Path: csiph.com!fu-berlin.de!uni-berlin.de!not-for-mail From: Kev Dwyer Newsgroups: comp.lang.python Subject: Re: Why do these statements evaluate the way they do? Date: Sat, 07 May 2016 08:04:04 +0100 Lines: 49 Message-ID: References: <9D4F2568-405C-419B-9B18-7376B34143CD@cajuntechie.org> Reply-To: kevin.p.dwyer@gmail.com Mime-Version: 1.0 Content-Type: text/plain; charset="ISO-8859-1" Content-Transfer-Encoding: 7Bit X-Trace: news.uni-berlin.de xVdFyv1xEXopuxAfYetykgwzlaj4mL2ia8d0oKgjwDcQ== 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; 'operator': 0.03; 'cpython': 0.05; 'revision': 0.05; 'false.': 0.07; '*is*': 0.09; 'received:80.91': 0.09; 'received:80.91.229': 0.09; 'received:gmane.org': 0.09; 'received:list': 0.09; 'statements': 0.09; 'subject:Why': 0.09; 'python': 0.10; 'caches': 0.16; 'equal.': 0.16; 'range,': 0.16; 'reason.': 0.16; 'received:80.91.229.3': 0.16; 'received:io': 0.16; 'received:plane.gmane.org': 0.16; 'received:psf.io': 0.16; 'wrote:': 0.16; 'tests': 0.18; '>>>': 0.20; 'fairly': 0.22; 'object.': 0.22; 'trying': 0.22; 'dec': 0.23; 'header:User- Agent:1': 0.26; 'example': 0.26; 'header:X-Complaints-To:1': 0.26; 'linux': 0.26; 'figure': 0.27; 'object,': 0.27; 'objects': 0.29; "i'm": 0.30; 'skip:[ 10': 0.31; 'int': 0.33; 'true.': 0.33; 'false': 0.35; 'identity': 0.35; 'but': 0.36; 'created': 0.36; 'to:addr:python-list': 0.36; 'subject:?': 0.36; 'subject:: ': 0.37; 'say': 0.37; 'received:org': 0.37; 'someone': 0.38; 'why': 0.39; 'subject:the': 0.39; 'to:addr:python.org': 0.40; 'some': 0.40; 'side': 0.62; 'more': 0.63; 'header:Reply-To:1': 0.67; 'reply-to:no real name:2**0': 0.71; 'evaluate': 0.72; 'reply- to:addr:gmail.com': 0.76; 'hoping': 0.77; '3.5.1': 0.84 X-Injected-Via-Gmane: http://gmane.org/ X-Gmane-NNTP-Posting-Host: cpc93820-hari18-2-0-cust1319.20-2.cable.virginm.net Mail-Copies-To: kevin.p.dwyer@gmail.com User-Agent: KNode/4.14.10 X-BeenThere: python-list@python.org X-Mailman-Version: 2.1.22 Precedence: list List-Id: General discussion list for the Python programming language List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-Mailman-Original-Message-ID: X-Mailman-Original-References: <9D4F2568-405C-419B-9B18-7376B34143CD@cajuntechie.org> Xref: csiph.com comp.lang.python:108257 Anthony Papillion wrote: > I'm trying to figure out why the following statements evaluate the way > they do and I'm not grasping it for some reason. I'm hoping someone can > help me. > > 40+2 is 42 #evaluates to True > But > 2**32 is 2**32 #evaluates to False > > This is an example taken from a Microsoft blog on the topic. They say the > reason is because the return is based on identity and not value but, to > me, these statements are fairly equal. > > Can someone clue me in? > > Anthony The *is* operator tests for identity, that is whether the objects on either side of the operator are the same object. CPython caches ints in the range -5 to 256 as an optimisation, so ints in this range are always the same object, and so the is operator returns True. Outside this range, a new int is created as required, and comparisons using is return False. This can be seen by looking at the id of the ints: Python 3.5.1 (default, Dec 29 2015, 10:53:52) [GCC 4.8.3 20140627 [gcc-4_8-branch revision 212064]] on linux Type "help", "copyright", "credits" or "license" for more information. >>> a = 42 >>> b = 42 >>> a is b True >>> id(a) 9186720 >>> id(b) 9186720 >>> c = 2 ** 32 >>> d = 2 ** 32 >>> c is d False >>> id(c) 140483107705136 >>> id(d) 140483107705168