Re: type(None)()

Path	csiph.com!usenet.pasdenom.info!weretis.net!feeder4.news.weretis.net!feeder2.ecngs.de!ecngs!feeder.ecngs.de!xlned.com!feeder1.xlned.com!newsfeed.xs4all.nl!newsfeed5.news.xs4all.nl!xs4all!newsgate.cistron.nl!newsgate.news.xs4all.nl!post.news.xs4all.nl!not-for-mail
Return-Path	<gandalf@shopzeus.com>
X-Original-To	python-list@python.org
Delivered-To	python-list@mail.python.org
X-Spam-Status	OK 0.002
X-Spam-Evidence	'H': 1.00; 'S': 0.00; 'none:': 0.05; 'that?': 0.05; 'raises': 0.07; 'bool': 0.09; 'instance.': 0.09; 'subject:None': 0.09; 'typeerror:': 0.09; 'subject:type': 0.16; 'wrote:': 0.17; 'instance': 0.17; '"",': 0.22; 'class.': 0.23; 'this:': 0.23; 'second': 0.24; 'allows': 0.25; 'header:In-Reply- To:1': 0.25; 'header:User-Agent:1': 0.26; '(most': 0.27; "d'aprano": 0.29; 'obj': 0.29; 'steven': 0.29; 'error': 0.30; 'file': 0.32; 'instances': 0.33; 'traceback': 0.33; 'to:addr :python-list': 0.33; 'useful': 0.36; 'two': 0.37; 'why': 0.37; 'subject:: ': 0.38; 'to:addr:python.org': 0.39; 'received:192': 0.39; 'received:192.168': 0.40; 'received:204': 0.72
Date	Thu, 16 Aug 2012 14:54:03 +0200
From	Laszlo Nagy <gandalf@shopzeus.com>
User-Agent	Mozilla/5.0 (X11; Linux x86_64; rv:14.0) Gecko/20120714 Thunderbird/14.0
MIME-Version	1.0
To	python-list@python.org
Subject	Re: type(None)()
References	<502cebf4$0$6905$e4fe514c@news2.news.xs4all.nl>
In-Reply-To	<502cebf4$0$6905$e4fe514c@news2.news.xs4all.nl>
Content-Type	text/plain; charset=ISO-8859-1; format=flowed
Content-Transfer-Encoding	7bit
X-BeenThere	python-list@python.org
X-Mailman-Version	2.1.12
Precedence	list
List-Id	General discussion list for the Python programming language <python-list.python.org>
List-Unsubscribe	<http://mail.python.org/mailman/options/python-list>, <mailto:python-list-request@python.org?subject=unsubscribe>
List-Archive	<http://mail.python.org/pipermail/python-list>
List-Post	<mailto:python-list@python.org>
List-Help	<mailto:python-list-request@python.org?subject=help>
List-Subscribe	<http://mail.python.org/mailman/listinfo/python-list>, <mailto:python-list-request@python.org?subject=subscribe>
Newsgroups	comp.lang.python
Message-ID	<mailman.3360.1345122462.4697.python-list@python.org> (permalink)
Lines	17
NNTP-Posting-Host	2001:888:2000:d::a6
X-Trace	1345122462 news.xs4all.nl 6983 [2001:888:2000:d::a6]:46801
X-Complaints-To	abuse@xs4all.nl
Xref	csiph.com comp.lang.python:27160

Show key headers only | View raw

On 2012-08-16 14:47, Hans Mulder wrote:
> On 8/08/12 04:14:01, Steven D'Aprano wrote:
>> NoneType raises an error if you try to create a second instance. bool
>> just returns one of the two singletons (doubletons?) again.
>>
>> py> type(None)()
>> Traceback (most recent call last):
>>    File "<stdin>", line 1, in <module>
>> TypeError: cannot create 'NoneType' instances
> Why is that?
Because None is a singleton. It is the only instance of its class. This 
is very useful because it allows you to write conditions like this:

if obj is None:
     do_something()

Thread

type(None)() Hans Mulder <hansmu@xs4all.nl> - 2012-08-16 14:47 +0200
  Re: type(None)() Laszlo Nagy <gandalf@shopzeus.com> - 2012-08-16 14:54 +0200
  Re: type(None)() Chris Angelico <rosuav@gmail.com> - 2012-08-16 23:37 +1000
  Re: type(None)() Ian Kelly <ian.g.kelly@gmail.com> - 2012-08-16 07:56 -0600
  Re: type(None)() Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2012-08-16 13:58 +0000
    Re: type(None)() Stefan Behnel <stefan_ml@behnel.de> - 2012-08-16 17:31 +0200
    Re: type(None)() Stefan Behnel <stefan_ml@behnel.de> - 2012-08-16 17:44 +0200
    Re: type(None)() Ethan Furman <ethan@stoneleaf.us> - 2012-08-16 09:06 -0700
  Re: type(None)() Robert Kern <robert.kern@gmail.com> - 2012-08-16 15:56 +0100
  Re: type(None)() MRAB <python@mrabarnett.plus.com> - 2012-08-16 16:13 +0100
  Re: type(None)() Ethan Furman <ethan@stoneleaf.us> - 2012-08-16 09:07 -0700

csiph-web