add classattribute to enum to handle non-Enum attributes

[ethanfurman]

BPO	26266
Nosy	@warsaw, @ethanfurman

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = 'https://github.com/ethanfurman'
closed_at = <Date 2016-08-20.07:03:00.445>
created_at = <Date 2016-02-02.16:27:30.956>
labels = ['type-feature']
title = 'add classattribute to enum to handle non-Enum attributes'
updated_at = <Date 2016-08-20.07:03:00.443>
user = 'https://github.com/ethanfurman'

bugs.python.org fields:

activity = <Date 2016-08-20.07:03:00.443>
actor = 'ethan.furman'
assignee = 'ethan.furman'
closed = True
closed_date = <Date 2016-08-20.07:03:00.445>
closer = 'ethan.furman'
components = []
creation = <Date 2016-02-02.16:27:30.956>
creator = 'ethan.furman'
dependencies = []
files = []
hgrepos = []
issue_num = 26266
keywords = []
message_count = 3.0
messages = ['259399', '266985', '273189']
nosy_count = 3.0
nosy_names = ['barry', 'eli.bendersky', 'ethan.furman']
pr_nums = []
priority = 'normal'
resolution = 'rejected'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue26266'
versions = ['Python 3.6']

[ethanfurman]

The rules for what objects in an Enum become members and which do not are fairly straight-forward:

__double_underscore__ do not (but is reserved for Python)
_single_underscore_ do not (but is reserved for Enum itself)
any descriptored object (such as functions) do not

Which means the proper way to add constants/attributes to an Enum is to write a descriptor, but most folks don't think about that when the Enum is not working properly they (okay, and me :/ ) just add the double-underscore.

This question has already come up a couple times on StackOverflow:

• http://stackoverflow.com/q/17911188/208880
• http://stackoverflow.com/q/34465739/208880

While this doesn't come up very often, that just means it is even more likely to have the attribute be __double_underscored__ instead of descriptored.

The solution is have a descriptor in the Enum module for this case. While it would be possible to have several (constant-unless-mutable, constant-even-if-mutable, not-constant, possibly others) I think the not-constant would be sufficient (aka a writable property), although I am not opposed to a constant-unless mutable version as well.

The not-constant version would look like this (I'll attach patch later):

class classattribute:
    def __init__(self, value):
        self.value = value
    def __get__(self, *args):
        return self.value
    def __set__(self, value):
        self.value = value
    def __repr__(self):
        return '%s(%r)' % (self.__class__.__name__, self.value)

[ethanfurman]

One possible downside to the classattribute route is that we have a descriptor whose only purpose is to shield the item from becoming a member; the up-side is that it's simple to implement.

Another possibility is skip:

class skip:
    """
    Protects item from becaming an Enum member during class creation.
    """
    def __init__(self, value):
        self.value = value

    def __get__(self, instance, ownerclass=None):
        return self.value

The advantage is that it is replaced by the metaclass with the stored value, so we have no extraneous descriptor after the Enum is created; the downside is that it requires change in the metaclass to do the right thing.

Thoughts?

[ethanfurman]

We'll keep the stdlib version simple.