[Python-Dev] PEP 435 -- Adding an Enum type to the Python standard library (original) (raw)

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 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(enumvalue) and to get back the original enum value, I just do self.enum[intvalue] 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 [].

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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