is implemented with id ?

[Ramchandra Apte]

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 at 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 at 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 at 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