Improve handling of type.__abstractmethods__ descriptor

[ncoghlan]

BPO	19022
Nosy	@ncoghlan, @benjaminp, @durban

^{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 = None
closed_at = None
created_at = <Date 2013-09-15.07:33:28.392>
labels = ['interpreter-core', 'type-bug', '3.9', '3.10', '3.11']
title = 'Improve handling of type.__abstractmethods__ descriptor'
updated_at = <Date 2021-11-29.17:56:16.479>
user = 'https://github.com/ncoghlan'

bugs.python.org fields:

activity = <Date 2021-11-29.17:56:16.479>
actor = 'iritkatriel'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Interpreter Core']
creation = <Date 2013-09-15.07:33:28.392>
creator = 'ncoghlan'
dependencies = []
files = []
hgrepos = []
issue_num = 19022
keywords = []
message_count = 2.0
messages = ['197755', '198290']
nosy_count = 4.0
nosy_names = ['ncoghlan', 'benjamin.peterson', 'daniel.urban', 'Yaroslav.Halchenko']
pr_nums = []
priority = 'normal'
resolution = None
stage = 'needs patch'
status = 'open'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue19022'
versions = ['Python 3.9', 'Python 3.10', 'Python 3.11']

[ncoghlan]

bpo-10006 pointed out that attempting to access object.__abstractmethods__ raises AttributeError. The solution was to ensure type.__abstractmethods__ *also* raises AttributeError.

This is thoroughly confusing, since the name is clearly visible in the output of dir(type). While it's technically legal for a descriptor to always raise AttributeError, breaking the following typical assumed invariant is *highly* dubious:

all(hasattr(obj, x) for x in dir(obj))

I see three main alternatives for improving the situation:

1. Make __abstractmethods__ a read-only descriptor that returns something meaningful (like a frozenset(), the same as you get if you inherit from abc.ABCMeta without defining any abstract methods)

2. Throw a better exception message that explains the broken invariant rather than the generic "AttributeError with attribute name" that is currently thrown.

3. Implement type.__dir__ to filter out the uncooperative descriptor

[durban]

I like the 3rd alternative the most. It seems to me, that __abstractmethods__ is currently an undocumented implementation detail of ABCs. The 1st alternative would cause every type to have an empty __abstractmethods__ (which is technically correct, but probably not very useful). It seems to me that the 2nd alternative would not help very much in the original problem mentioned in bpo-10006 (automatically looking up every name found in dir). So I think, that the best would be to fix type.__dir__ this way. (Although, if we really want to document and use __abstractmethods__ on every type, then of course we should do the 1st alternative.)

[devdanzin]

This looks like a prime candidate for a discuss.python.org topic, if there's still interest in pursuing this issue.