[Python-ideas] constant/enum type in stdlib

[Michael Foord]

On 30 January 2013 15:22, Michael Foord <fuzzyman at gmail.com> wrote:

>
>
> On 30 January 2013 07:26, Antoine Pitrou <solipsis at pitrou.net> wrote:
>
>> On Wed, 30 Jan 2013 17:58:37 +1300
>> Greg Ewing <greg.ewing at canterbury.ac.nz> wrote:
>> > Guido van Rossum wrote:
>> >
>> > > class color(enum):
>> > >   RED = value()
>> > >   WHITE = value()
>> > >   BLUE = value()
>> >
>> > We could do somewhat better than that:
>> >
>> >     class Color(Enum):
>> >        RED, WHITE, BLUE = range(3)
>>
>
>
>
> With a Python 3 metaclass that provides default values for *looked up*
> entries you could have this:
>
> class Color(Enum):
>     RED, WHITE, BLUE
>
> The lookup would create the member - with the appropriate value.
>
>


class values(dict):
    def __init__(self):
        self.value = 0
    def __getitem__(self, key):
        try:
            return dict.__getitem__(self, key)
        except KeyError:
            value = self[key] = self.value
            self.value += 1
            return value

class EnumMeta(type):

     @classmethod
     def __prepare__(metacls, name, bases):
        return values()

     def __new__(cls, name, bases, classdict):
        result = type.__new__(cls, name, bases, dict(classdict))
        return result


class Enum(metaclass=EnumMeta):
    pass

class Color(Enum):
    RED, WHITE, BLUE




> Michael
>
>
>
>
>
>> >
>> > However, it's still slightly annoying that you have to
>> > specify how many values there are in the range() call.
>> > It would be even nicer it we could just use an infinite
>> > iterator, such as
>> >
>> >     class Color(Enum):
>> >        RED, WHITE, BLUE = values()
>>
>> Well, how about:
>>
>> class Color(Enum):
>>     values = ('RED', 'WHITE', 'BLUE')
>>
>> ?
>>
>> (replace values with __values__ if you prefer)
>>
>> Regards
>>
>> Antoine.
>>
>>
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>> Python-ideas at python.org
>> http://mail.python.org/mailman/listinfo/python-ideas
>>
>
>
>
> --
>
> http://www.voidspace.org.uk/
>
> May you do good and not evil
> May you find forgiveness for yourself and forgive others
>
> May you share freely, never taking more than you give.
> -- the sqlite blessing http://www.sqlite.org/different.html
>
>


-- 

http://www.voidspace.org.uk/

May you do good and not evil
May you find forgiveness for yourself and forgive others
May you share freely, never taking more than you give.
-- the sqlite blessing http://www.sqlite.org/different.html
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