Re: id == vs is
In article , Cameron Simpson wrote: > The "is" test is more direct and less subject to iffiness because the longer > expression using id() leaves more scope/time for things to change, and of > course "id" itself can be rebound to something weird. Not to mention that Python is case-sensitive and in the code as presented by the OP: Id(x) == id(y) those are two different functions :-) -- https://mail.python.org/mailman/listinfo/python-list
Re: id == vs is
Ben Finney writes: > Dan Stromberg writes: > > Are the following two expressions the same? […] > > It depends what you mean by “the same”. My apologies, I mis-read the question. My answers were for a different question (one you didn't ask). Please ignore that. -- \ “If you ever reach total enlightenment while you're drinking a | `\ beer, I bet it makes beer shoot out your nose.” —Jack Handey | _o__) | Ben Finney -- https://mail.python.org/mailman/listinfo/python-list
Re: id == vs is
On 27Oct2014 00:41, MRAB wrote: On 2014-10-27 00:24, Ethan Furman wrote: On 10/26/2014 05:23 PM, Ethan Furman wrote: On 10/26/2014 05:12 PM, Dan Stromberg wrote: Are the following two expressions the same? x is y Id(x) == id(y) ? Listen to MRAB, ignore me. That is all. Well, apart of Joshua's qualifications, that is! :-) Let's make it clear for the OP, since this thread seems to have wandered into the realms of the confusing, for what is a basic question. The short answer is "yes". The longer answer is: if nobody else is fiddling with "x" and "y" and if id() is the normal builtin python "id" function Then yes. The "is" test is more direct and less subject to iffiness because the longer expression using id() leaves more scope/time for things to change, and of course "id" itself can be rebound to something weird. Cheers, Cameron Simpson The significant problems we face cannot be solved at the same level of thinking we were at when we created them. - Albert Einstein -- https://mail.python.org/mailman/listinfo/python-list
Re: id == vs is
On Sun, 26 Oct 2014 17:12:29 -0700, Dan Stromberg wrote: > Are the following two expressions the same? > > x is y > > Id(x) == id(y) No, although if "Id" and "id" were the same function, they might be equivalent in some cases. -- Denis McMahon, denismfmcma...@gmail.com -- https://mail.python.org/mailman/listinfo/python-list
Re: id == vs is
Dan Stromberg writes: > Are the following two expressions the same? > > x is y > > Id(x) == id(y) It depends what you mean by “the same”. Do they give the same result? Sometimes yes, sometimes no. It depends on what the types of the values are. Do they express the same intent? Always no. The first queries object identity, the second queries object equality. -- \ “Ridicule is the only weapon which can be used against | `\ unintelligible propositions.” —Thomas Jefferson, 1816-07-30 | _o__) | Ben Finney -- https://mail.python.org/mailman/listinfo/python-list
Re: id == vs is
On 2014-10-27 00:24, Ethan Furman wrote: On 10/26/2014 05:23 PM, Ethan Furman wrote: On 10/26/2014 05:12 PM, Dan Stromberg wrote: Are the following two expressions the same? x is y Id(x) == id(y) ? Listen to MRAB, ignore me. That is all. Well, apart of Joshua's qualifications, that is! :-) -- https://mail.python.org/mailman/listinfo/python-list
Re: id == vs is
On 10/26/2014 05:12 PM, Dan Stromberg wrote: Are the following two expressions the same? x is y Id(x) == id(y) ? Nope. If the value if `id(x)` is not interned, then the two value could be different objects that still represent the same value. -- ~Ethan~ -- https://mail.python.org/mailman/listinfo/python-list
Re: id == vs is
On 10/26/2014 05:23 PM, Ethan Furman wrote: On 10/26/2014 05:12 PM, Dan Stromberg wrote: Are the following two expressions the same? x is y Id(x) == id(y) ? Listen to MRAB, ignore me. That is all. -- ~Ethan~ -- https://mail.python.org/mailman/listinfo/python-list
Re: id == vs is
On 27 October 2014 00:12, Dan Stromberg wrote:
> Are the following two expressions the same?
>
> x is y
>
> Id(x) == id(y)
Much of the time, but not all the time. The obvious exception is if
"id" is redefined, but that one's kind of boring. The real thing to
watch out for is if the object that "x" points to is garbage collected
before "y" is evaluated:
nothing = "!"
id("hello" + nothing) == id("hello" + nothing)
#>>> True
("hello" + nothing) is ("hello" + nothing)
#>>> False
Since in the first case the ("hello" + nothing) gets garbage
collected, CPython is allowed to re-use its id. If instead you assign
them outside of the expression:
nothing = "!"
x = "hello" + nothing
y = "hello" + nothing
id(x) == id(y)
#>>> False
the collection cannot happen.
Note that in this case CPython is allowed to deduplicate these strings
anyway (although in this case it does not), so using "is" here is not
safe.
--
https://mail.python.org/mailman/listinfo/python-list
Re: id == vs is
On 2014-10-27 00:12, Dan Stromberg wrote: Are the following two expressions the same? x is y Id(x) == id(y) ? Yes. I ported some Java code to Python, and it was using Java's idea of equality (via ==) in some places. Right now, I have a suite of unit tests working using the second expression above, but I'm thinking about switching to the first. Thanks! -- https://mail.python.org/mailman/listinfo/python-list
