[Python-Dev] Boolean value of an Enum member

[Ethan Furman]

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~