Path: csiph.com!usenet.pasdenom.info!aioe.org!news.stack.nl!newsfeed.xs4all.nl!newsfeed2.news.xs4all.nl!xs4all!post.news.xs4all.nl!not-for-mail
Return-Path: <gherron@digipen.edu>
X-Original-To: python-list@python.org
Delivered-To: python-list@mail.python.org
X-Spam-Status: OK 0.071
X-Spam-Evidence: '*H*': 0.86; '*S*': 0.00; 'tutorial': 0.03; 'modified': 0.07; '[1,': 0.09; 'def': 0.12; 'dictionary,': 0.16; 'mutable': 0.16; 'once.': 0.16; 'retained': 0.16; 'successive': 0.16; 'wrote:': 0.18; 'slightly': 0.19; 'print': 0.22; 'header :User-Agent:1': 0.23; '(or': 0.24; 'code:': 0.26; 'header:In- Reply-To:1': 0.27; 'function': 0.29; '[1]': 0.29; "i'm": 0.30; 'gary': 0.31; 'once,': 0.31; 'python.org': 0.32; 'classes': 0.35; 'created': 0.35; 'instances': 0.36; 'list': 0.37; 'received:10': 0.37; 'to:addr:python-list': 0.38; 'list,': 0.38; 'pm,': 0.38; 'rather': 0.38; 'does': 0.39; 'to:addr:python.org': 0.39; 'how': 0.40; 'most': 0.60; 'such': 0.63; 'more': 0.64; 'between': 0.67; 'default': 0.69; 'institute': 0.72; 'obvious': 0.74; 'received:10.10': 0.74; 'received:204': 0.75; 'dr.': 0.77; 'subject:Value': 0.84; 'why?': 0.91
Date: Wed, 19 Jun 2013 12:57:48 -0700
From: Gary Herron <gherron@digipen.edu>
User-Agent: Mozilla/5.0 (X11; Linux i686; rv:17.0) Gecko/20130510 Thunderbird/17.0.6
MIME-Version: 1.0
To: python-list@python.org
Subject: Re: Default Value
References: <7e6361d5-6619-4aaa-adda-8b5f01bde57f@googlegroups.com>
In-Reply-To: <7e6361d5-6619-4aaa-adda-8b5f01bde57f@googlegroups.com>
Content-Type: text/plain; charset=ISO-8859-1; format=flowed
Content-Transfer-Encoding: 7bit
X-BeenThere: python-list@python.org
X-Mailman-Version: 2.1.15
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.3597.1371672307.3114.python-list@python.org>
Lines: 43
NNTP-Posting-Host: 2001:888:2000:d::a6
X-Trace: 1371672307 news.xs4all.nl 15882 [2001:888:2000:d::a6]:40667
X-Complaints-To: abuse@xs4all.nl
Xref: csiph.com comp.lang.python:48747

On 06/19/2013 12:17 PM, Ahmed Abdulshafy wrote:
> I'm reading the Python.org tutorial right now, and I found this part rather strange and incomprehensible to me>
>
> Important warning: The default value is evaluated only once. This makes a difference when the default is a mutable object such as a list, dictionary, or instances of most classes

This code:
> def f(a, L=[]):
>      L.append(a)
>      return L

does the same as this code:

M=[]
def f(a, L=M):
     L.append(a)
     return L

where it's slightly more obvious that the list is created once, and 
modified with each call to the function (or rather with each call to the 
function that does not supply its own value for L).

Gary Herron


>
> print(f(1))
> print(f(2))
> print(f(3))
>
> This will print
> [1]
> [1, 2]
> [1, 2, 3]
>
> How the list is retained between successive calls? And why?


-- 
Dr. Gary Herron
Department of Computer Science
DigiPen Institute of Technology
(425) 895-4418