[issue19022] Improve handling of type.__abstractmethods__ descriptor

[Nick Coghlan]

New submission from Nick Coghlan:

Issue 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

----------
messages: 197755
nosy: ncoghlan
priority: normal
severity: normal
stage: needs patch
status: open
title: Improve handling of type.__abstractmethods__ descriptor
type: behavior
versions: Python 3.4

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue19022>
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