Puzzled by "is"

[Dick Moores]

At 06:13 PM 8/9/2007, Ben Finney wrote:
>Content-Transfer-Encoding: base64Grzegorz 
>SÅ‚odkowicz <jergosh at wp.plÜš]\΂‚ˆ]  Üȝ@st 
>theorisation but I'd rather expect the interpreter
> > simply not to create a second tuple while there already is an
> > identical one.
>
>Others have already said that it's an implementation optimisation,
>which seems to partly answer your question.
>
>It's important to also realise that the language is *deliberately*
>non-committal on whether any given value will have this behaviour;
>that is, it's entirely left to the language implementation which
>optimisation trade-offs to make, and the language user (that's you and
>I) should *not* expect any particular behaviour to hold between
>different implementations.

I'm not clear on the meaning of "implementations" 
here.  Would 2.5 for Windows, Mac, Linux all be 
different implementations? Would Iron Python be another? ActivePython?

Thanks,

Dick