New submission from Kevin Shweh:

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):
        def abstract(self):


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.)

assignee: docs@python
components: Documentation, Library (Lib)
messages: 299810
nosy: Kevin Shweh, docs@python
priority: normal
severity: normal
status: open
title: Abstract classes derived from built-in classes don't block instance 
type: behavior
versions: Python 2.7, Python 3.6

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