[Python-Dev] constant/enum type in stdlib

[Michael Foord]

On 23/11/2010 15:37, Ben.Cottrell at nominum.com wrote:
> On Tue, 23 Nov 2010 15:15:29 +0000, Michael Foord wrote:
>> There are still two reasonable APIs (unless you have changed your mind
>> and think that sticking with plain integers is best), of which I prefer
>> the latter:
>>
>> SOME_CONST = Constant('SOME_CONST', 1)
>> OTHER_CONST = Constant('OTHER_CONST', 2)
>>
>> or:
>>
>> Constants = make_constants('Constants', 'SOME_CONST OTHER_CONST', start=1)
>> SOME_CONST = Constants.SOME_CONST
>> OTHER_CONST = Constants.OTHER_CONST
> I prefer the latter too, because that makes it possible to have
> 'Constants' be a rendezvous point for making sure that you're
> passing something valid. Perhaps using 'in':
>
> def func(foo):
>      if foo not in Constants:
>          raise ValueError('foo must be SOME_CONST or OTHER_CONST')
>      ...
>
> I know this is probably not going to happen, but I would *so much*
> like it if functions would start rejecting "the wrong kind of 2".
> Constants that are valid, integer-wise, but which aren't part of
> the set of constants allowed for that argument. I'd prefer not to
> think of the number of times I've made the following mistake:
>
> s = socket.socket(socket.SOCK_DGRAM, socket.AF_INET)

Well it would be perfectly possible for the __contains__ method (on the 
metaclass so that a Constants class can act as a container) to permit a 
*raw integer* (to be backwards compatible with code using hard coded 
values) but not permit other constants that aren't valid. Code that is 
*deliberately* using the wrong constants would be screwed of course...

All the best,

Michael
> 	~Ben
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