+
+    def __init__(self, fget=None, fset=None, fdel=None, doc=None):
+        self.fget = fget
+        self.fset = fset
+        self.fdel = fdel
+        if doc is None and fget is not None:
+            doc = fget.__doc__
+        self.__doc__ = doc
+
+    def __get__(self, instance, ownerclass):
+        return self.fget(ownerclass)
+
+
 # tests
 
 class TestEnum(unittest.TestCase):
@@ -1701,6 +1717,102 @@ class TestEnum(unittest.TestCase):
             third = auto()
         self.assertEqual([Dupes.first, Dupes.second, Dupes.third], list(Dupes))
 
+    def test_multiple_mixin(self):
+        class MaxMixin:
+            @classproperty
+            def MAX(cls):
+                max = len(cls)
+                cls.MAX = max
+                return max
+        class StrMixin:
+            def __str__(self):
+                return self._name_.lower()
+        class SomeEnum(Enum):
+            def behavior(self):
+                return 'booyah'
+        class AnotherEnum(Enum):
+            def behavior(self):
+                return 'nuhuh!'
+            def social(self):
+                return "what's up?"
+        class Color(MaxMixin, Enum):
+            RED = auto()
+            GREEN = auto()
+            BLUE = auto()
+        self.assertEqual(Color.RED.value, 1)
+        self.assertEqual(Color.GREEN.value, 2)
+        self.assertEqual(Color.BLUE.value, 3)
+        self.assertEqual(Color.MAX, 3)
+        self.assertEqual(str(Color.BLUE), 'Color.BLUE')
+        class Color(MaxMixin, StrMixin, Enum):
+            RED = auto()
+            GREEN = auto()
+            BLUE = auto()
+        self.assertEqual(Color.RED.value, 1)
+        self.assertEqual(Color.GREEN.value, 2)
+        self.assertEqual(Color.BLUE.value, 3)
+        self.assertEqual(Color.MAX, 3)
+        self.assertEqual(str(Color.BLUE), 'blue')
+        class Color(StrMixin, MaxMixin, Enum):
+            RED = auto()
+            GREEN = auto()
+            BLUE = auto()
+        self.assertEqual(Color.RED.value, 1)
+        self.assertEqual(Color.GREEN.value, 2)
+        self.assertEqual(Color.BLUE.value, 3)
+        self.assertEqual(Color.MAX, 3)
+        self.assertEqual(str(Color.BLUE), 'blue')
+        class CoolColor(StrMixin, SomeEnum, Enum):
+            RED = auto()
+            GREEN = auto()
+            BLUE = auto()
+        self.assertEqual(CoolColor.RED.value, 1)
+        self.assertEqual(CoolColor.GREEN.value, 2)
+        self.assertEqual(CoolColor.BLUE.value, 3)
+        self.assertEqual(str(CoolColor.BLUE), 'blue')
+        self.assertEqual(CoolColor.RED.behavior(), 'booyah')
+        class CoolerColor(StrMixin, AnotherEnum, Enum):
+            RED = auto()
+            GREEN = auto()
+            BLUE = auto()
+        self.assertEqual(CoolerColor.RED.value, 1)
+        self.assertEqual(CoolerColor.GREEN.value, 2)
+        self.assertEqual(CoolerColor.BLUE.value, 3)
+        self.assertEqual(str(CoolerColor.BLUE), 'blue')
+        self.assertEqual(CoolerColor.RED.behavior(), 'nuhuh!')
+        self.assertEqual(CoolerColor.RED.social(), "what's up?")
+        class CoolestColor(StrMixin, SomeEnum, AnotherEnum):
+            RED = auto()
+            GREEN = auto()
+            BLUE = auto()
+        self.assertEqual(CoolestColor.RED.value, 1)
+        self.assertEqual(CoolestColor.GREEN.value, 2)
+        self.assertEqual(CoolestColor.BLUE.value, 3)
+        self.assertEqual(str(CoolestColor.BLUE), 'blue')
+        self.assertEqual(CoolestColor.RED.behavior(), 'booyah')
+        self.assertEqual(CoolestColor.RED.social(), "what's up?")
+        class ConfusedColor(StrMixin, AnotherEnum, SomeEnum):
+            RED = auto()
+            GREEN = auto()
+            BLUE = auto()
+        self.assertEqual(ConfusedColor.RED.value, 1)
+        self.assertEqual(ConfusedColor.GREEN.value, 2)
+        self.assertEqual(ConfusedColor.BLUE.value, 3)
+        self.assertEqual(str(ConfusedColor.BLUE), 'blue')
+        self.assertEqual(ConfusedColor.RED.behavior(), 'nuhuh!')
+        self.assertEqual(ConfusedColor.RED.social(), "what's up?")
+        class ReformedColor(StrMixin, IntEnum, SomeEnum, AnotherEnum):
+            RED = auto()
+            GREEN = auto()
+            BLUE = auto()
+        self.assertEqual(ReformedColor.RED.value, 1)
+        self.assertEqual(ReformedColor.GREEN.value, 2)
+        self.assertEqual(ReformedColor.BLUE.value, 3)
+        self.assertEqual(str(ReformedColor.BLUE), 'blue')
+        self.assertEqual(ReformedColor.RED.behavior(), 'booyah')
+        self.assertEqual(ConfusedColor.RED.social(), "what's up?")
+        self.assertTrue(issubclass(ReformedColor, int))
+
 
 class TestOrder(unittest.TestCase):
 
@@ -2064,6 +2176,49 @@ class TestFlag(unittest.TestCase):
             d = 6
         self.assertEqual(repr(Bizarre(7)), '<Bizarre.d|c|b: 7>')
 
+    def test_multiple_mixin(self):
+        class AllMixin:
+            @classproperty
+            def ALL(cls):
+                members = list(cls)
+                all_value = None
+                if members:
+                    all_value = members[0]
+                    for member in members[1:]:
+                        all_value |= member
+                cls.ALL = all_value
+                return all_value
+        class StrMixin:
+            def __str__(self):
+                return self._name_.lower()
+        class Color(AllMixin, Flag):
+            RED = auto()
+            GREEN = auto()
+            BLUE = auto()
+        self.assertEqual(Color.RED.value, 1)
+        self.assertEqual(Color.GREEN.value, 2)
+        self.assertEqual(Color.BLUE.value, 4)
+        self.assertEqual(Color.ALL.value, 7)
+        self.assertEqual(str(Color.BLUE), 'Color.BLUE')
+        class Color(AllMixin, StrMixin, Flag):
+            RED = auto()
+            GREEN = auto()
+            BLUE = auto()
+        self.assertEqual(Color.RED.value, 1)
+        self.assertEqual(Color.GREEN.value, 2)
+        self.assertEqual(Color.BLUE.value, 4)
+        self.assertEqual(Color.ALL.value, 7)
+        self.assertEqual(str(Color.BLUE), 'blue')
+        class Color(StrMixin, AllMixin, Flag):
+            RED = auto()
+            GREEN = auto()
+            BLUE = auto()
+        self.assertEqual(Color.RED.value, 1)
+        self.assertEqual(Color.GREEN.value, 2)
+        self.assertEqual(Color.BLUE.value, 4)
+        self.assertEqual(Color.ALL.value, 7)
+        self.assertEqual(str(Color.BLUE), 'blue')
+
     @support.reap_threads
     def test_unique_composite(self):
         # override __eq__ to be identity only
@@ -2439,6 +2594,49 @@ class TestIntFlag(unittest.TestCase):
         for f in Open:
             self.assertEqual(bool(f.value), bool(f))
 
+    def test_multiple_mixin(self):
+        class AllMixin:
+            @classproperty
+            def ALL(cls):
+                members = list(cls)
+                all_value = None
+                if members:
+                    all_value = members[0]
+                    for member in members[1:]:
+                        all_value |= member
+                cls.ALL = all_value
+                return all_value
+        class StrMixin:
+            def __str__(self):
+                return self._name_.lower()
+        class Color(AllMixin, IntFlag):
+            RED = auto()
+            GREEN = auto()
+            BLUE = auto()
+        self.assertEqual(Color.RED.value, 1)
+        self.assertEqual(Color.GREEN.value, 2)
+        self.assertEqual(Color.BLUE.value, 4)
+        self.assertEqual(Color.ALL.value, 7)
+        self.assertEqual(str(Color.BLUE), 'Color.BLUE')
+        class Color(AllMixin, StrMixin, IntFlag):
+            RED = auto()
+            GREEN = auto()
+            BLUE = auto()
+        self.assertEqual(Color.RED.value, 1)
+        self.assertEqual(Color.GREEN.value, 2)
+        self.assertEqual(Color.BLUE.value, 4)
+        self.assertEqual(Color.ALL.value, 7)
+        self.assertEqual(str(Color.BLUE), 'blue')
+        class Color(StrMixin, AllMixin, IntFlag):
+            RED = auto()
+            GREEN = auto()
+            BLUE = auto()
+        self.assertEqual(Color.RED.value, 1)
+        self.assertEqual(Color.GREEN.value, 2)
+        self.assertEqual(Color.BLUE.value, 4)
+        self.assertEqual(Color.ALL.value, 7)
+        self.assertEqual(str(Color.BLUE), 'blue')
+
     @support.reap_threads
     def test_unique_composite(self):
         # override __eq__ to be identity only
@@ -2524,6 +2722,7 @@ class TestUnique(unittest.TestCase):
             value = 4
 
 
+
 expected_help_output_with_docs = """\
 Help on class Color in module %s:
 
diff --git a/Misc/NEWS.d/next/Library/2018-09-14-20-00-47.bpo-29577.RzwKFD.rst b/Misc/NEWS.d/next/Library/2018-09-14-20-00-47.bpo-29577.RzwKFD.rst
new file mode 100644
index 0000000000..bd71ac496a
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2018-09-14-20-00-47.bpo-29577.RzwKFD.rst
@@ -0,0 +1 @@
+Support multiple mixin classes when creating Enums.