Re: lambda in list comprehension acting funny

Path	csiph.com!usenet.pasdenom.info!news.albasani.net!newsfeed.freenet.ag!news2.euro.net!newsgate.cistron.nl!newsgate.news.xs4all.nl!post.news.xs4all.nl!not-for-mail
Return-Path	<python-python-list@m.gmane.org>
X-Original-To	python-list@python.org
Delivered-To	python-list@mail.python.org
X-Spam-Status	OK 0.001
X-Spam-Evidence	'H': 1.00; 'S': 0.00; 'expressions': 0.07; 'parameter': 0.07; 'subject:skip:c 10': 0.07; 'python': 0.09; 'received:80.91': 0.09; 'received:80.91.229': 0.09; 'received:gmane.org': 0.09; 'received:list': 0.09; 'references.': 0.09; 'stored': 0.10; "'at": 0.16; 'bind': 0.16; 'funcs': 0.16; 'helps.': 0.16; 'lambda': 0.16; 'message-id:@dough.gmane.org': 0.16; 'received:80.91.229.3': 0.16; 'received:plane.gmane.org': 0.16; 'simplest': 0.16; 'variable': 0.20; 'references': 0.23; 'this:': 0.23; 'header:In-Reply-To:1': 0.25; 'header:User- Agent:1': 0.26; 'header:X-Complaints-To:1': 0.28; 'subject:list': 0.28; 'print': 0.32; 'anyone': 0.33; 'to:addr:python-list': 0.33; 'received:org': 0.36; 'does': 0.37; 'subject:: ': 0.38; 'gives': 0.39; 'to:addr:python.org': 0.39; 'header:Received:5': 0.40; 'more': 0.63; 'importantly,': 0.65; 'received:93': 0.72; '(they': 0.84; 'received:hr': 0.84; 'why?': 0.84; 'subject:funny': 0.91
X-Injected-Via-Gmane	http://gmane.org/
To	python-list@python.org
From	Jurko Gospodnetić <jurko.gospodnetic@pke.hr>
Subject	Re: lambda in list comprehension acting funny
Date	Wed, 11 Jul 2012 09:01:18 +0200
Organization	PKE sistemi d.o.o.
References	<CADjSo4QvTnGKy_ZYOO5qZu3tSKovwAMGjku4L+RXEkYmwXPz4g@mail.gmail.com> <CADjSo4SeuMJOP_iEM9rjVsFtVh84HaVUtPkUCZGKiEdQBWuerw@mail.gmail.com>
Mime-Version	1.0
Content-Type	text/plain; charset=UTF-8; format=flowed
Content-Transfer-Encoding	8bit
X-Gmane-NNTP-Posting-Host	93-139-12-146.adsl.net.t-com.hr
User-Agent	Mozilla/5.0 (Windows NT 6.1; WOW64; rv:13.0) Gecko/20120614 Thunderbird/13.0.1
In-Reply-To	<CADjSo4SeuMJOP_iEM9rjVsFtVh84HaVUtPkUCZGKiEdQBWuerw@mail.gmail.com>
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.2008.1341990101.4697.python-list@python.org> (permalink)
Lines	30
NNTP-Posting-Host	2001:888:2000:d::a6
X-Trace	1341990101 news.xs4all.nl 6872 [2001:888:2000:d::a6]:40629
X-Complaints-To	abuse@xs4all.nl
Xref	csiph.com comp.lang.python:25168

Show key headers only | View raw

   Hi.

>> funcs = [ lambda x: x**i for i in range( 5 ) ]
>> print funcs[0]( 2 )
>>
>> This gives me
>> 16
>>
>> When I was excepting
>> 1
>>
>> Does anyone know why?

   Just the way Python lambda expressions bind their variable 
references. Inner 'i' references the outer scope's 'i' variable and not 
its value 'at the time the lambda got defined'.


> And more importantly, what's the simplest way to achieve the latter? :)

   Try giving the lambda a default parameter (they get calculated and 
have their value stored at the time the lambda is defined) like this:
   funcs = [ lambda x, i=i: x**i for i in range( 5 ) ]


   Hope this helps.

   Best regards,
     Jurko Gospodnetić

Back to comp.lang.python | Previous | Next | Find similar | Unroll thread

Thread

Re: lambda in list comprehension acting funny Jurko Gospodnetić <jurko.gospodnetic@pke.hr> - 2012-07-11 09:01 +0200

csiph-web