Знам, че is се използва за сравнение, ако два обекта са еднакви, но == е за равенство. От моя опит is винаги работеше за числа, защото Python използва повторно числа. например:
>>>a = 3
>>>a is 3
True
И съм свикнал да използвам is винаги, когато сравнявам нещо с число. Но is не работи за тази програма по-долу:
from collections import namedtuple
# Code taken directly from [Udacity site][1].
# make a basic Link class
Link = namedtuple('Link', ['id', 'submitter_id', 'submitted_time', 'votes',
'title', 'url'])
# list of Links to work with
links = [
Link(0, 60398, 1334014208.0, 109,
"C overtakes Java as the No. 1 programming language in the TIOBE index.",
"http://pixelstech.net/article/index.php?id=1333969280"),
Link(1, 60254, 1333962645.0, 891,
"This explains why technical books are all ridiculously thick and overpriced",
"http://prog21.dadgum.com/65.html"),
Link(23, 62945, 1333894106.0, 351,
"Learn Haskell Fast and Hard",
"http://yannesposito.com/Scratch/en/blog/Haskell-the-Hard-Way/"),
Link(2, 6084, 1333996166.0, 81,
"Announcing Yesod 1.0- a robust, developer friendly, high performance web framework for Haskell",
"http://www.yesodweb.com/blog/2012/04/announcing-yesod-1-0"),
Link(3, 30305, 1333968061.0, 270,
"TIL about the Lisp Curse",
"http://www.winestockwebdesign.com/Essays/Lisp_Curse.html"),
Link(4, 59008, 1334016506.0, 19,
"The Downfall of Imperative Programming. Functional Programming and the Multicore Revolution",
"http://fpcomplete.com/the-downfall-of-imperative-programming/"),
Link(5, 8712, 1333993676.0, 26,
"Open Source - Twitter Stock Market Game - ",
"http://www.twitstreet.com/"),
Link(6, 48626, 1333975127.0, 63,
"First look: Qt 5 makes JavaScript a first-class citizen for app development",
"http://arstechnica.com/business/news/2012/04/an-in-depth-look-at-qt-5-making-javascript-a-first-class-citizen-for-native-cross-platform-developme.ars"),
Link(7, 30172, 1334017294.0, 5,
"Benchmark of Dictionary Structures", "http://lh3lh3.users.sourceforge.net/udb.shtml"),
Link(8, 678, 1334014446.0, 7,
"If It's Not on Prod, It Doesn't Count: The Value of Frequent Releases",
"http://bits.shutterstock.com/?p=165"),
Link(9, 29168, 1334006443.0, 18,
"Language proposal: dave",
"http://davelang.github.com/"),
Link(17, 48626, 1334020271.0, 1,
"LispNYC and EmacsNYC meetup Tuesday Night: Large Scale Development with Elisp ",
"http://www.meetup.com/LispNYC/events/47373722/"),
Link(101, 62443, 1334018620.0, 4,
"research!rsc: Zip Files All The Way Down",
"http://research.swtch.com/zip"),
Link(12, 10262, 1334018169.0, 5,
"The Tyranny of the Diff",
"http://michaelfeathers.typepad.com/michael_feathers_blog/2012/04/the-tyranny-of-the-diff.html"),
Link(13, 20831, 1333996529.0, 14,
"Understanding NIO.2 File Channels in Java 7",
"http://java.dzone.com/articles/understanding-nio2-file"),
Link(15, 62443, 1333900877.0, 1244,
"Why vector icons don't work",
"http://www.pushing-pixels.org/2011/11/04/about-those-vector-icons.html"),
Link(14, 30650, 1334013659.0, 3,
"Python - Getting Data Into Graphite - Code Examples",
"http://coreygoldberg.blogspot.com/2012/04/python-getting-data-into-graphite-code.html"),
Link(16, 15330, 1333985877.0, 9,
"Mozilla: The Web as the Platform and The Kilimanjaro Event",
"https://groups.google.com/forum/?fromgroups#!topic/mozilla.dev.planning/Y9v46wFeejA"),
Link(18, 62443, 1333939389.0, 104,
"github is making me feel stupid(er)",
"http://www.serpentine.com/blog/2012/04/08/github-is-making-me-feel-stupider/"),
Link(19, 6937, 1333949857.0, 39,
"BitC Retrospective: The Issues with Type Classes",
"http://www.bitc-lang.org/pipermail/bitc-dev/2012-April/003315.html"),
Link(20, 51067, 1333974585.0, 14,
"Object Oriented C: Class-like Structures",
"http://cecilsunkure.blogspot.com/2012/04/object-oriented-c-class-like-structures.html"),
Link(10, 23944, 1333943632.0, 188,
"The LOVE game framework version 0.8.0 has been released - with GLSL shader support!",
"https://love2d.org/forums/viewtopic.php?f=3&t=8750"),
Link(22, 39191, 1334005674.0, 11,
"An open letter to language designers: Please kill your sacred cows. (megarant)",
"http://joshondesign.com/2012/03/09/open-letter-language-designers"),
Link(21, 3777, 1333996565.0, 2,
"Developers guide to Garage48 hackatron",
"http://martingryner.com/developers-guide-to-garage48-hackatron/"),
Link(24, 48626, 1333934004.0, 17,
"An R programmer looks at Julia",
"http://www.r-bloggers.com/an-r-programmer-looks-at-julia/")]
# links is a list of Link objects. Links have a handful of properties. For
# example, a Link's number of votes can be accessed by link.votes if "link" is a
# Link.
# make the function query() return a list of Links submitted by user 62443, by
# submission time ascending
def query():
print "hello"
print [link for link in links if link.submitter_id == 62443] # is does not work
return sorted([link for link in links if link.submitter_id == 62443],key = lambda x: x[2])
query()
Когато използвах is във функцията за заявка като тази [link for link in links if link.submitter_id is 62443], ще получа празен списък. Но ако използвам ==, работи добре.
В по-голямата си част кодът беше взет директно от сайта udacity, но аз също го изпробвах на моята локална машина. Същият резултат. Така че мисля, че числата сега са различни обекти в този случай, но защо? Има ли нужда от това?
РЕДАКТИРАНЕ: Да. Признавам, че този въпрос е дублиран и трябва да бъде затворен. Но е дубликат с първата публикация не вторият. Не знаех този въпрос, преди да публикувам това.
Проблемът ми беше, че мислех, че числовите обекти винаги ще се използват повторно.
Благодаря на всички, отървах се от лошия навик.
==според намерението на Гуидо ;-) - person Tim Peters schedule 19.12.2013FalseвместоTrue. - person Matthias schedule 19.12.2013