[Python-Dev] PEP 435 -- Adding an Enum type to the Python standard library

R. David Murray rdmurray at bitdance.com
Wed Apr 24 04:12:22 CEST 2013

On Wed, 24 Apr 2013 11:37:16 +1200, Greg Ewing <greg.ewing at canterbury.ac.nz> wrote:
> R. David Murray wrote:
> > The first False looks correct to me, I would not expect an enum value to
> > be an instance of the class that holds it as an attribute.  The second
> > certainly looks odd, but what does it even mean to have an instance of
> > an Enum class?
> This attitude baffles me. In most other languages having a
> notion of an enum, when you define an enum, you're defining
> a type. The name of the enum is the name of the type, and
> its values are instances of that type.
> Why should our enums be any different?

Obviously they don't need to be, since people have discussed how to
implement this.

But I am primarily a Python programmer, so my intuition is based on my
Python experience, not on what other languages do.


       a = 1
       b = 2

I understood that 'a' and 'b' are class attributes, where the int value
had been transformed into instances of a (separate) value class rather
than being ints.  The fact that there was a specific value class
had been discussed.

If 'a' is now an instance of MyEnum, then I would expect that:


would be valid (b being an attribute of the MyEnum class which should
therefore be accessible from an instance of that class).  That seems
a bit odd, and based on my Python-only mindset, I saw no particular
reason why an enum value *should* be instance of the enum class, since
it would lead to that oddity.  (Well, I'm not sure I was concious of
that *particular* oddity, but in general it seemed like an odd thing to
have class attributes of a class be instances of that class when the set
of class attributes was the most important thing about that class...).
It seemed more natural for the values to be instances of a separate
value class.

Now, given that I was viewing the Enum as being a collection of attributes
whose values were instances of a different class, what would it mean
to create an instance of the Enum class itself?  You'd have an instance
with access to those class attributes...but the getitem wouldn't work,
because that's on the metaclass.  You'd really want the Enum class to
be a singleton...the important thing was that it was an instance of the
metaclass, its instances would be pretty useless.

I don't have any *problem* with enum values being instances of the class.
If you make the attribute values instances of the enum class, then
yes instances of enum class have a meaning.  And then having attributes
of the class be instances of the class makes perfect sense in

It's just not where *my* Python-only intuition, or my understanding of
the discussion, led me.

I feel like I'm revealing my ignorance or stupidity here, but what the
heck, that's what was going through my brain and I might as well own up
to it :).


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