Hi all,

Sorry to jump into this discussion so late, but I just finished reading through this thread and had a few thoughts..

(Sorry this message is a bit long.  TL;DR: Please check the list of required/desired/undesired properties at the end and let me know if I've gotten anything seriously wrong with my interpretation of the discussion thus far)

It seems to me that this particular thread started out as a call to step away from existing implementations and take a look at this issue from the direction of "what do we want/need" instead, but then it quickly got sidetracked back into discussing all the details of various existing/proposed implementations.  I'd like to try to take a step back (again) for a minute and raise the question:  What do we actually want to get out of this whole endeavor?

First of all, as I see it, there are two main (and fairly distinct) use cases for enums in Python:
  1. Predefined "unique values" for passing around within Python code
  2. API-defined constants for interacting with non-Python libraries, etc (i.e. C defines/enums that need to be represented in Python, or database field values, etc)
In non-Python code, typically enums have always been represented under the covers as ints, and therefore must be passed back and forth as numbers.  The fact that they have an integer value, however, is purely an implementation artifact.  It comes from the fact that C and some other languages don't have a rich enough type system to properly make enums their own distinct types, but Python does not have this limitation, and I think we should be careful not to constrain the way we do things within Python just because of the limitations of other languages.

Where possible I believe we should conceptually be thinking of enums not as "sequences of ints" but more as "collections of singletons".  That is, they are simply objects, with a defined name and type, which compare equal to themselves but not to others, and are generally related to others by some sort of grouping mechanism (and the same name always maps to the same object).  In this context, the idea of assigning a "value" to an enum makes little sense and is arguably completely unnecessary.  (and if we eliminate this aspect, it mitigates many of the issues that have been brought up about evaluation order and accidental duplication, in addition to potentially making the base design a lot simpler)

Obviously, the second use case does require an association between enums and (typically int) values, but that could be viewed as simply a special case of the larger definition of "enums", rather than the other way around.  I do think one thing worth noting, however, is that (at least in my experience) the cases which require associating names with values pretty much also always require that every name has a specific value, so the value for each and every enum within the group should probably be being defined explicitly anyway (I have encountered very few cases where it's actually useful to mix-and-match "I care about this value but I don't care about those ones").  It doesn't seem unreasonable, therefore, to define two different categories of enums: one that has no concept of "value" (for pure-Python), and one which does have associated values but all values have to be specified explicitly (for the "mapping constants" case).

On a related note, to be honest, I'm not really sure I can think of any realistic use cases for "string enums" (or really anything other than ints in general).  Does anybody have an example of where this would actually be useful as opposed to just using "pure" (valueless) enums (which would already have string names)?

Anyway, in the interest of trying to get the discussion back onto more theoretical ground, I also wanted to try to summarize the more general thoughts/impressions I've gleaned from the discussions up to this point.  From what I can tell, these are some of the properties that there seems to be some general consensus enums probably should or shouldn't have:

Required properties (without these, any implementation is not generally useful, or is at least something different than an "enum"):
  1. Enums must be groupable in some way (i.e. "Colors", or "Error values")
  2. Enums within the same group must not compare equal to each other (unless two names are intentionally defined to map to the same enum (i.e. "aliases"))
  3. (Within reason and the limitations of Python) Once defined, an enum's properties (i.e. its name, identity, group membership, relationships to other objects, etc) must be treated as immutable (i.e. not change out from under the programmer unexpectedly).  Conceptually they should be considered to be "constants".
Desirable properties (which ones are more or less desirable will vary for different people, but from what I've heard I think everybody sorta agrees that all of these could be good things as long as they don't cause other problems):
  1. Enums should represent themselves (str/repr) by symbolic names, not as ints, etc.
  2. Enums from different groups should preferably not compare equal to each other (even if they have the same associated int value).
  3. It should be possible to determine what group an enum belongs to.
  4. Enums/groups should be definable inline using a fairly simple Python syntax.
  5. It should also be relatively easy to define enums/groups programmatically.
  6. By default, enums should be referenceable as relatively simple identifiers (i.e. no need for quoting, function-calls, etc, just variables/attributes/etc)
  7. If the programmer doesn't care about the value of an enum, she shouldn't have to explicitly state a meaningless value.
  8. (If an enum does have an associated value) it should be easy to compare with and/or convert back and forth between enums and values (so that they can be used with existing APIs).
  9. It would be nice to be able to associate docstrings, and possibly other metadata with enums.
Undesirable properties:
  1. Enum syntax should not be "too magic".  (In particular, it's pretty clear at this point that creating new enums as a side-effect of name lookups (even as convenient as it does make the syntax) is ultimately not gonna fly)
  2. The syntax for defining enums should not be so onerous or verbose that it's significantly harder to use than the existing idioms people are already using.
  3. The syntax for defining enums should not be so alien that it will completely baffle programmers who are already used to typical Python constructs.
  4. It shouldn't be necessary to quote enum names when defining them (since they won't be quoted when they're used)
I want to check: Is this a valid summary of things?  Anything I missed, or do people have substantial objections to any of the required/desirable/undesirable points I mentioned?

Obviously, it may not be possible to achieve all of the desirable properties at the same time, but I think it's useful to start with an idea of what we'd ideally like, and then we can sit down and see how close we can actually get to it..

(Actually, on pondering these properties, I've started to put together a slightly different enum implementation which I think has some potential (it's somewhat a cross between Barry's and Tim's with a couple of ideas of my own).  I think I'll flesh it out a little more and then put it up for comment as a separate thread, if people don't mind..)

--Alex