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Groups > comp.lang.python > #98452
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| From | Chris Angelico <rosuav@gmail.com> |
| Newsgroups | comp.lang.python |
| Subject | Re: Question about math.pi is mutable |
| Date | Mon, 9 Nov 2015 02:11:26 +1100 |
| Lines | 67 |
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On Mon, Nov 9, 2015 at 12:42 AM, BartC <bc@freeuk.com> wrote:
> Sorry, you'll have to assume I'm very stupid.
>
> What exactly is being looked up, and in what?
>
> From what I can understand in your example:
>
> * You are calling shutil.copyfileobj with two arguments, which happen to be
> instances of classes Source and Dest.
>
> * Let's say these are known as src and dst inside .copyfileobj.
>
> * .copyfileobj then calls methods src.read() dst.write().
>
> * Each of these method calls has the form A.B() which seems to me to be
> little different from shutil.copyfileobj().
>
> So to get back to what I was saying, does this lookup involving searching
> for method B in object A, and if so, does it actually do a search by name?
>
> (Someone mentioned a precalculated hash, of "A", or of "src" or "dst", use
> used, but still, it's looking something up in a table, and a hash table
> lookup I believe still requires an string compare to check if you've got the
> right entry.)
The lookups I'm talking about happen pretty much anywhere. Firstly,
this function:
def f():
shutil.copyfileobj(src, dst)
looks up the global name 'shutil' (that's one dict lookup - module
globals are a dictionary), then performs an attribute lookup on the
resulting object with the name 'copyfileobj', then looks up the two
global names 'src' and 'dst'.
In fact[1], those strings can be found in f.__code__.co_names, and the
byte-code identifies them by their indices in that tuple. These are
str objects (note that this means they can be any legal Unicode
string; "шутил.цопыфилеобй(срц, дст)" is just as valid), and their
hashes will normally be precomputed to save time. Now, things are a
bit different with function-locals; the compiler always knows which
names they are, and it can compile things differently. Compare:
def f(src):
shutil.copyfileobj(src, dst)
Instead of looking up a global name 'src', this now looks up a local
name. They're identified by slot positions, so the compiler simply
knows that "src" is in slot #0, and instead of looking anything up,
simply says "load the thing in slot #0". There can also be other
levels of indirection, such as __getattr__ and __getattribute__, which
affect how stuff gets looked up. But otherwise, name lookups generally
involve poking a dict with a string and taking what comes back
(possibly replacing KeyError with NameError or AttributeError as the
case may be).
ChrisA
[1] Assuming we're using CPython, or something which uses the same
byte-code; this is NOT a language requirement - none of this is.
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Question about math.pi is mutable "wangq@travelsky.com" <wangq@travelsky.com> - 2015-11-06 10:33 +0800
Re: Question about math.pi is mutable Bartc <bc@freeuk.com> - 2015-11-06 12:30 +0000
Re: Question about math.pi is mutable Lorenzo Sutton <lorenzofsutton@gmail.com> - 2015-11-06 17:12 +0100
Re: Question about math.pi is mutable Bartc <bc@freeuk.com> - 2015-11-06 19:30 +0000
Re: Question about math.pi is mutable Chris Angelico <rosuav@gmail.com> - 2015-11-07 06:40 +1100
Re: Question about math.pi is mutable Christian Gollwitzer <auriocus@gmx.de> - 2015-11-06 20:59 +0100
Re: Question about math.pi is mutable Thomas 'PointedEars' Lahn <PointedEars@web.de> - 2015-11-06 23:19 +0100
Re: Question about math.pi is mutable Thomas 'PointedEars' Lahn <PointedEars@web.de> - 2015-11-07 00:48 +0100
Re: Question about math.pi is mutable Steven D'Aprano <steve@pearwood.info> - 2015-11-07 13:00 +1100
Re: Question about math.pi is mutable Ben Finney <ben+python@benfinney.id.au> - 2015-11-07 14:43 +1100
Re: Question about math.pi is mutable Bartc <bc@freeuk.com> - 2015-11-07 11:23 +0000
Re: Question about math.pi is mutable Ben Finney <ben+python@benfinney.id.au> - 2015-11-07 22:35 +1100
Re: Question about math.pi is mutable Bartc <bc@freeuk.com> - 2015-11-07 11:57 +0000
Re: Question about math.pi is mutable Ben Finney <ben+python@benfinney.id.au> - 2015-11-07 23:15 +1100
Re: Question about math.pi is mutable Bartc <bc@freeuk.com> - 2015-11-07 13:00 +0000
Re: Question about math.pi is mutable Laura Creighton <lac@openend.se> - 2015-11-07 15:09 +0100
Re: Question about math.pi is mutable Marko Rauhamaa <marko@pacujo.net> - 2015-11-07 16:23 +0200
Re: Question about math.pi is mutable Marko Rauhamaa <marko@pacujo.net> - 2015-11-07 16:28 +0200
Re: Question about math.pi is mutable Bartc <bc@freeuk.com> - 2015-11-07 15:01 +0000
Re: Question about math.pi is mutable Marko Rauhamaa <marko@pacujo.net> - 2015-11-07 19:46 +0200
Re: Question about math.pi is mutable Steven D'Aprano <steve@pearwood.info> - 2015-11-08 14:07 +1100
Re: Question about math.pi is mutable Random832 <random832@fastmail.com> - 2015-11-08 01:29 -0500
Re: Question about math.pi is mutable Steven D'Aprano <steve@pearwood.info> - 2015-11-08 13:59 +1100
Re: Question about math.pi is mutable Bartc <bc@freeuk.com> - 2015-11-08 10:40 +0000
Re: Question about math.pi is mutable Marko Rauhamaa <marko@pacujo.net> - 2015-11-08 13:02 +0200
Re: Question about math.pi is mutable BartC <bc@freeuk.com> - 2015-11-08 11:19 +0000
Re: Question about math.pi is mutable Marko Rauhamaa <marko@pacujo.net> - 2015-11-08 13:50 +0200
Re: Question about math.pi is mutable BartC <bc@freeuk.com> - 2015-11-08 12:39 +0000
Re: Question about math.pi is mutable Marko Rauhamaa <marko@pacujo.net> - 2015-11-08 14:43 +0200
Re: Question about math.pi is mutable BartC <bc@freeuk.com> - 2015-11-08 13:42 +0000
Re: Question about math.pi is mutable Marko Rauhamaa <marko@pacujo.net> - 2015-11-08 16:33 +0200
Re: Question about math.pi is mutable Chris Angelico <rosuav@gmail.com> - 2015-11-09 02:11 +1100
Re: Question about math.pi is mutable Steven D'Aprano <steve@pearwood.info> - 2015-11-09 11:58 +1100
Re: Question about math.pi is mutable Chris Angelico <rosuav@gmail.com> - 2015-11-08 23:28 +1100
Re: Question about math.pi is mutable Steven D'Aprano <steve@pearwood.info> - 2015-11-09 11:35 +1100
Re: Question about math.pi is mutable Michael Torrie <torriem@gmail.com> - 2015-11-08 08:59 -0700
Re: Question about math.pi is mutable BartC <bc@freeuk.com> - 2015-11-08 17:54 +0000
Re: Question about math.pi is mutable Chris Angelico <rosuav@gmail.com> - 2015-11-09 05:01 +1100
Re: Question about math.pi is mutable BartC <bc@freeuk.com> - 2015-11-08 18:58 +0000
Re: Question about math.pi is mutable Ian Kelly <ian.g.kelly@gmail.com> - 2015-11-08 12:51 -0700
Re: Question about math.pi is mutable Dennis Lee Bieber <wlfraed@ix.netcom.com> - 2015-11-08 18:13 -0500
Re: Question about math.pi is mutable BartC <bc@freeuk.com> - 2015-11-08 23:54 +0000
Re: Question about math.pi is mutable Ben Finney <ben+python@benfinney.id.au> - 2015-11-09 11:00 +1100
Re: Question about math.pi is mutable BartC <bc@freeuk.com> - 2015-11-09 00:11 +0000
Re: Question about math.pi is mutable Dennis Lee Bieber <wlfraed@ix.netcom.com> - 2015-11-09 08:18 -0500
Re: Question about math.pi is mutable Chris Angelico <rosuav@gmail.com> - 2015-11-09 11:04 +1100
Re: Question about math.pi is mutable Ben Finney <ben+python@benfinney.id.au> - 2015-11-09 11:26 +1100
Re: Question about math.pi is mutable Gregory Ewing <greg.ewing@canterbury.ac.nz> - 2015-11-09 21:29 +1300
Re: Question about math.pi is mutable Chris Angelico <rosuav@gmail.com> - 2015-11-09 11:36 +1100
Re: Question about math.pi is mutable Ben Finney <ben+python@benfinney.id.au> - 2015-11-09 11:50 +1100
Re: Question about math.pi is mutable Chris Angelico <rosuav@gmail.com> - 2015-11-09 11:56 +1100
Re: Question about math.pi is mutable Ben Finney <ben+python@benfinney.id.au> - 2015-11-09 12:04 +1100
Re: Question about math.pi is mutable Steven D'Aprano <steve@pearwood.info> - 2015-11-09 22:22 +1100
Re: Question about math.pi is mutable Random832 <random832@fastmail.com> - 2015-11-09 10:32 -0500
Re: Question about math.pi is mutable Ben Finney <ben+python@benfinney.id.au> - 2015-11-10 06:45 +1100
Re: Question about math.pi is mutable Steven D'Aprano <steve@pearwood.info> - 2015-11-10 13:37 +1100
Re: Question about math.pi is mutable Ben Finney <ben+python@benfinney.id.au> - 2015-11-10 17:10 +1100
Re: Question about math.pi is mutable Steven D'Aprano <steve@pearwood.info> - 2015-11-10 22:34 +1100
Re: Question about math.pi is mutable BartC <bc@freeuk.com> - 2015-11-10 13:26 +0000
Re: Question about math.pi is mutable Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2015-11-11 18:38 +1100
Re: Question about math.pi is mutable Marko Rauhamaa <marko@pacujo.net> - 2015-11-11 10:30 +0200
Re: Question about math.pi is mutable Steven D'Aprano <steve@pearwood.info> - 2015-11-11 22:20 +1100
Re: Question about math.pi is mutable Terry Reedy <tjreedy@udel.edu> - 2015-11-10 03:30 -0500
Re: Question about math.pi is mutable Laura Creighton <lac@openend.se> - 2015-11-10 11:33 +0100
Re: Question about math.pi is mutable Ben Finney <ben+python@benfinney.id.au> - 2015-11-10 23:14 +1100
Re: Question about math.pi is mutable Steven D'Aprano <steve@pearwood.info> - 2015-11-11 12:10 +1100
Re: Question about math.pi is mutable Laura Creighton <lac@openend.se> - 2015-11-09 21:07 +0100
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Re: Question about math.pi is mutable BartC <bc@freeuk.com> - 2015-11-09 16:54 +0000
Re: Question about math.pi is mutable Ben Finney <ben+python@benfinney.id.au> - 2015-11-10 06:52 +1100
Re: Question about math.pi is mutable Ben Finney <ben+python@benfinney.id.au> - 2015-11-09 08:00 +1100
Re: Question about math.pi is mutable BartC <bc@freeuk.com> - 2015-11-08 22:35 +0000
Re: Question about math.pi is mutable Ben Finney <ben+python@benfinney.id.au> - 2015-11-09 09:54 +1100
Re: Question about math.pi is mutable Steven D'Aprano <steve@pearwood.info> - 2015-11-09 13:23 +1100
Re: Question about math.pi is mutable BartC <bc@freeuk.com> - 2015-11-09 12:22 +0000
Re: Question about math.pi is mutable Chris Angelico <rosuav@gmail.com> - 2015-11-09 23:36 +1100
Re: Question about math.pi is mutable Steven D'Aprano <steve@pearwood.info> - 2015-11-08 22:30 +1100
Re: Question about math.pi is mutable BartC <bc@freeuk.com> - 2015-11-08 12:24 +0000
Re: Question about math.pi is mutable Grant Edwards <invalid@invalid.invalid> - 2015-11-07 16:16 +0000
Re: Question about math.pi is mutable Marko Rauhamaa <marko@pacujo.net> - 2015-11-07 20:00 +0200
Re: Question about math.pi is mutable Grant Edwards <invalid@invalid.invalid> - 2015-11-08 03:31 +0000
Re: Question about math.pi is mutable Marko Rauhamaa <marko@pacujo.net> - 2015-11-08 09:45 +0200
Re: Question about math.pi is mutable Christian Gollwitzer <auriocus@gmx.de> - 2015-11-08 09:00 +0100
Re: Question about math.pi is mutable Paul Rubin <no.email@nospam.invalid> - 2015-11-08 00:08 -0800
Re: Question about math.pi is mutable Marko Rauhamaa <marko@pacujo.net> - 2015-11-08 11:49 +0200
Re: Question about math.pi is mutable Grant Edwards <invalid@invalid.invalid> - 2015-11-09 14:49 +0000
Re: Question about math.pi is mutable Bartc <bc@freeuk.com> - 2015-11-07 15:19 +0000
Re: Question about math.pi is mutable Steven D'Aprano <steve@pearwood.info> - 2015-11-08 14:50 +1100
Re: Question about math.pi is mutable Marko Rauhamaa <marko@pacujo.net> - 2015-11-08 11:44 +0200
Re: Question about math.pi is mutable Steven D'Aprano <steve@pearwood.info> - 2015-11-08 22:11 +1100
Re: Question about math.pi is mutable Marko Rauhamaa <marko@pacujo.net> - 2015-11-08 13:25 +0200
Re: Question about math.pi is mutable BartC <bc@freeuk.com> - 2015-11-08 11:06 +0000
Re: Question about math.pi is mutable Antoon Pardon <antoon.pardon@rece.vub.ac.be> - 2015-11-09 15:49 +0100
Re: Question about math.pi is mutable Ben Finney <ben+python@benfinney.id.au> - 2015-11-10 10:29 +1100
Re: Question about math.pi is mutable Antoon Pardon <antoon.pardon@rece.vub.ac.be> - 2015-11-10 11:03 +0100
Re: Question about math.pi is mutable Michael Torrie <torriem@gmail.com> - 2015-11-13 20:11 -0700
Re: Question about math.pi is mutable Antoon Pardon <antoon.pardon@rece.vub.ac.be> - 2015-11-14 16:02 +0100
Re: Question about math.pi is mutable Thomas 'PointedEars' Lahn <PointedEars@web.de> - 2015-11-06 23:26 +0100
Re: Question about math.pi is mutable Terry Reedy <tjreedy@udel.edu> - 2015-11-06 20:49 -0500
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Re: Question about math.pi is mutable Bartc <bc@freeuk.com> - 2015-11-07 23:02 +0000
Re: Question about math.pi is mutable Mark Lawrence <breamoreboy@yahoo.co.uk> - 2015-11-07 23:27 +0000
Re: Question about math.pi is mutable Ben Finney <ben+python@benfinney.id.au> - 2015-11-08 10:32 +1100
Re: Question about math.pi is mutable Chris Angelico <rosuav@gmail.com> - 2015-11-08 10:52 +1100
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Re: Question about math.pi is mutable Denis McMahon <denismfmcmahon@gmail.com> - 2015-11-13 09:19 +0000
Re: Question about math.pi is mutable Larry Hudson <orgnut@yahoo.com> - 2015-11-13 18:30 -0800
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Re: Question about math.pi is mutable Chris Angelico <rosuav@gmail.com> - 2015-11-13 09:52 +1100
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