[Python-ideas] Boolean value of an Enum member

[Guido van Rossum]

Honestly I think it's too late to change. The proposal to change plain
Enums to False when their value is zero (or falsey) would be a huge
backward incompatibility. I don't think there's a reasonable path forward,
and also don't think there's a big reason to regret the current semantics.

On Fri, Jan 15, 2016 at 10:22 AM, Ethan Furman <ethan at stoneleaf.us> wrote:

> When Enum was being designed one of the questions considered was where to
> start autonumbering: zero or one.
>
> As I remember the discussion we chose not to start with zero because we
> didn't want an enum member to be False by default, and having a member with
> value 0 be True was discordant.  So the functional API starts with 1 unless
> overridden.  In fact, according to the Enum docs:
>
>    The reason for defaulting to ``1`` as the starting number and
>    not ``0`` is that ``0`` is ``False`` in a boolean sense, but
>    enum members all evaluate to ``True``.
>
> However, if the Enum is combined with some other type (str, int, float,
> etc), then most behaviour is determined by that type -- including boolean
> evaluation.  So the empty string, 0 values, etc, will cause that Enum
> member to evaluate as False.
>
> So the question now is:  for a standard Enum (meaning no other type
> besides Enum is involved) should __bool__ look to the value of the Enum
> member to determine True/False, or should we always be True by default and
> make the Enum creator add their own __bool__ if they want something
> different?
>
> On the one hand we have backwards compatibility, which will take a version
> to change.
>
> On the other hand we have a pretty basic difference in how zero/empty is
> handled between "pure" Enums and "mixed" Enums.
>
> On the gripping hand we have . . .
>
> Please respond with your thoughts on changing pure Enums to match mixed
> Enums or any experience you have had with relying on the "always True"
> behaviour or if you have implemented your own __bool__ to match the
> standard True/False meanings or if you have implemented your own __bool__
> to match some other scheme entirely.
>
> --
> ~Ethan~
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-- 
--Guido van Rossum (python.org/~guido)
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