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Groups > comp.lang.python > #103879
| From | Jussi Piitulainen <jussi.piitulainen@helsinki.fi> |
|---|---|
| Newsgroups | comp.lang.python |
| Subject | Re: Explaining names vs variables in Python |
| Date | 2016-03-02 17:37 +0200 |
| Organization | A noiseless patient Spider |
| Message-ID | <lf57fhk3n6y.fsf@ling.helsinki.fi> (permalink) |
| References | <a894d5ed-d906-4ff7-a537-32bf0187e062@googlegroups.com> <lf5d1rdrzmz.fsf@ling.helsinki.fi> <mailman.105.1456931638.20602.python-list@python.org> |
Ian Kelly writes:
> On Wed, Mar 2, 2016 at 2:35 AM, Jussi Piitulainen wrote:
>> The following are too delicate for me. I suppose the answers could have
>> been different, but I can't guess what mechanism actually leads to these
>> results. Just idle curiosity on my part.
>>
>>>>> 890 is 890
>> True
>>>>> id(890) == id(890)
>> True
>
> This has to do with the way code blocks are compiled. In the
> interactive interpreter, a single line like '890 is 890' is compiled
> to a single code object. The constant 890 appears twice in the same
> code block, so the optimizer uses the same constant for both. Note in
> the following that the same index appears for both, so they're
> actually the same object reference.
No wonder my guesses failed. This is different.
>>>> import dis
>>>> dis.dis('890 is 890')
> 1 0 LOAD_CONST 0 (890)
> 3 LOAD_CONST 0 (890)
> 6 COMPARE_OP 8 (is)
> 9 RETURN_VALUE
>>>> compile('890 is 890', '', 'exec').co_consts
> (890, None)
>
>
> As for the earlier example of:
>
>>>>> 1 + 1 is 2
>> True
>>>>> 800 + 90 + 0 is 890
>> False
>
> This one actually surprises me a little, because the optimizer is also
> smart enough to evaluate '800 + 90 + 0' and just store a constant of
> 890:
>
>>>> dis.dis('800 + 90 + 0 is 890')
> 1 0 LOAD_CONST 5 (890)
> 3 LOAD_CONST 3 (890)
> 6 COMPARE_OP 8 (is)
> 9 RETURN_VALUE
>>>> compile('800 + 90 + 0 is 890', '', 'exec').co_consts
> (800, 90, 0, 890, None, 890, 890)
>
> Not smart enough to reuse the existing reference in this case
> apparently, or even to prune out the original constants that are no
> longer used in the code.
So it has access to three 890-objects and compares two of them :)
Thanks.
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Explaining names vs variables in Python Salvatore DI DIO <salvatore.didio@gmail.com> - 2016-03-02 00:32 -0800
Re: Explaining names vs variables in Python Jesper K Brogaard <jesper@brogAAaard.eu> - 2016-03-02 10:03 +0100
Re: Explaining names vs variables in Python Steven D'Aprano <steve@pearwood.info> - 2016-03-02 21:32 +1100
Re: Explaining names vs variables in Python Marko Rauhamaa <marko@pacujo.net> - 2016-03-02 14:34 +0200
Re: Explaining names vs variables in Python Chris Angelico <rosuav@gmail.com> - 2016-03-02 23:50 +1100
Re: Explaining names vs variables in Python Jussi Piitulainen <jussi.piitulainen@helsinki.fi> - 2016-03-02 15:11 +0200
Re: Explaining names vs variables in Python Marko Rauhamaa <marko@pacujo.net> - 2016-03-02 15:39 +0200
Re: Explaining names vs variables in Python Chris Angelico <rosuav@gmail.com> - 2016-03-03 00:48 +1100
Re: Explaining names vs variables in Python Marko Rauhamaa <marko@pacujo.net> - 2016-03-02 16:11 +0200
Re: Explaining names vs variables in Python Rustom Mody <rustompmody@gmail.com> - 2016-03-02 07:08 -0800
Re: Explaining names vs variables in Python Steven D'Aprano <steve@pearwood.info> - 2016-03-03 04:23 +1100
Re: Explaining names vs variables in Python Rustom Mody <rustompmody@gmail.com> - 2016-03-02 09:28 -0800
Re: Explaining names vs variables in Python Marko Rauhamaa <marko@pacujo.net> - 2016-03-02 20:12 +0200
Re: Explaining names vs variables in Python Steven D'Aprano <steve@pearwood.info> - 2016-03-03 12:52 +1100
Re: Explaining names vs variables in Python Rustom Mody <rustompmody@gmail.com> - 2016-03-03 09:03 -0800
Re: Explaining names vs variables in Python Ian Kelly <ian.g.kelly@gmail.com> - 2016-03-03 12:53 -0700
Re: Explaining names vs variables in Python Mark Lawrence <breamoreboy@yahoo.co.uk> - 2016-03-02 21:49 +0000
Re: Explaining names vs variables in Python Steven D'Aprano <steve@pearwood.info> - 2016-03-03 13:05 +1100
Re: Explaining names vs variables in Python Mark Lawrence <breamoreboy@yahoo.co.uk> - 2016-03-03 16:09 +0000
Re: Explaining names vs variables in Python Chris Angelico <rosuav@gmail.com> - 2016-03-03 08:52 +1100
Re: Explaining names vs variables in Python Rustom Mody <rustompmody@gmail.com> - 2016-03-02 17:23 -0800
Re: Explaining names vs variables in Python Mark Lawrence <breamoreboy@yahoo.co.uk> - 2016-03-02 22:51 +0000
Re: Explaining names vs variables in Python Steven D'Aprano <steve@pearwood.info> - 2016-03-03 04:10 +1100
Re: Explaining names vs variables in Python Antoon Pardon <antoon.pardon@rece.vub.ac.be> - 2016-03-02 10:08 +0100
Effects of caching frequently used objects, was Re: Explaining names vs variables in Python Peter Otten <__peter__@web.de> - 2016-03-02 10:12 +0100
Re: Explaining names vs variables in Python Jussi Piitulainen <jussi.piitulainen@helsinki.fi> - 2016-03-02 11:35 +0200
Re: Explaining names vs variables in Python Ian Kelly <ian.g.kelly@gmail.com> - 2016-03-02 08:13 -0700
Re: Explaining names vs variables in Python Jussi Piitulainen <jussi.piitulainen@helsinki.fi> - 2016-03-02 17:37 +0200
Re: Explaining names vs variables in Python Steven D'Aprano <steve@pearwood.info> - 2016-03-02 21:16 +1100
Re: Explaining names vs variables in Python "ast" <nomail@invalid.com> - 2016-03-02 11:52 +0100
Re: Explaining names vs variables in Python Salvatore DI DIO <salvatore.didio@gmail.com> - 2016-03-02 02:58 -0800
Re: Effects of caching frequently used objects, was Re: Explaining names vs variables in Python Dennis Lee Bieber <wlfraed@ix.netcom.com> - 2016-03-02 09:16 -0500
Re: Explaining names vs variables in Python Ben Finney <ben+python@benfinney.id.au> - 2016-03-03 04:53 +1100
RE: Effects of caching frequently used objects, was Re: Explaining names vs variables in Python Albert-Jan Roskam <sjeik_appie@hotmail.com> - 2016-03-25 13:03 +0000
Re: Effects of caching frequently used objects, was Re: Explaining names vs variables in Python Chris Angelico <rosuav@gmail.com> - 2016-03-26 00:22 +1100
Re: Effects of caching frequently used objects, was Re: Explaining names vs variables in Python Ethan Furman <ethan@stoneleaf.us> - 2016-03-25 09:45 -0700
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