Strange behaviour of 'is'

[Duncan Booth]

Steve Holden <steve at holdenweb.com> wrote:

> Absolutely correct. It would be more interesting to discuss how the 
> output from these statements varied between (say) CPython, Jython and 
> Iron Python. At the moment the discussion is indeed about insignificant 
> implementation trivia.

CPython seems to collapse identical float values if they are in the same 
compilation unit:

>>> x = 2.
>>> y = 2.
>>> x is y
False
>>> x = 2.; y = 2.
>>> x is y
True
>>> y = [2., 2.]
>>> y[0] is y[1]
True

IronPython doesn't collapse them even when they are in expression:

IronPython 1.0.60816 on .NET 2.0.50727.42
Copyright (c) Microsoft Corporation. All rights reserved.
>>> x = 2.
>>> y = 2.
>>> x is y
False
>>> x = 2.; y = 2.
>>> x is y
False
>>> y = [2., 2.]
>>> y[0] is y[1]
False

JPython seems to behave in a similar manner to CPython:

Python command console - JPython 2.1

>>> x = 2.
>>> y = 2.
>>> x is y
0

>>> x = 2.; y = 2.
>>> x is y
1

>>> y = [2., 2.]
>>> y[0] is y[1]
1

>>> 

Sorry, I don't have a more recent Jython implementation to hand to complete 
the comparison.