Path: csiph.com!usenet.pasdenom.info!weretis.net!feeder1.news.weretis.net!feeder.erje.net!eu.feeder.erje.net!newsfeed.xs4all.nl!newsfeed2a.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 0.000 X-Spam-Evidence: '*H*': 1.00; '*S*': 0.00; 'third-party': 0.04; 'column': 0.07; 'nested': 0.07; 'subject:two': 0.07; 'expression:': 0.09; 'imported': 0.09; 'lines.': 0.09; 'matched': 0.09; 'cc:addr:python-list': 0.11; 'python': 0.11; 'wrote': 0.14; 'mostly': 0.14; '"for"': 0.16; '(there': 0.16; '1400': 0.16; '23,': 0.16; 'correlation': 0.16; 'crude': 0.16; 'doing,': 0.16; 'expressions)': 0.16; 'grep': 0.16; 'ivan': 0.16; 'merely': 0.16; 'naming': 0.16; 'nesting': 0.16; 'otoh,': 0.16; 'pity': 0.16; 'received:166.84': 0.16; 'received:166.84.1': 0.16; 'received:166.84.1.89': 0.16; 'received:mailbackend.panix.com': 0.16; 'received:panix.com': 0.16; 'roy': 0.16; 'sure.': 0.16; ':-)': 0.16; 'wrote:': 0.18; 'trying': 0.19; 'received:10.0.1': 0.19; 'received:166': 0.19; 'work,': 0.20; 'cc:addr:python.org': 0.22; 'rapidly': 0.24; 'skip': 0.24; 'stick': 0.24; 'cc:2**0': 0.24; 'source': 0.25; 'header:In-Reply-To:1': 0.27; 'leave': 0.29; 'am,': 0.29; 'generally': 0.29; "i'm": 0.30; 'code': 0.31; 'regular': 0.32; 'subject:with': 0.35; "can't": 0.35; "who's": 0.35; 'but': 0.35; 'next': 0.36; 'shows': 0.36; 'charset:us- ascii': 0.36; 'received:10.0': 0.36; 'experience,': 0.37; 'two': 0.37; 'list': 0.37; 'received:10': 0.37; 'nov': 0.38; 'subject:needed': 0.38; 'needed': 0.38; 'enough': 0.39; 'read': 0.60; 'full': 0.61; 'subject:skip:C 10': 0.61; 'simply': 0.61; 'header:Message-Id:1': 0.63; 'name': 0.63; 'such': 0.63; 'our': 0.64; 'more': 0.64; 'to:addr:gmail.com': 0.65; 'between': 0.67; 'smith': 0.68; 'glad': 0.83; '11:45': 0.84; '2014,': 0.84; 'email addr:panix.com': 0.84 Subject: Re: Comprehension with two variables - explanation needed Mime-Version: 1.0 (Apple Message framework v1283) Content-Type: text/plain; charset=us-ascii From: Roy Smith In-Reply-To: Date: Sun, 23 Nov 2014 12:08:19 -0500 Content-Transfer-Encoding: quoted-printable References: <002501d00733$6995dce0$3cc196a0$@gmail.com> To: Skip Montanaro X-Mailer: Apple Mail (2.1283) Cc: Python 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: 58 NNTP-Posting-Host: 2001:888:2000:d::a6 X-Trace: 1416762503 news.xs4all.nl 2881 [2001:888:2000:d::a6]:50816 X-Complaints-To: abuse@xs4all.nl Xref: csiph.com comp.lang.python:81392 We seem to be somewhat more liberal about nested comprehensions here, = but I can't say I'm proud of that :-) 906 Python source files, 109k lines.=20 $ find . -name '*.py' | xargs grep '\[.* for .*\]' | wc -l 729 $ find . -name '*.py' | xargs grep '\[.* for .* for .*\]' | wc -l 46 Without naming names, a quick look into our nested comprehensions with = "hg blame" shows a very strong correlation between "people who wrote = many nested comprehensions" and "people who's code I find difficult to = read in general". I see one of these that has my name on the commit: report =3D [keys] + [[line[key] for key in keys] for line in results] but I'm glad to report that this was merely some third-party code that I = imported into our repo. Whew, that was close! On Nov 23, 2014, at 11:45 AM, Skip Montanaro wrote: > On Sun, Nov 23, 2014 at 9:57 AM, Roy Smith wrote: >>=20 >> If it was complicated enough that you needed to loopify it to >> understand what it's doing, have pity on the next person who has to >> maintain your code and leave it as a loop >=20 > Well, sure. I was mostly trying to give Ivan a path out of the weeds. > Poking through the code I'm involved with at work, using a crude > measuring stick (a couple regular expressions) I found over 1400 one > line list comprehensions, those which matched this regular expression: >=20 > \[.* for .*\] >=20 > OTOH, I only found 14 with two "for" keywords: >=20 > \[.* for .* for .*\] >=20 > (There may well have been more nested loop list comprehensions, simply > because it's hard to cram the full listcomp into 80 columns, my > self-imposed column limit.) >=20 > In my experience, list comprehensions rapidly get unwieldy once you > get to nesting them. I generally unravel such items into a nested set > of for loops. >=20 > Skip >=20 -- Roy Smith roy@panix.com