[issue5996] abstract class instantiable when subclassing dict

[R. David Murray]

R. David Murray added the comment:

Closed issue 31127 as a duplicate of this one.

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[issue5996] abstract class instantiable when subclassing dict

[R. David Murray]

R. David Murray added the comment:

My apologies, I added Nathaniel to nosy here when I closed the duplicate, but 
forgot to add a link to the closed issue: issue 28537.

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[issue5996] abstract class instantiable when subclassing dict

[Guido van Rossum]

Guido van Rossum added the comment:

Oh sorry. I received the emails in a strange order. I guess it can stay open.

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resolution: wont fix -> 
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[issue5996] abstract class instantiable when subclassing dict

[Guido van Rossum]

Guido van Rossum added the comment:

Where did you report that? I don't see your name on this bug -- it has a patch 
that's been unapplied for 5 years, so I doubt it's very important.

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[issue5996] abstract class instantiable when subclassing dict

[Nathaniel Manista]

Nathaniel Manista added the comment:

Wait, really? My report came out of a real bug that I had in my system and 
shipped to my users; it wasn't academic or contrived at all.

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[issue5996] abstract class instantiable when subclassing dict

[Guido van Rossum]

Guido van Rossum added the comment:

Honestly let's just forget about this.

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resolution:  -> wont fix
status: open -> closed

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[issue5996] abstract class instantiable when subclassing dict

[R. David Murray]

Changes by R. David Murray :


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[issue5996] abstract class instantiable when subclassing dict

[Luiz Poleto]

Changes by Luiz Poleto :


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[issue5996] abstract class instantiable when subclassing dict

[Xiang Zhang]

Xiang Zhang added the comment:

I think subclassing builtin types and making it abstract is rare. And when 
there is a need, we can mimic this in application level (this may also apply to 
types having custom __new__):

In [2]: class CustomDict(dict, metaclass=abc.ABCMeta):
   ...: def __new__(cls, *args, **kwargs):
   ...: if getattr(cls, '__abstractmethods__', None):
   ...: raise TypeError
   ...: return super().__new__(cls, *args, **kwargs)
   ...: @abc.abstractmethod
   ...: def f(self):
   ...: pass

Adding the abstract class checking in tp_alloc or builtin types' tp_new maybe 
degrade performance.

Is it necessary to add this support?

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[issue5996] abstract class instantiable when subclassing dict

[Raymond Hettinger]

Changes by Raymond Hettinger :


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[issue5996] abstract class instantiable when subclassing dict

[Xiang Zhang]

Changes by Xiang Zhang :


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[issue5996] abstract class instantiable when subclassing dict

[Maciej Szulik]

Changes by Maciej Szulik :


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[issue5996] abstract class instantiable when subclassing dict

[Jakub Wilk]

Changes by Jakub Wilk jw...@jwilk.net:


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[issue5996] abstract class instantiable when subclassing dict

[Mark Dickinson]

Changes by Mark Dickinson dicki...@gmail.com:


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[issue5996] abstract class instantiable when subclassing dict

[Mark Dickinson]

Changes by Mark Dickinson dicki...@gmail.com:


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versions: +Python 3.4

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[issue5996] abstract class instantiable when subclassing dict

[Raymond Hettinger]

Raymond Hettinger raymond.hettin...@gmail.com added the comment:

IIRC, classes weren't supposed to be able inherit from two classes that had 
different metaclasses (since differing metaclasses don't necessarily play well 
with one another).

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[issue5996] abstract class instantiable when subclassing dict

[Eugene Toder]

Eugene Toder elto...@gmail.com added the comment:

They are, when there's a most specific metaclass -- the one which is a subclass 
of all others (described here 
http://www.python.org/download/releases/2.2.3/descrintro/#metaclasses, 
implemented here 
http://hg.python.org/cpython/file/ab162f925761/Objects/typeobject.c#l1956). 
Since ABCMeta is a subclass of type this holds.

Also, in the original example there's no multiple inheritance at all.

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[issue5996] abstract class instantiable when subclassing dict

[Raymond Hettinger]

Changes by Raymond Hettinger raymond.hettin...@gmail.com:


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priority: normal - low

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[issue5996] abstract class instantiable when subclassing dict

[Eric Snow]

Changes by Eric Snow ericsnowcurren...@gmail.com:


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[issue5996] abstract class instantiable when subclassing dict

[Eugene Toder]

Eugene Toder elto...@gmail.com added the comment:

Anyone has any thoughts on this?

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[issue5996] abstract class instantiable when subclassing dict

[Eugene Toder]

Eugene Toder elto...@gmail.com added the comment:

This patch fixes the problem by moving the check from object_new to 
PyType_GenericAlloc. The check is very cheap, so this should not be an issue.

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nosy: +eltoder
Added file: http://bugs.python.org/file21600/abc.patch

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[issue5996] abstract class instantiable when subclassing dict

[Nadeem Vawda]

Changes by Nadeem Vawda nadeem.va...@gmail.com:


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[issue5996] abstract class instantiable when subclassing dict

[Éric Araujo]

Changes by Éric Araujo mer...@netwok.org:


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versions: +Python 3.3 -Python 2.6

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[issue5996] abstract class instantiable when subclassing dict

[Antoine Pitrou]

Changes by Antoine Pitrou pit...@free.fr:


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[issue5996] abstract class instantiable when subclassing dict

[Daniel Urban]

Daniel Urban urban.dani...@gmail.com added the comment:

I think, that the reason is that, object.__new__ checks, if the class is 
instantiable (object_new in Objects/typeobject.c ). dict.__new__ (and 
tuple.__new__, and I guess the __new__ method of other built-in types) doesn't 
call object.__new__, but user defined types typically either doesn't have a 
__new__, or call object.__new__ from it (directly or with super).

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[issue5996] abstract class instantiable when subclassing dict

[Terry J. Reedy]

Terry J. Reedy tjre...@udel.edu added the comment:

I presume you claim the dict example to be a bug in relation to A class
that has a metaclass derived from ABCMeta cannot be instantiated unless
all of its abstract methods and properties are overridden.

There is the same difference with @abstractproperty
Windows, 3.0.1

class C(metaclass=abc.ABCMeta):
@abc.abstractmethod
def f(self): return True

class C2(dict,metaclass=abc.ABCMeta):
@abc.abstractmethod
def f(self): return True

c2=C2()
print(c2.f())

c=C()
# prints
True
...
TypeError: Can't instantiate abstract class C with abstract methods f

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[issue5996] abstract class instantiable when subclassing dict

[johannes raggam]

New submission from johannes raggam raggam...@adm.at:

when declaring a abstract base class with an abstract property or method
and subclassing from dict, the class is instantiable (instanceable?).

 import abc
 class A(object):
... __metaclass__ = abc.ABCMeta
... @abc.abstractproperty
... def abstract(self): return True
... 
 a = A()
Traceback (most recent call last):
  File stdin, line 1, in module
TypeError: Can't instantiate abstract class A with abstract methods abstract
 
 class A2(dict):
... __metaclass__ = abc.ABCMeta
... @abc.abstractproperty
... def abstract(self): return True
... 
 a2 = A2()
 


although, when using the dict definition from __builtin__.pi directly,
the abc behaves like expected. but this may be a bug in the
c-implementation from dict.

platform:
Python 2.6.2 (r262:71600, Apr 25 2009, 21:56:41) 
[GCC 4.3.2] on linux2

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components: Library (Lib)
messages: 87574
nosy: thet
severity: normal
status: open
title: abstract class instantiable when subclassing dict
type: behavior
versions: Python 2.6

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