Groups | Search | Server Info | Keyboard shortcuts | Login | Register [http] [https] [nntp] [nntps]

Groups > comp.lang.python > #44200

Re: Nested iteration?

Path csiph.com!usenet.pasdenom.info!gegeweb.org!de-l.enfer-du-nord.net!feeder1.enfer-du-nord.net!cs.uu.nl!news.stack.nl!newsfeed.xs4all.nl!newsfeed1.news.xs4all.nl!xs4all!newsgate.cistron.nl!newsgate.news.xs4all.nl!post.news.xs4all.nl!not-for-mail
Return-Path <ian.g.kelly@gmail.com>
X-Original-To python-list@python.org
Delivered-To python-list@mail.python.org
X-Spam-Status OK 0.005
X-Spam-Evidence '*H*': 0.99; '*S*': 0.00; 'handler': 0.05; '*not*': 0.07; 'lines,': 0.07; 'exit': 0.09; 'function,': 0.09; 'iterate': 0.09; 'propagate': 0.09; 'raises': 0.09; '*other*': 0.16; '23,': 0.16; 'caveat': 0.16; 'guarded': 0.16; 'handler.': 0.16; 'happily': 0.16; 'ignoring': 0.16; 'letting': 0.16; 'loop.': 0.16; 'looping': 0.16; 'loops': 0.16; 'pairs': 0.16; 'pairs,': 0.16; 'exception': 0.16; 'wrote:': 0.18; 'stack': 0.19; 'instance,': 0.24; 'file.': 0.24; 'this:': 0.26; 'header:In-Reply-To:1': 0.27; 'chris': 0.29; 'am,': 0.29; 'reaches': 0.30; 'message- id:@mail.gmail.com': 0.30; 'code': 0.31; 'catching': 0.31; 'exceptions': 0.31; 'object.': 0.31; 'raised': 0.31; 'terminate': 0.31; 'run': 0.32; '(i.e.': 0.33; 'there,': 0.34; "i'd": 0.34; 'could': 0.34; 'problem': 0.35; 'received:209.85': 0.35; 'received:209.85.220': 0.35; 'definition': 0.35; 'received:google.com': 0.35; 'there': 0.35; 'returning': 0.36; 'method': 0.36; 'possible': 0.36; 'subject:?': 0.36; 'should': 0.36; 'received:209': 0.37; 'being': 0.38; 'to:addr:python-list': 0.38; 'little': 0.38; 'to:addr:python.org': 0.39; 'called': 0.40; 'even': 0.60; 'catch': 0.60; 'ian': 0.60; 'break': 0.61; 'mentioned': 0.61; 'simply': 0.61; 'simple': 0.61; '(that': 0.65; 'here': 0.66; 'between': 0.67; 'unusual': 0.74; '"simply': 0.84; 'prevailing': 0.84; '2013': 0.98
DKIM-Signature v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20120113; h=x-received:mime-version:in-reply-to:references:from:date:message-id :subject:to:content-type; bh=9wT/keayZizHot3EFxsjwr9CwrbE5E4ULZunFm9mGRQ=; b=E/pwcFzWhtHFWvLE0Zk6R7nIXnn6XwwhI8j/rpZMToEweeD9q7/LDoAuLsIYYsjMuI R/erZXrwOPkEJMBqS/cFqnQeQ0QhAeVX5wqRIE4OE2ppunHf01PDL9CIOexPrEFfU++j gxOPuFff4cWOj2a0J4rQmmlHE5JC9sB45FojfPB4X3cv54q4Lc3nVdVXExmaRbiC3Myj r7mvk/jLcPkilgWhTN2dVsDJJOXeS1hacoq68hN77cm/bK7M7uX04puHuaGmkDaBFEwu 65J3Kv1A1CMxrCp92cwenAC7qgW6oRiiFtjFTW6z1Svgp7TxdSN1XiCxq8cRje58f125 QiXw==
X-Received by 10.66.121.227 with SMTP id ln3mr14203290pab.165.1366735206572; Tue, 23 Apr 2013 09:40:06 -0700 (PDT)
MIME-Version 1.0
In-Reply-To <CALwzidk6qz4qHOFWYqv_cDEtjbhvS_kjG0vXJZmeB0dJ07doTQ@mail.gmail.com>
References <kl6a1f$k2l$1@panix2.panix.com> <CAPTjJmpKR8z7Cub1WaurmsuugNgCvPYFdh9WYqJd_1OTfpLw6Q@mail.gmail.com> <CALwzidk6qz4qHOFWYqv_cDEtjbhvS_kjG0vXJZmeB0dJ07doTQ@mail.gmail.com>
From Ian Kelly <ian.g.kelly@gmail.com>
Date Tue, 23 Apr 2013 10:39:26 -0600
Subject Re: Nested iteration?
To Python <python-list@python.org>
Content-Type text/plain; charset=ISO-8859-1
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.985.1366735209.3114.python-list@python.org> (permalink)
Lines 33
NNTP-Posting-Host 2001:888:2000:d::a6
X-Trace 1366735209 news.xs4all.nl 2266 [2001:888:2000:d::a6]:45506
X-Complaints-To abuse@xs4all.nl
Xref csiph.com comp.lang.python:44200

Show key headers only | View raw

On Tue, Apr 23, 2013 at 10:30 AM, Ian Kelly <ian.g.kelly@gmail.com> wrote:
> On Tue, Apr 23, 2013 at 10:21 AM, Chris Angelico <rosuav@gmail.com> wrote:
>> The definition of the for loop is sufficiently simple that this is
>> safe, with the caveat already mentioned (that __iter__ is just
>> returning self). And calling next() inside the loop will simply
>> terminate the loop if there's nothing there, so I'd not have a problem
>> with code like that - for instance, if I wanted to iterate over pairs
>> of lines, I'd happily do this:
>>
>> for line1 in f:
>>   line2=next(f)
>>   print(line2)
>>   print(line1)
>>
>> That'll happily swap pairs, ignoring any stray line at the end of the
>> file. Why bother catching StopIteration just to break?
>
> The next() there will *not* "simply terminate the loop" if it raises a
> StopIteration; for loops do not catch StopIteration exceptions that
> are raised from the body of the loop.  The StopIteration will continue
> to propagate until it is caught or it reaches the sys.excepthook.  In
> unusual circumstances, it is even possible that it could cause some
> *other* loop higher in the stack to break (i.e. if the current code is
> being run as a result of the next() method being called by the looping
> construct).

To expand on this, the prevailing wisdom here is that calls to next()
should always be guarded with a StopIteration exception handler.  The
one exception to this is when the next() call is inside the body of a
generator function, and the exception handler would cause the
generator to exit anyway; in that case there is little difference
between "except StopIteration: return" and letting the StopIteration
propagate to the generator object.

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

Thread

Nested iteration? roy@panix.com (Roy Smith) - 2013-04-23 11:40 -0400
  Re: Nested iteration? Oscar Benjamin <oscar.j.benjamin@gmail.com> - 2013-04-23 17:05 +0100
  Re: Nested iteration? Ian Kelly <ian.g.kelly@gmail.com> - 2013-04-23 10:05 -0600
  Re: Nested iteration? Peter Otten <__peter__@web.de> - 2013-04-23 18:15 +0200
  Re: Nested iteration? Chris Angelico <rosuav@gmail.com> - 2013-04-24 02:21 +1000
  Re: Nested iteration? Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2013-04-23 16:35 +0000
  Re: Nested iteration? Ian Kelly <ian.g.kelly@gmail.com> - 2013-04-23 10:30 -0600
  Re: Nested iteration? Ian Kelly <ian.g.kelly@gmail.com> - 2013-04-23 10:39 -0600
  Re: Nested iteration? Chris Angelico <rosuav@gmail.com> - 2013-04-24 02:42 +1000
    Re: Nested iteration? Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2013-04-23 16:53 +0000
  Re: Nested iteration? Terry Jan Reedy <tjreedy@udel.edu> - 2013-04-23 16:49 -0400
  Re: Nested iteration? Joshua Landau <joshua.landau.ws@gmail.com> - 2013-04-23 22:14 +0100
  Re: Nested iteration? Oscar Benjamin <oscar.j.benjamin@gmail.com> - 2013-04-23 22:29 +0100
  Re: Nested iteration? Joshua Landau <joshua.landau.ws@gmail.com> - 2013-04-23 22:41 +0100
  Re: Nested iteration? Oscar Benjamin <oscar.j.benjamin@gmail.com> - 2013-04-23 23:42 +0100

csiph-web