[Python-ideas] constant/enum type in stdlib

[Terry Reedy]

On 1/30/2013 2:26 AM, Antoine Pitrou 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)
>>
>> However, it's still slightly annoying that you have to
>> specify how many values there are in the range() call.

For small enumerations, not much of a problem. Or, if one does not want 
to take the time to count, allow

RED, WHITE, BLUE, _extras = range(12)  # any number >= n

and have a metaclass delete _extras.

> Well, how about:
>
> class Color(Enum):
>      values = ('RED', 'WHITE', 'BLUE')
> ?
> (replace values with __values__ if you prefer)

I had the same idea, and having never written a metaclass that I can 
remember, decided to try it.

class EnumMeta(type):
     def __new__(cls, name, bases, dic):
         for i, name in enumerate(dic['_values']):
             dic[name] = i
         del dic['_values']
         return type.__new__(cls, name, bases, dic)

class Enum(metaclass=EnumMeta):
     _values = ()

class Color(Enum):
     _values = 'RED', 'GREEN', 'BLUE'

print(Color.RED, Color.GREEN, Color.BLUE)
 >>>
0 1 2

So this syntax is at least feasible -- today.

-- 
Terry Jan Reedy