[Python-ideas] constant/enum type in stdlib

[Terry Reedy]

On 1/30/2013 10:30 AM, Michael Foord wrote:
> On 30 January 2013 15:22, Michael Foord

>     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):

Adding  'print(self.value, key)' here prints

0 __name__
0 __name__
1 RED
2 WHITE
3 BLUE

(I do not understand why it is the second and not first lookup of 
__name__ that increments the counter, but...)

>          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

So RED, WHITE, BLUE are 1, 2, 3; not 0, 1, 2 as I and many readers might 
expect. That aside (which can be fixed), this is very nice.

-- 
Terry Jan Reedy