[Python-ideas] PEP for enum library type?

[Ethan Furman]

On 02/12/2013 11:13 AM, Guido van Rossum wrote:
> On Tue, Feb 12, 2013 at 10:31 AM, Ethan Furman <ethan at stoneleaf.us> wrote:
>> On 02/12/2013 10:08 AM, Guido van Rossum wrote:
>>>
>>> I'm torn. I like the clean look of Tim's:
>>>
>>> class Flag(Enum):
>>>      RED, WHITE, BLUE
>>>
>>> However, I don't like the magic needed for its implementation -- and
>>> anybody who thinks they know Python pretty well and thinks about this
>>> syntax for more than two seconds has to wonder how on earth it's done
>>> (and the answer is pretty horrible). It is also pretty brittle to
>>> depend on the lookup order -- and I suspect it will be prone to
>>> masking other bugs (any typo in a name used in class-level code will
>>> essentially generate a bogus new enum value).
>>
>>
>> It seems to me the point of an enum is to give names to an order, so why
>> would the lookup order be a problem?
>
> Because the language requirement that the expressions in a tuple are
> evaluated left-to-right is fairly weak. (There are other, similar
> contexts where the order is not l2r.)

Ah.


>> If you don't mind verbose, we could always go for something like:
>>
>> class Color(Enum):
>>      start_defs()
>>      BLACK
>>      RED, GREEN, BLUE
>>      CYAN, YELLOW, MAGENTA
>>      end_defs()
>>
>> and if we mandate that the enum names must come first we can even ditch the
>> 'start_defs' call.
>
> If that's where you're leaning I would much rather do this:
>
> class Color(Enum):
>    BLACK = val()
>    RED = val()
>    # etc.
>
> EIBTI.

Personally, I'd be fine with:

class Color(Enum):
    BLACK
    RED
    BLUE
    PURPLE

which avoids the l2r issue, and the repetitive use of val() (or whatever 
it's called).


>> As far as typos go, I don't think that's a new problem (it certainly isn't
>> for me, anyway ;) and my best defense is plenty of tests.
>
> So with Tim's implementation, what happens here:
>
> class Color(Enum):
>    RED, GREEN, BLUE
>    if sys.platform == 'win32':
>      MAGENTA
>
> and you happen to have no "import sys" in your module? The sys lookup
> will succeed, create a new enum, and then you will get an error
> something like this:
>
> AttributeError: 'Enum' object has no attribute 'platform'

We could minimize that issue by requiring that enum names be ALLCAPS -- 
if something comes through that isn't, don't supply a value and let the 
AttributeError perculate up.

--
~Ethan~