[Python-ideas] Enums

[Guido van Rossum]

On Thu, Jul 28, 2011 at 10:12 AM, Barry Warsaw <barry at python.org> wrote:
> On Jul 27, 2011, at 07:21 PM, Guido van Rossum wrote:
>
>>It's just possible that there's no way to define enums that neither
>>introduced a new (class) scope nor requires a lot of redundant typing.
>
> Ezio had a very nice suggestion, which I've implemented in my experimental
> branch, e.g.:
>
>    >>> foo = sys.modules['foo']
>    >>> Colors.inject(foo)
>
> .inject() takes anything that implements the setattr() protocol.  So that you
> could then do:
>
>    >>> import foo
>    >>> foo.red
>    Colors.red

Nice, but it still hides the definitions from static analyzers.

It seems I am almost unique in my insistence that Python's dynamic
features be used very sparingly. :-)

> In my experimental branch I've also made EnumValues subclasses of ints.  This
> breaks one test in my Mailman 3 test suite, since I was using a custom encoder
> for json.dumps() which knew how to encode EnumValues.  However, the json
> module doesn't allow you to override the encoding of basic types, so whereas
> before my extended encoder's .default() method got called for all the
> EnumValues being encoded in a REST response, they are now encoded
> *incorrectly* as their integer values by default.  I'd need to figure out a
> workaround for that, but it does show that EnumValue-as-int is a backward
> incompatible change.

Sure. Almost any change is backward incompatible...

> A few other things I've done in my experimental branch:
>
> - Added an optional argument `iterable` to make_enum() so that you can use the
>  convenience function to auto-assign integer values other than sequentially
>  incrementing from 1.  E.g.
>
>    >>> def enumiter():
>    ...     start = 1
>    ...     while True:
>    ...         yield start
>    ...         start <<= 1
>    >>> make_enum('Flags', 'a b c d e f g', enumiter())
>    <Flags {a: 1, b: 2, c: 4, d: 8, e: 16, f: 32, g: 64}>
>
> - Renamed attributes .enumclass -> .enum and .enumname -> .name (hey, if
>  you're going to be backward incompatible...)
>
> If you want to play with it:
>
>    $ bzr branch lp:~barry/flufl.enum/asint
>
> or take a look at:
>
>    http://bazaar.launchpad.net/~barry/flufl.enum/asint/files
>
> The new using.txt file is at:
>
>    http://tinyurl.com/3s85oq7
>
> Let me know what you think, while I figure out a workaround for the
> incompatible change.
>
> Cheers,
> -Barry
>



-- 
--Guido van Rossum (python.org/~guido)