Re: [Python-ideas] constant/enum type in stdlib

Jan. 30, 2013


      On 1/30/2013 2:26 AM, Antoine Pitrou wrote:
...
On Wed, 30 Jan 2013 17:58:37 +1300
Greg Ewing <greg.ewing@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

Re: [Python-ideas] constant/enum type in stdlib

Terry Reedy