On Wednesday, 5 September 2012 14:44:23 UTC+5:30, Steven D'Aprano wrote: > On Wed, 05 Sep 2012 08:30:31 +0200, Franck Ditter wrote: > > > > > Hi ! > > > a is b <==> id(a) == id(b) in builtin classes. Is that true ? > > > > Not just for builtin classes, for any objects, provided that they are >
Seeing this thread, I think the is statment should be removed. It has a replacement syntax of id(x) == id(y) and "a==True" should be automatically changed into memory comparison. > alive at the same time. > > > > There is no guarantee whether IDs will be re-used. Some versions of > > Python do re-use IDs, e.g. CPython: > > > > steve@runes:~$ python > > Python 2.6.6 (r266:84292, Dec 27 2010, 00:02:40) > > [GCC 4.4.5] on linux2 > > Type "help", "copyright", "credits" or "license" for more information. > > >>> a = ["some", "object"] > > >>> id(a) > > 3074285228L > > >>> del a > > >>> b = [100, 200] > > >>> id(b) > > 3074285228L > > > > but others do not, e.g. Jython and IronPython: > > > > steve@runes:~$ jython > > Jython 2.5.1+ (Release_2_5_1, Aug 4 2010, 07:18:19) > > [OpenJDK Client VM (Sun Microsystems Inc.)] on java1.6.0_18 > > Type "help", "copyright", "credits" or "license" for more information. > > >>> a = ["some", "object"] > > >>> id(a) > > 1 > > >>> del a > > >>> b = [100, 200] > > >>> id(b) > > 2 > > > > > > steve@runes:~$ ipy > > IronPython 2.6 Beta 2 DEBUG (2.6.0.20) on .NET 2.0.50727.1433 > > Type "help", "copyright", "credits" or "license" for more information. > > >>> a = ["some", "object"] > > >>> id(a) > > 43 > > >>> del a > > >>> b = [100, 200] > > >>> id(b) > > 44 > > > > > > CPython especially has the most complicated behaviour with IDs and object > > identity: > > > > >>> a = 99.99 > > >>> b = 99.99 > > >>> a is b > > False > > >>> a = 99.99; b = 99.99; a is b > > True > > > > > > In general, you almost never need to care about IDs and object identity. > > The main exception is testing for None, which should always be written as: > > > > if x is None > > > > > > -- > > Steven -- http://mail.python.org/mailman/listinfo/python-list
