Le Tuesday 16 September 2008 16:57:26 Grant Edwards, vous avez écrit : > On 2008-09-16, Maric Michaud <[EMAIL PROTECTED]> wrote: > > Le Tuesday 16 September 2008 15:57:53 Grant Edwards, vous avez écrit : > >> On 2008-09-16, Maric Michaud <[EMAIL PROTECTED]> wrote: > >> > all expressions that return something, return a new object, > >> > >> That's not _quite_ true: > >> >>> a=1 > >> >>> b=a.__add__(0) > >> >>> a is b > >> > >> True > >> > >> ;) > > > > This is implementation specific, > > Indeed. > > > the specification of the language says that it should be > > false, > > That's curious. If so, then the implimentation is in violating > the specification.
But it doesn't matter as the "is" operator is not relevant for any use case with scalars and even strings and the optimization is not negligible. In fact, I'd rather say it's unspecified, each implementation does it its own way. [EMAIL PROTECTED] 17:48:03:~$ ipython >>>[1]: int() is 0 # contructor honour cache ...[1]: True >>>[2]: 256 is 255+1 # still caching ...[1]: True >>>[3]: 257 is 255+2 # here it stops ...[3]: False [EMAIL PROTECTED] 17:48:08:~$ jython Jython 2.2.1 on java1.6.0_07 Type "copyright", "credits" or "license" for more information. >>> int() is 0 # not caching when using constructor 0 >>> 2 is 1+1 # caching on expression 1 >>> 500 is 250*2 # caching occurs beyond my CPython 1 >>> 5000 is 5000 # compile-time optimization 1 >>> 5000 is 2500*2 # not caching too big numbers 0 > Where is that in the specification? Each literal creates a new instance, and instances of int are not supposed to be unique for each value (at less it's not specified so), so "1 is not 1", but implementation can cache integers, short strings, to avoid creating an new instance each time they appear in code. jython do this also for integer (see above), but not for strings : [EMAIL PROTECTED] 17:33:52:~$ jython Jython 2.2.1 on java1.6.0_07 Type "copyright", "credits" or "license" for more information. >>> a = "aaaa" >>> a is "aaaa" 0 [EMAIL PROTECTED] 17:36:48:~$ ipython >>>[1]: a = "aaaa" >>>[2]: a is "aaaa" ...[2]: True BTW, I'm talking of Jython 2.2, Jython 2.4, which quite ready I think, can change his mind as well. > I > suspect the statement in the specification should only apply to > mutable objects. > No, CPython also have a unique empty tuple, but tuple literals are not cached nor there are compile-time optimization : >>>[5]: () is () ...[5]: True >>>[6]: (1,) is (1,) ...[6]: False > > and it is for higher numbers : > >>>>[15]: a=1000 > >>>> > >>>>[16]: b=a.__add__(0) > >>>> > >>>>[17]: a is b > > > > ...[17]: False > > > > Don't disturb our OP, with side questions, please, it was > > enough hard like this ;) > > I'm not trying to confuse the OP, but to quote Einstein: > "Everything should be made as simple as possible, but not > simpler." -- _____________ Maric Michaud -- http://mail.python.org/mailman/listinfo/python-list
