[docs] [issue31127] Abstract classes derived from built-in classes don't block instance creation

Kevin Shweh report at bugs.python.org
Sun Aug 6 15:00:42 EDT 2017


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):
	@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.)

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

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue31127>
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