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

Jan. 30, 2013


      On 30 January 2013 15:22, Michael Foord <fuzzyman@gmail.com> wrote:
...
On 30 January 2013 07:26, Antoine Pitrou <solipsis@pitrou.net> 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)
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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May you share freely, never taking more than you give.
-- the sqlite blessing http://www.sqlite.org/different.html