Abstract classes derived from built-in classes don't block instance creation

[KevinShweh]

BPO	31127
Nosy	@bitdancer
Superseder	bpo-5996: abstract class instantiable when subclassing built-in types

^{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 = <Date 2017-08-06.21:39:14.711>
created_at = <Date 2017-08-06.19:00:42.367>
labels = ['type-bug', 'library', 'docs']
title = "Abstract classes derived from built-in classes don't block instance creation"
updated_at = <Date 2017-08-06.21:39:14.709>
user = 'https://bugs.python.org/KevinShweh'

bugs.python.org fields:

activity = <Date 2017-08-06.21:39:14.709>
actor = 'r.david.murray'
assignee = 'docs@python'
closed = True
closed_date = <Date 2017-08-06.21:39:14.711>
closer = 'r.david.murray'
components = ['Documentation', 'Library (Lib)']
creation = <Date 2017-08-06.19:00:42.367>
creator = 'Kevin Shweh'
dependencies = []
files = []
hgrepos = []
issue_num = 31127
keywords = []
message_count = 2.0
messages = ['299810', '299814']
nosy_count = 3.0
nosy_names = ['r.david.murray', 'docs@python', 'Kevin Shweh']
pr_nums = []
priority = 'normal'
resolution = 'duplicate'
stage = 'resolved'
status = 'closed'
superseder = '5996'
type = 'behavior'
url = 'https://bugs.python.org/issue31127'
versions = ['Python 2.7', 'Python 3.6']

[KevinShweh]

The only check that prevents instantiating abstract classes is in object.__new__, but most built-in classes never actually call object.__new__. That means you can do stuff like

import abc

class Foo(list, metaclass=abc.ABCMeta):
	@abc.abstractmethod
	def abstract(self):
		pass

Foo()

and the Foo() call will silently succeed.

Ideally, the Foo() call should fail. Other options include having the Foo class definition itself fail, or just making a note in the documentation describing the limitation. (As far as I can see, this is currently undocumented.)

[bitdancer]

This is a duplidate of bpo-5996. It is not clear if we are going to treat it as a bug or a doc bug.