[Python-Dev] enum discussion: can someone please summarize open issues?

Eric V. Smith eric at trueblade.com
Sat May 4 16:01:12 CEST 2013

On 5/4/2013 2:42 AM, Nick Coghlan wrote:
> On Sat, May 4, 2013 at 4:10 PM, Georg Brandl <g.brandl at gmx.net> wrote:
>> Am 04.05.2013 01:22, schrieb Antoine Pitrou:
>>> On Sat, 04 May 2013 11:15:17 +1200
>>> Greg Ewing <greg.ewing at canterbury.ac.nz> wrote:
>>>> Eli Bendersky wrote:
>>>>> I'm just curious what it is about enums that sets everyone on a "let's
>>>>> make things safer" path. Python is about duck typing, it's absolutely
>>>>> "unsafe" in the static typing sense, in the most fundamental ways
>>>>> imaginable.
>>>> This isn't about catching bugs in the program, it's
>>>> about validating user input. That's a common enough
>>>> task that it deserves to have a convenient way to
>>>> do it correctly.
>>> +1. An enum is basically a bidirectional mapping between some raw
>>> values and some "nice" instances, so it deserves a well-defined lookup
>>> operation in each direction.
> As I see it, there are 3 possible ways forward here:
> 1. The current PEP, offering only "getattr(MyEnum, name)".
> If code needs to ensure non-enum values are detected immediately (such
> as during translation of user input entered at a command prompt), then
> they can either create a separate mapping using:
>     lookup = {m.name, m for m in (getattr(MyEnum, name) for name in
> dir(MyEnum)) if isinstance(m, MyEnum)}
> or else create a lookup function:
>     def getmember(enum, name):
>         m = getattr(enum, name, None)
>         if not isinstance(m, enum):
>             raise KeyError("{!r} is not a member of {!r}".format(name, enum))
>         return m
> 2. We restore __getitem__ on EnumMetaclass *solely* for member lookup
> by name (the "getmember" functionality above). This would leave
> __call__ used for the reverse lookup (value to member and hence name)
> and __getitem__ for the forward lookup (name to member and hence
> value) (Note: given Ethan's comments about his current implementation,
> I believe this actually fits nicely with the way
> EnumMetaclass.__getattr__ is already implemented)
> 3. We offer my earlier suggestion of an "as_dict()" method on the
> metaclass, which implements the mapping calculation above. As others
> pointed out, this has the same name clash problem as offering
> additional non-special methods on namedtuple objects.
> I'm now -1 on my own as_dict() suggestion, due to the general name
> clash problem for arbitrary enums.

To avoid the name collision, namedtuple calls this _asdict().


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