Path: csiph.com!x330-a1.tempe.blueboxinc.net!newsfeed.hal-mli.net!feeder1.hal-mli.net!goblin1!goblin2!goblin.stu.neva.ru!newsfeed.xs4all.nl!newsfeed5.news.xs4all.nl!xs4all!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 0.000
X-Spam-Evidence: '*H*': 1.00; '*S*': 0.00; 'parameter': 0.05; 'subject:two': 0.07; 'terry': 0.07; 'python': 0.08; 'received:80.91': 0.09; 'received:80.91.229': 0.09; 'received:80.91.229.12': 0.09; 'received:gmane.org': 0.09; 'received:list': 0.09; 'received:lo.gmane.org': 0.09; 'similarly,': 0.09; 'subclass': 0.09; 'subject:parameters': 0.09; 'am,': 0.14; 'wrote:': 0.14; 'arrays.': 0.16; 'lisp': 0.16; 'reedy': 0.16; 'retrieves': 0.16; 'subject:function': 0.16; 'sublist': 0.16; 'tuples,': 0.16; 'wrapping': 0.16; 'call.': 0.19; 'programming': 0.19; 'jan': 0.20; 'header:In-Reply-To:1': 0.21; 'objects,': 0.23; 'function': 0.25; 'expect': 0.25; 'parameters': 0.26; 'object': 0.26; 'class': 0.29; 'do.': 0.30; 'objects.': 0.30; 'functional': 0.31; 'header:X-Complaints-To:1': 0.32; 'to:addr:python-list': 0.33; 'list': 0.33; 'list.': 0.33; 'header :User-Agent:1': 0.35; 'languages': 0.35; 'lists,': 0.36; 'received:org': 0.38; 'creates': 0.38; 'linked': 0.38; 'subject:: ': 0.38; 'subject: (': 0.39; 'perhaps': 0.39; 'header:Mime- Version:1': 0.39; 'to:addr:python.org': 0.39; 'everyone': 0.40; 'really': 0.40; 'easily': 0.60; 'back': 0.63; 'free': 0.63; 'discipline': 0.67
X-Injected-Via-Gmane: http://gmane.org/
To: python-list@python.org
From: Terry Reedy <tjreedy@udel.edu>
Subject: Re: scope of function parameters (take two)
Date: Tue, 31 May 2011 11:59:21 -0400
References: <F8395F78-615E-4FBD-B6FC-1D6173EAEA45@mcgill.ca>	<F4EAD1ED-563D-4D6E-A50C-68308A9F26B7@mcgill.ca>	<201105310713.24842.wolfgang@rohdewald.de> <D6671B7A-C8B1-4235-A50F-31D9F2AD7DD7@mcgill.ca>
Mime-Version: 1.0
Content-Type: text/plain; charset=ISO-8859-1; format=flowed
Content-Transfer-Encoding: 7bit
X-Gmane-NNTP-Posting-Host: rain.gmane.org
User-Agent: Mozilla/5.0 (Windows; U; Windows NT 5.1; en-US; rv:1.9.2.17) Gecko/20110414 Lightning/1.0b2 Thunderbird/3.1.10
In-Reply-To: <D6671B7A-C8B1-4235-A50F-31D9F2AD7DD7@mcgill.ca>
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.2317.1306857576.9059.python-list@python.org>
Lines: 22
NNTP-Posting-Host: 82.94.164.166
X-Trace: 1306857577 news.xs4all.nl 49179 [::ffff:82.94.164.166]:36438
X-Complaints-To: abuse@xs4all.nl
Xref: x330-a1.tempe.blueboxinc.net comp.lang.python:6733

On 5/31/2011 2:37 AM, Henry Olders wrote:

> what I want is a function that is free of side effects back through
> the parameters passed in the function call.

You can get that by refraining from mutating parameter objects.
Simple as that.
Just do not expect Python to enforce that discipline on everyone else.
To be really functional, and never mutate objects, do not use Python 
lists, which are arrays. Use linked-list trees, like Lisp languages and 
perhaps others do. One can easily do this with tuples, or a subclass of 
tuples, or a class wrapping tuples.

Linked-lists and functional programming go together because prepending 
to a linked list creates a new object while appending to a Python list 
mutates an existing list. Similarly, popping from a linked list 
retrieves an item and an existing sublist while popping from a Python 
list retrieves and item and mutates the list.

-- 
Terry Jan Reedy