Path: csiph.com!v102.xanadu-bbs.net!xanadu-bbs.net!news.mixmin.net!newsreader4.netcologne.de!news.netcologne.de!xlned.com!feeder7.xlned.com!newsfeed.xs4all.nl!newsfeed3.news.xs4all.nl!xs4all!post.news.xs4all.nl!not-for-mail Return-Path: X-Original-To: python-list@python.org Delivered-To: python-list@mail.python.org X-Spam-Status: OK 0.008 X-Spam-Evidence: '*H*': 0.98; '*S*': 0.00; 'else:': 0.03; 'explicit': 0.07; 'processing.': 0.07; 'stops': 0.07; 'subject:PEP': 0.07; 'function:': 0.09; 'lines.': 0.09; 'cc:addr:python-list': 0.11; 'def': 0.12; 'expression.': 0.16; 'wrote:': 0.18; 'file,': 0.19; '8bit%:5': 0.22; 'example': 0.22; 'cc:addr:python.org': 0.22; 'load': 0.23; 'header': 0.24; 'cc:2**0': 0.24; 'cc:no real name:2**0': 0.24; 'header:In-Reply-To:1': 0.27; 'point': 0.28; 'am,': 0.29; 'raise': 0.29; 'message-id:@mail.gmail.com': 0.30; 'code': 0.31; '25,': 0.31; 'bad.': 0.31; 'breaking': 0.31; 'file': 0.32; 'another': 0.32; 'checking': 0.33; 'but': 0.35; 'received:google.com': 0.35; 'raising': 0.36; 'possible': 0.36; '8bit%:4': 0.38; '8bit%:6': 0.40; 'even': 0.60; 'expression': 0.60; 'subject:? ': 0.60; 'more': 0.64; 'to:addr:gmail.com': 0.65; 'within': 0.65; 'to:charset:iso-8859-1': 0.74; 'subject:this': 0.83; 'condition.': 0.84; '2013': 0.98 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20120113; h=mime-version:in-reply-to:references:date:message-id:subject:from:to :cc:content-type; bh=uFK5NItjmw/uqDQImnng2fbxiLQmSMoxE0NVbQTWSpU=; b=CANr0xeUFRYdShLbQbN7tFTPEhMmrrYIu4QpelcpciBJYC2j678OKtlm4OVLYQxngx XnCECHSH2yma5IW0yWECMSbuSvqlMI3zllS0S5EvAhmpNKUZwGXeLPSJim1mm+sdvo4g r+rGwVwPSg23ETkhej2bbztmiP39nDRWFAgldlCS2VLEPRmoXu+PqRzXwvN/l4QvT29D Mo48dnbQiz7Y7ylwOP3YnteC3Ml6/IduYD3OFY1BpZAR8gnvG6PYcpKHqFkkLsCfQGI3 u8hH/5i+u0kky2ERE46781OLYpld3eyEJ9dnmuEIh8jzoXdHkWC4fgQWvi4r0eoKm8j8 hpMA== MIME-Version: 1.0 X-Received: by 10.220.73.135 with SMTP id q7mr12666215vcj.33.1372120873940; Mon, 24 Jun 2013 17:41:13 -0700 (PDT) In-Reply-To: References: <8D03F2B8CF0E7BE-1864-1796B@webmail-m103.sysops.aol.com> Date: Tue, 25 Jun 2013 10:41:13 +1000 Subject: Re: Is this PEP-able? fwhile From: wu wei To: =?ISO-8859-1?Q?F=E1bio_Santos?= Content-Type: multipart/alternative; boundary=001a11c2917c47c1fe04dfefc9b4 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 List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Newsgroups: comp.lang.python Message-ID: Lines: 66 NNTP-Posting-Host: 2001:888:2000:d::a6 X-Trace: 1372120877 news.xs4all.nl 15885 [2001:888:2000:d::a6]:45675 X-Complaints-To: abuse@xs4all.nl Xref: csiph.com comp.lang.python:49115 --001a11c2917c47c1fe04dfefc9b4 Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: quoted-printable On Tue, Jun 25, 2013 at 10:19 AM, F=E1bio Santos wrote: > for X in (i for i in open('largefile') if is_part_of_header(i)): > > The above code would be wasting time on IO and processing. It would load > another line and calculate the condition for every line of the large file > and I just wanted to loop over the few header lines. > > itertools.takewhile and fwhile/for..while actually stops the loop when th= e > condition is not meant, while your example keeps checking the condition > until the end of file, even though it is a generator expression. > Ah yes, of course, my bad. It's still possible by raising a StopIteration within the condition function: def is_part_of_header(x): if header_condition: return True else: raise StopIteration But yes, by this point any clarity of the generator expression approach comes with the cost of more explicit setup of the breaking condition. --001a11c2917c47c1fe04dfefc9b4 Content-Type: text/html; charset=ISO-8859-1 Content-Transfer-Encoding: quoted-printable
On Tue, Jun 25, 2013 at 10:19 AM, F=E1bio Santos=A0<fabiosantosart@gmail.com>=A0wrote:

for X in (i for i in open('largefile= 9;) if is_part_of_header(i)):

The above code would be wasting time on IO and processin= g. It would load another line and calculate the condition for every line of= the large file and I just wanted to loop over the few header lines.

itertools.takewhile and fwhile/for..while actually stops the loop when the = condition is not meant, while your example keeps checking the condition unt= il the end of file, even though it is a generator expression.

Ah yes, of course, my bad.

=
It's still possible by raising a StopIterati= on within the condition function:

=A0 =A0 def is_part_of_header(x):
=A0 =A0 = =A0 =A0 if header_condition:
=A0 =A0 =A0 = =A0 =A0 =A0 return True
=A0 =A0 =A0 =A0 els= e:
=A0 =A0 =A0 =A0 =A0 =A0 raise StopIterat= ion

But yes, by= this point any clarity of the generator expression approach comes with the= cost of more explicit setup of the breaking condition.
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