[Python-Dev] PEP 435: initial values must be specified? Yes

[Tim Delaney]

On 7 May 2013 14:54, Ethan Furman <ethan at stoneleaf.us> wrote:

> On 05/06/2013 07:58 PM, Tim Delaney wrote:
>
>>
>> Considering that doesn't actually work with the reference implementation
>> (AutoNumber.__new__ is never called) ... no.
>>
>
> Two points:
>
>   1) Did you grab the latest code?  That exact implementation passes in
> the tests.
>

D'oh! I had my default path being my forked repo ... so didn't see the
changes. BTW I can't see how that exact implementation passes ... not
enough parameters declared in AutoNumber.__new__ ...


>   2) You can write your __new__ however you want -- use ... !  ;)


    class AutoNumber(Enum):
        def __new__(cls, value):
            if value is Ellipsis:
                try:
                    value = cls._auto_number
                except AttributeError:
                    value = cls._auto_number = 0
            else:
                cls._auto_number = int(value)

            obj = object.__new__(cls)
            obj._value = value
            cls._auto_number += 1
            return obj

        def __int__(self):
            return self._value

    class Color(AutoNumber):
        red = ...
        green = 3
        blue = ...

    print(repr(Color.red))
    print(repr(Color.green))
    print(repr(Color.blue))

---------- Run Python3 ----------
<Color.red: 0>
<Color.green: 3>
<Color.blue: 4>

Unfortunately, if you subclass AutoNumber from IntEnum it breaks.

---------- Run Python3 ----------
Traceback (most recent call last):
  File "D:\home\repos\mercurial\ref435\ref435.py", line 346, in <module>
    class Color(AutoNumber):
  File "D:\home\repos\mercurial\ref435\ref435.py", line 184, in __new__
    enum_item = __new__(enum_class, *args)
TypeError: int() argument must be a string or a number, not 'ellipsis'

I would probably also suggest 2 changes:

1. Set enum_item._name before calling enum_item.__init__.

2. Don't pass any arguments to enum_item.__init__ - the value should be set
in enum_item.__new__.

Tim Delaney
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