On Fri, Apr 12, 2013 at 12:26 PM, Barry Warsaw <ba...@python.org> wrote: > On Apr 12, 2013, at 09:43 AM, Guido van Rossum wrote: > >>I do wonder about this passage in the PEP: >> >>> Let's say you wanted to encode an enumeration value in a database. You >>> might want to get the enumeration class object from an enumeration value:: >>> >>> >>> cls = Colors.red.enum >>> >>> print(cls.__name__) >>> Colors >> >>I don't understand what this has to do with storing enums in a >>database. > > Not much, really. It's just hold over text from the original motivation for > exposing the enum class as an attribute on the values. In Mailman, I store > these values in my database and they get reconstituted correctly by the ORM > layer. Anyway, in this particular case, I think the motivation is unnecessary > for describing the API, so I'll remove that from the PEP.
Yeah, it looked like an editing mistake. :-) >>But it reminded me that for the purpose of storing enums in a database, it >>would be nice to have two examples: one that stores the names and looks them >>up (do you really have to use getattr() for that?), and one that stores the >>values and looks them up (how do you do that at all?). > > It's going to be dependent on how you store and retrieve enum values. > > As an example, in my database layer I store the enum values in an integer > column, with the ORM layer knowing which Enum subclass to use. So all I need > to do to store the value is ``int(enum_value)`` and to get back the original > enum value, I just do ``self._enum[int_value]`` where self._enum is the Enum > subclass. To me, that's probably the most common way of doing it. Agreed. I can't easily find that in the PEP though. It doesn't mention __getitem__ and I can't find any examples of using <enumclass>[<int>]. > If you store by name though, yes, you'd have to use > ``getattr(self._enum, name)``. At one point Enums also supported getitem > syntax for lookup by name, but consider this case: > > class Fruit(Enum): > apple = 'red' > banana = 'yellow' > tangerine = 'orange' > orange = 'reddish yellow' > > What should Fruit['orange'] return? In private email Nick pointed out that > using getattr() for lookup by name works fine, and getitem for look up by > value has been in the API since the beginning, so now Fruit['orange'] is > documented to return Fruit.tangerine, i.e. lookup by value only. (Actually, > in flufl.enum, lookup-by-name is still there but deprecated. We can just drop > it for Python 3.4). Yeah, getattr() is good enough, and it is documented in the PEP. >>Should the metaclass-based API used to create IntEnum be documented, >>so strongly motivated people can write their own crazy variants? > > I think you recommended against that in python-ideas :). I have changed my mind; I am now at least +0 on documenting the metaclass craziness. -- --Guido van Rossum (python.org/~guido) _______________________________________________ Python-Dev mailing list Python-Dev@python.org http://mail.python.org/mailman/listinfo/python-dev Unsubscribe: http://mail.python.org/mailman/options/python-dev/archive%40mail-archive.com
