[Python-ideas] Enums

[Guido van Rossum]

On Fri, Jul 29, 2011 at 7:55 AM, Nick Coghlan <ncoghlan at gmail.com> wrote:
> On Fri, Jul 29, 2011 at 7:37 PM, M.-A. Lemburg <mal at egenix.com> wrote:
>> Paul Colomiets wrote:
>>> The problem with named value is that:
>>>
>>>     namedvalue(1, "red")
>>>
>>> Will give you are repr of "red". And you don't know whether it's
>>> TerminalColors.red or WebSafe.red or BuildbotState.red. And
>>> most of us will be too lazy to add group name or module name
>>> directly into the named value (partially because it's repeating
>>> yourself).
>>>
>>> So the big feature of flufl.enum is classname (call it group
>>> name) in repr and isinstance check.
>>
>> A meta class variant could easily add class and module
>> names to the attrbute name, if needed/wanted.
>
> Given that the status quo is for "TerminalColors.red" et al to just
> display as "1" (or whatever integers they're assigned), I'm finding it
> hard to consider this a particularly significant criticism. If we
> don't even know what *kind* of values are being passed to a debugging
> message, we have bigger problems than whether or not those values have
> been given names (and searching for all named values of 'red' with a
> value of '1' is going to create a much smaller resulting set than
> searching for all values of '1' ).
>
>> Just to make sure:
>>
>> namedvalue() will just change the repr() of the value, not
>> the str() of it, right ?
>
> Yeah, I figured out the mechanics needed to make that work properly
> when writing up the recipe for the cookbook
> (http://code.activestate.com/recipes/577810-named-values/)
>
>> I think that's essential for making namedvalue()s useful
>> in practice, since you mostly need this for debugging
>> and error reporting and don't want the namevalue aspect
>> of a constant to get in the way of its normal use in
>> e.g. conversion to text formats such as JSON, XML, etc.
>
> Yeah, the case I first thought of was writing numbers to CSV files,
> but it's really any data export operation.

That's assuming the data export format doesn't support enums.

I would fine enums whose str() is just an int pretty annoying. Note
that str(True) == repr(True) == 'True' and that's how I'd like it to
be for enums in general. If you want the int value, use int(e). If you
want the class name, use e.__class__.__name__ (maybe e.__class__ is
what you're after anyway). My observation is that most enums have
sufficiently unique names that during a debugging session the class is
not a big mystery; it's the mapping from value to name that's hard to
memorize. As a compromise, I could live with str(e) == 'red' and
repr(e) == 'Color.red'. But I don't think I could stomach str(e) == 1.

-- 
--Guido van Rossum (python.org/~guido)