Groups | Search | Server Info | Keyboard shortcuts | Login | Register [http] [https] [nntp] [nntps]

Groups > comp.lang.python > #103851

Re: Explaining names vs variables in Python

Show all headers | View raw

Salvatore DI DIO writes:

[- -]

> But where is the consistency ? if I try :
>
>>>> v = 890
>>>> w = 890
>>>> v is w
> False

I think it goes as follows.

Python keeps a cached pool of some numbers that may occur relatively
often. When a numerical expression evaluates to a cached value, it
returns the cached object instead.

>>> 1 + 1 is 2
True
>>> 800 + 90 + 0 is 890
False

This way programs don't fill the memory with a large number of copies of
frequently occurring numbers like 2, and also don't keep rare numbers
like 890 around in the cache unnecessarily.

Python doesn't keep track of how often numbers actually occur. I think
it considers 0, 1, 2, ..., n, up to some rather small n, heuristically
frequent, and that's it. Also, this is an implementation detail.

Hm, -5, -4, -3, -2, -1 also seem cached when I try them, but -6 not.

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

>>> 890 is 891
False
>>> id(890) == id(891)
False

Python 3.4.3 (default, Oct 14 2015, 20:33:09) 
[GCC 4.8.4] on linux

Back to comp.lang.python | Previous | Next — Previous in thread | Next in thread | Find similar | Unroll thread

Thread

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

csiph-web