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.list@tim.thechases.com>
X-Original-To: python-list@python.org
Delivered-To: python-list@mail.python.org
X-Spam-Status: OK 0.003
X-Spam-Evidence: '*H*': 0.99; '*S*': 0.00; 'iterate': 0.09; 'lst': 0.09; 'cc:addr:python-list': 0.10; 'def': 0.10; ':-)': 0.13; '"calling': 0.16; '-tkc': 0.16; 'foo():': 0.16; 'foo(object):': 0.16; 'from:addr:python.list': 0.16; 'from:addr:tim.thechases.com': 0.16; 'from:name:tim chase': 0.16; 'rewritten': 0.16; 'roy': 0.16; 'stumbled': 0.16; 'subject:optimization': 0.16; 'trivia': 0.16; 'wrote:': 0.17; 'skip': 0.17; 'bit': 0.21; 'cc:2**0': 0.23; 'seems': 0.23; 'cc:no real name:2**0': 0.24; 'testing': 0.24; 'cc:addr:python.org': 0.25; 'header:In-Reply-To:1': 0.25; 'subject:list': 0.28; 'skip:_ 10': 0.29; 'class': 0.29; 'call.': 0.30; 'lists': 0.31; 'print': 0.32; 'version': 0.34; 'agree': 0.34; 'list': 0.35; 'charset:us- ascii': 0.36; 'subject:: ': 0.38; 'skip:l 20': 0.38; 'shows': 0.38; 'performance': 0.39; 'little': 0.39; 'subject:-': 0.40; 'today.': 0.69; 'smith': 0.71; 'costly': 0.84; 'received:50.22': 0.84; 'angel': 0.93
Date: Wed, 6 Mar 2013 21:57:35 -0600
From: Tim Chase <python.list@tim.thechases.com>
To: Roy Smith <roy@panix.com>
Subject: Re: Interesting list() un-optimization
In-Reply-To: <roy-572C99.22201106032013@70-1-84-166.pools.spcsdns.net>
References: <roy-572C99.22201106032013@70-1-84-166.pools.spcsdns.net>
X-Mailer: Claws Mail 3.7.6 (GTK+ 2.20.1; x86_64-pc-linux-gnu)
Mime-Version: 1.0
Content-Type: text/plain; charset=US-ASCII
Content-Transfer-Encoding: 7bit
X-AntiAbuse: This header was added to track abuse, please include it with any abuse report
X-AntiAbuse: Primary Hostname - boston.accountservergroup.com
X-AntiAbuse: Original Domain - python.org
X-AntiAbuse: Originator/Caller UID/GID - [47 12] / [47 12]
X-AntiAbuse: Sender Address Domain - tim.thechases.com
Cc: python-list@python.org
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.2976.1362628560.2939.python-list@python.org>
Lines: 35
NNTP-Posting-Host: 2001:888:2000:d::a6
X-Trace: 1362628560 news.xs4all.nl 6982 [2001:888:2000:d::a6]:36657
X-Complaints-To: abuse@xs4all.nl
Xref: csiph.com comp.lang.python:40699

On 2013-03-06 22:20, Roy Smith wrote:
> I stumbled upon an interesting bit of trivia concerning lists and
> list comprehensions today.

I agree with Dave Angel that this is interesting.  A little testing
shows that this can be rewritten as

  my_objects = list(iter(my_query_set))

which seems to then skip the costly __len__ call.  Performance geeks
are welcome to time it against the list-comprehension version :-)

-tkc


class Foo(object):
    def __init__(self):
        self.items = range(10)
    def __iter__(self):
        return iter(self.items)
    def __len__(self):
        print "Calling costly __len__"
        return len(self.items)

print "Ensuring we can iterate over it:"
for x in Foo():
    print x

print "\nJust call list():"
lst = list(Foo())
print lst

print "\nCall list(iter())"
lst = list(iter(Foo()))
print lst