[Python-ideas] constant/enum type in stdlib

Wed Jan 30 22:09:31 CET 2013

On 1/30/2013 2:26 AM, Antoine Pitrou wrote:
> On 2013年1月30日 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