Now that I actually had the chance to play with the implementation, I see most of my worst fears were justified. :-( Look:

    >>> import enum
    >>> class B(enum.Flag):
            b = 3
            c = 4
            d = 6
    >>> B.b | B.c
    Traceback (most recent call last): ...
    ValueError: 7 is not a valid B
    >>> t = B.b | B.c
    >>> t
    <B.d|1: 7>
    >>> B.b | B.c
    >>> B.b | B.c
    <B.d|1: 7>
    >>> ~B.d
    <B.0: 0>

Do you really find this behavior acceptable?? I see at least three bugs here.

At least you did say in the documentation

> Individual flags should have values that are powers of two (1, 2, 4, 8, ...)

but it seems to me it's not prominent enough.

---

Also, auto, despite parentheses, works exactly as it shouldn't.

    >>> class C(enum.Enum):
	a = b = enum.auto()	
    >>> C.a
    <C.a: 1>
    >>> C.b
    <C.b: 2>

    >>> def f():
            return enum.auto()
    >>> class E(enum.Enum):
            a = b = f()
            c = d = 2
    >>> E.a is E.b
    False
    >>> E.c is E.d
    True
    >>> E.b is E.c
    True

In my opinion, this is simply horrible. Even _you_ said (http://bugs.python.org/issue23591#msg275093) this would be unacceptable. I'm really, really sad.