[Python-Dev] PEP 435 -- Adding an Enum type to the Python standard library

Guido van Rossum guido at python.org
Fri Apr 12 21:56:03 CEST 2013

On Fri, Apr 12, 2013 at 12:26 PM, Barry Warsaw <barry at 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
> 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)

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