James Stroud a écrit : > Hello all, > > What /is/ identity in python?
A unique identifier associated with each and every object in the process. What exactly is this identifier is left to the implementation - FWIW and IIRC, CPython uses the memory address of the C 'object' datastructure. > For example, we can always count on > > py> None is None > True Yes. None is a singleton. > But I have noticed that this works for strings: > > py> "none" is "none" > True > > and, for example, integers: > > py> 42 is 42 > True This is an implementation detail of CPython - which does some caching, and is by no way specified by the language. You should *never* rely on this. FWIW, try: a = 3900000 b = 3900000 a is b > And I have noticed that this works for equivalent expressions: > > py> 42 is (40 + 2) > True > > So, I'm guessing that the integer 42, or the string "none" is cached > somewhere Bingo !-) > and python looks to see if it is in the cache when evaluating > 'is'. I don't think the evaluation of 'is' as anything to do with this. > My guess is supported with this test: > > py> id(42) > 149679044 > py> id(40+2) > 149679044 > py> id(7*6) > 149679044 Now what about this: >>> id(600) 134745616 >>> id(601) 134745616 id of an object is unique *for the lifetime of this object*. Nothing prevents it from being reused later. > So, I guess my question is to what extent is this equivalency > implementation dependent? cf above > Is this equivalency a requirement for a > legitimate python implementation? cf above > Won't these checks slow down > evaluation of 'is' considerably? I really doubt that comparing memory addresses could be slow... > Does '==' ever fall back and use 'is' > (or 'id') for evaluation? Seems so : >>> class Foo(object): pass ... >>> f1 = Foo() >>> f2 = Foo() >>> f3 = f1 >>> f1 == f2 False >>> f1 == f3 True -- http://mail.python.org/mailman/listinfo/python-list
