2010/11/24 Steven D'Aprano <st...@pearwood.info>:
> Nick Coghlan wrote:
>>
>> On Wed, Nov 24, 2010 at 10:30 PM, Michael Foord
>> <fuzzy...@voidspace.org.uk> wrote:
>>>
>>> Based on a non-exhaustive search, Python standard library modules
>>> currently
>>> using integers for constants:
>>
>> Thanks for that review. I think following up on the "NamedConstant"
>> idea may make more sense than pursuing enums in their own right.
>
> Pardon me if I've missed something in this thread, but when you say
> "NamedConstant", do you mean actual constants that can only be bound once
> but not re-bound? If so, +1. If not, what do you mean?
>
> I thought PEP 3115 could be used to implement such constants, but I can't
> get it to work...
>
> class readonlydict(dict):
> def __setitem__(self, key, value):
> if key in self:
> raise TypeError("can't rebind constant")
> dict.__setitem__(self, key, value)
> # Need to also handle updates, del, pop, etc.
>
> class MetaConstant(type):
> �...@classmethod
> def __prepare__(metacls, name, bases):
> return readonlydict()
> def __new__(cls, name, bases, classdict):
> assert type(classdict) is readonlydict
> return type.__new__(cls, name, bases, classdict)
>
> class Constant(metaclass=MetaConstant):
> a = 1
> b = 2
> c = 3
>
>
> What I expect is that Constant.a should return 1, and Constant.a=2 should
> raise TypeError, but what I get is a normal class __dict__.
The construction namespace can be customized, but class.__dict__ must
always be a real dict.
--
Regards,
Benjamin
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