On Thu, Aug 5, 2010 at 16:15, Rhodri James <rho...@wildebst.demon.co.uk> wrote: > On Thu, 05 Aug 2010 17:07:53 +0100, wheres pythonmonks > <wherespythonmo...@gmail.com> wrote:
> You're not testing for equivalence there, you're testing for identity. "is" > and "is not" test whether the two objects concerned are (or are not) the > same object. Two objects can have the same value, but be different objects. > The interpreter can fool you by caching and reusing objects which have the > same value when it happens to know about it, in particular for small > integers, but this is just a happy accident of the implementation and in no > way guaranteed by the language. For example: > >>>> "spam, eggs, chips and spam" is "spam, eggs, chips and spam" > > True >>>> >>>> a = "spam, eggs, chips and spam" >>>> b = "spam, eggs, chips and spam" >>>> a is b > > False >>>> >>>> a == b > > True > I'm wondering if there isn't considerable predictability to that "happy accident". Note how 1 'word' is treated versus 2: >>> x = >>> 'alksjdhflkajshdflkajhdflkjahsdflkjshadflkjhsadlfkjhaslkdjfhslkadhflkjshdflkjshdflkjshdfk' >>> y = >>> 'alksjdhflkajshdflkajhdflkjahsdflkjshadflkjhsadlfkjhaslkdjfhslkadhflkjshdflkjshdflkjshdfk' >>> x is y True >>> x = 'alksjdhflkajshdflkajhdflkjahsdflkj >>> hadflkjhsadlfkjhaslkdjfhslkadhflkjshdflkjshdflkjshdfk' >>> y = 'alksjdhflkajshdflkajhdflkjahsdflkj >>> hadflkjhsadlfkjhaslkdjfhslkadhflkjshdflkjshdflkjshdfk' >>> x is y False >>> (Python 3.1 on Vista.) Dick Moores -- http://mail.python.org/mailman/listinfo/python-list
