[issue26988] Add AutoNumberedEnum to stdlib

[John Hagen]

New submission from John Hagen:

I suggest that the AutoNumberedEnum be added to the standard library for the following reasons:

1) Provides a fundamental tool for defining a unique, abstract set of coupled members
2) Avoids boilerplate @enum.unique for a very common use case of enumerations
3) The code already exists in the Python documentation, so it has been vetted at some level

The AutoNumberedEnum also allows the developer to make a clearer distinction
between enumerations whose values have special meaning and those that do not.

Consider:

@enum.unique
class Color(enum.Enum):
    red = 1
    blue = 2
    green = 3

@enum.unique
class Shape(enum.Enum):
    """Member values denote number of sides."""
    circle = 1
    triangle = 3
    square = 4

With AutoNumberedEnum it's possible to better express the intent that 
the value of Color members does not hold special meaning, while
Shape members do:

class Color(enum.AutoNumberedEnum):
    red = ()
    blue = ()
    green = ()

@enum.unique
class Shape(enum.Enum):
    """Member values denote number of sides."""
    circle = 1
    triangle = 3
    square = 4

For enumerations with many members (10s), there becomes a maintenance
issue when inserting new enumerations into the list:

@enum.unique
class Color(enum.Enum):
    aquamarine = 1
    blue = 2
    fushia = 3
    # inserting a member here (perhaps because it's clearest to keep these in alphabetic order)
    # results in having to increment all following members
    ...
    green = 40
    red = 41

Most other languages have support for naming enumerations
without explicitly declaring their values (albeit with 
specialized syntax that makes it cleaner):

C++: http://en.cppreference.com/w/cpp/language/enum
C#: https://msdn.microsoft.com/en-us/library/sbbt4032.aspx
Java: https://docs.oracle.com/javase/tutorial/java/javaOO/enum.html
Rust: https://doc.rust-lang.org/book/enums.html

Currently, a developer can copy the code from the Python docs into
his or her project, or add a dependency on aenum.  I would argue
that it belongs in the standard library.

If there are objections to it being too magical, I would argue
it's already spelled out in the docs, so it's being prescribed
as a solution already.  I think it should be very clear when you
derive from "AutoNumberedEnum" what is going on.

The code is very simple, so I would
hope maintenance would not be difficult.

----------
components: Library (Lib)
messages: 265214
nosy: John Hagen, ethan.furman
priority: normal
severity: normal
status: open
title: Add AutoNumberedEnum to stdlib
type: enhancement
versions: Python 3.6

_______________________________________
Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue26988>
_______________________________________