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Groups > comp.lang.python > #66319
| From | Marko Rauhamaa <marko@pacujo.net> |
|---|---|
| Newsgroups | comp.lang.python |
| Subject | Re: Explanation of list reference |
| Date | 2014-02-14 21:56 +0200 |
| Organization | A noiseless patient Spider |
| Message-ID | <871tz5piy0.fsf@elektro.pacujo.net> (permalink) |
| References | <13208de8-0f85-4e60-b059-dc087c8fda41@googlegroups.com> <qoteh35ft4q.fsf@ruuvi.it.helsinki.fi> <917ede6d-db7c-4a8c-8203-27677283776b@googlegroups.com> |
dave em <daveandem2000@gmail.com>:
> Case 1: Example of variable with a specific value from P 170 of IYOCGWP
>
>>>> spam = 42
>>>> cheese = spam
>>>> spam = 100
>>>> spam
> 100
>>>> cheese
> 42
>
> Case 2: Example of variable with a list reference from p 170
>
>>>> spam = [0, 1, 2, 3, 4, 5]
>>>> cheese = spam
>>>> cheese[1] = 'Hello!'
>>>> spam
> [0, 'Hello!', 2, 3, 4, 5]
>>>> cheese
> [0, 'Hello!', 2, 3, 4, 5]
>
> What I am trying to explain is this, why in case 1 when acting on spam
> (changing the value from 42 to 100) only affects spam and not cheese.
> Meanwhile, in case two acting on cheese also affects spam.
A very good question! Elementary and advanced at the same time.
There are two fundamentally different kinds of values in Python: "small"
values and "big" values. A variable can only hold a small value. A list
element can only hold a small value. A dictionary entry can only hold a
small value. The same is true for an object member (aka field).
So we have four kinds of (memory) slots: variables, list elements,
dictionary entries and fields. Any slot can only hold a small value.
The small values include numbers, booleans (True or False) and
references. All other values are big, too big to fit in a slot. They
have to be stored in a "vault" big enough to hold them. This vault is
called the heap. Big values cannot be stored in slots directly; instead,
references to big values are used.
Let me now annotate your excellent example:
spam = 42 # put the small value 42 (number) in a memory slot,
# namely a variable named "spam"
cheese = spam # copy the contents of the variable "spam" into
# another memory slot, a variable named "cheese;"
# now both variables contain the same small value 42
spam = 100 # replace the contents of the variable "spam" with the
# small value 100; leave the contents of the variable
# "cheese" intact
spam
> 100 # as expected
cheese
> 42 # ditto
spam = [0, 1, 2, 3, 4, 5]
# a list is a "big" value; the statement creates a
# list of six slots in the heap an puts a number in
# each slot; then, a reference to the list is placed
# in the variable "spam"
cheese = spam # copy the reference to the six-element list from the
# variable "spam" into the variable "cheese;" the heap
# still contains only one list, and the two variables
# refer to the same one
# (rationale: big values take time and space to copy
# in full, and almost always copying references is
# good for the problem at hand; if a full copy is
# needed, Python has ways to do that, too)
cheese[1] = 'Hello!'
# a character string (text snippet) is a "big" value;
# the statement creates the six-character string
# 'Hello!' in the heap; then, a reference to the
# string is placed in the second element of the list
# referred to by the variable "cheese"
# (that's a complicated sentence with lots to chew
# even though the Python statement looks so innocently
# simple)
# there still is a single list in the heap; the list
# is still referred to by both variables; however the
# second slot of the list, which used to hold the
# number 1, has been replaced with a reference to the
# "big" string 'Hello!'
spam
> [0, 'Hello!', 2, 3, 4, 5]
# as expected, right?
cheese
> [0, 'Hello!', 2, 3, 4, 5]
# right?
The final situation is represented by this picture of Python's memory:
spam cheese
+-----+ +-----+
| . | | . |
+--+--+ +--+--+
| |
| | VARIABLES
= = =|= = = = = = =|= = = = = = = = = = = = = = = = = = =
| / THE HEAP
| ---------
| /
| |
v v
+-----+-----+-----+-----+-----+-----+
| 0 | . | 2 | 3 | 4 | 5 | a list
+-----+--+--+-----+-----+-----+-----+
|
|
|
|
v a string
+--------+
| Hello! | a string
+--------+
Marko
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Explanation of list reference dave em <daveandem2000@gmail.com> - 2014-02-14 10:08 -0800
Re: Explanation of list reference Jussi Piitulainen <jpiitula@ling.helsinki.fi> - 2014-02-14 20:26 +0200
Re: Explanation of list reference dave em <daveandem2000@gmail.com> - 2014-02-14 10:54 -0800
Re: Explanation of list reference Denis McMahon <denismfmcmahon@gmail.com> - 2014-02-14 19:20 +0000
Re: Explanation of list reference Marko Rauhamaa <marko@pacujo.net> - 2014-02-14 21:56 +0200
Re: Explanation of list reference Ian Kelly <ian.g.kelly@gmail.com> - 2014-02-14 13:17 -0700
Re: Explanation of list reference Marko Rauhamaa <marko@pacujo.net> - 2014-02-14 22:58 +0200
Re: Explanation of list reference Chris Angelico <rosuav@gmail.com> - 2014-02-15 08:16 +1100
Re: Explanation of list reference Marko Rauhamaa <marko@pacujo.net> - 2014-02-14 23:43 +0200
Re: Explanation of list reference Ned Batchelder <ned@nedbatchelder.com> - 2014-02-14 19:00 -0500
Re: Explanation of list reference dave em <daveandem2000@gmail.com> - 2014-02-14 16:26 -0800
Re: Explanation of list reference Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2014-02-15 06:07 +0000
Re: Explanation of list reference Marko Rauhamaa <marko@pacujo.net> - 2014-02-15 11:00 +0200
Re: Explanation of list reference Chris Angelico <rosuav@gmail.com> - 2014-02-15 11:57 +1100
Re: Explanation of list reference Rustom Mody <rustompmody@gmail.com> - 2014-02-14 17:55 -0800
Re: Explanation of list reference Chris Angelico <rosuav@gmail.com> - 2014-02-15 13:08 +1100
Re: Explanation of list reference Rustom Mody <rustompmody@gmail.com> - 2014-02-14 18:34 -0800
Re: Explanation of list reference Chris Angelico <rosuav@gmail.com> - 2014-02-15 13:42 +1100
Re: Explanation of list reference Rustom Mody <rustompmody@gmail.com> - 2014-02-14 19:14 -0800
Re: Explanation of list reference Chris Angelico <rosuav@gmail.com> - 2014-02-15 14:33 +1100
Re: Explanation of list reference Rustom Mody <rustompmody@gmail.com> - 2014-02-14 20:24 -0800
Re: Explanation of list reference Chris Angelico <rosuav@gmail.com> - 2014-02-15 15:37 +1100
Re: Explanation of list reference Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2014-02-15 06:54 +0000
Re: Explanation of list reference Chris Angelico <rosuav@gmail.com> - 2014-02-15 18:19 +1100
Re: Explanation of list reference Ian Kelly <ian.g.kelly@gmail.com> - 2014-02-14 22:20 -0700
Re: Explanation of list reference Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2014-02-15 07:03 +0000
Re: Explanation of list reference Chris Angelico <rosuav@gmail.com> - 2014-02-15 16:31 +1100
Re: Explanation of list reference Rustom Mody <rustompmody@gmail.com> - 2014-02-14 22:07 -0800
Re: Explanation of list reference Ben Finney <ben+python@benfinney.id.au> - 2014-02-15 17:19 +1100
Re: Explanation of list reference Rustom Mody <rustompmody@gmail.com> - 2014-02-14 22:31 -0800
Re: Explanation of list reference Chris Angelico <rosuav@gmail.com> - 2014-02-15 17:23 +1100
Re: Explanation of list reference Ian Kelly <ian.g.kelly@gmail.com> - 2014-02-15 01:08 -0700
Re: Explanation of list reference Mark Lawrence <breamoreboy@yahoo.co.uk> - 2014-02-15 12:42 +0000
Re: Explanation of list reference Marko Rauhamaa <marko@pacujo.net> - 2014-02-15 15:44 +0200
Re: Explanation of list reference Rustom Mody <rustompmody@gmail.com> - 2014-02-16 06:00 -0800
Re: Explanation of list reference Ned Batchelder <ned@nedbatchelder.com> - 2014-02-16 11:29 -0500
Re: Explanation of list reference Rustom Mody <rustompmody@gmail.com> - 2014-02-16 08:50 -0800
Re: Explanation of list reference Marko Rauhamaa <marko@pacujo.net> - 2014-02-16 20:01 +0200
Re: Explanation of list reference Mark Lawrence <breamoreboy@yahoo.co.uk> - 2014-02-16 18:36 +0000
Re: Explanation of list reference Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2014-02-17 05:52 +0000
Re: Explanation of list reference Terry Reedy <tjreedy@udel.edu> - 2014-02-15 15:40 -0500
Re: Explanation of list reference Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2014-02-16 02:27 +0000
Re: Explanation of list reference Grant Edwards <invalid@invalid.invalid> - 2014-02-15 19:02 +0000
Re: Explanation of list reference Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2014-02-15 06:44 +0000
Re: Explanation of list reference Chris Angelico <rosuav@gmail.com> - 2014-02-15 18:05 +1100
Re: Explanation of list reference Gregory Ewing <greg.ewing@canterbury.ac.nz> - 2014-02-16 00:29 +1300
Re: Explanation of list reference Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2014-02-15 06:15 +0000
Re: Explanation of list reference Rustom Mody <rustompmody@gmail.com> - 2014-02-14 22:24 -0800
Re: Explanation of list reference Gregory Ewing <greg.ewing@canterbury.ac.nz> - 2014-02-16 00:32 +1300
Re: Explanation of list reference Chris Angelico <rosuav@gmail.com> - 2014-02-15 22:37 +1100
Re: Explanation of list reference Marko Rauhamaa <marko@pacujo.net> - 2014-02-15 11:10 +0200
Re: Explanation of list reference Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2014-02-15 10:31 +0000
Re: Explanation of list reference Christian Gollwitzer <auriocus@gmx.de> - 2014-02-15 10:44 +0100
Re: Explanation of list reference Marko Rauhamaa <marko@pacujo.net> - 2014-02-15 12:10 +0200
Re: Explanation of list reference Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2014-02-15 10:53 +0000
Re: Explanation of list reference Gregory Ewing <greg.ewing@canterbury.ac.nz> - 2014-02-17 11:40 +1300
Re: Explanation of list reference Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2014-02-17 05:57 +0000
Re: Explanation of list reference Chris Angelico <rosuav@gmail.com> - 2014-02-15 22:21 +1100
Re: Explanation of list reference Alister <alister.ware@ntlworld.com> - 2014-02-16 19:16 +0000
Re: Explanation of list reference Ben Finney <ben+python@benfinney.id.au> - 2014-02-15 15:20 +1100
Re: Explanation of list reference Marko Rauhamaa <marko@pacujo.net> - 2014-02-15 11:31 +0200
Re: Explanation of list reference Marko Rauhamaa <marko@pacujo.net> - 2014-02-15 12:13 +0200
Re: Explanation of list reference Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2014-02-15 11:17 +0000
Re: Explanation of list reference Marko Rauhamaa <marko@pacujo.net> - 2014-02-15 14:07 +0200
Re: Explanation of list reference Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2014-02-15 14:41 +0000
Re: Explanation of list reference Marko Rauhamaa <marko@pacujo.net> - 2014-02-15 18:29 +0200
Re: Explanation of list reference Roy Smith <roy@panix.com> - 2014-02-15 12:02 -0500
Re: Explanation of list reference Rustom Mody <rustompmody@gmail.com> - 2014-02-15 09:30 -0800
Re: Explanation of list reference Ian Kelly <ian.g.kelly@gmail.com> - 2014-02-15 11:37 -0700
Re: Explanation of list reference Ian Kelly <ian.g.kelly@gmail.com> - 2014-02-15 11:45 -0700
Re: Explanation of list reference Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2014-02-15 19:05 +0000
Re: Explanation of list reference Ian Kelly <ian.g.kelly@gmail.com> - 2014-02-15 10:11 -0700
Re: Explanation of list reference Marko Rauhamaa <marko@pacujo.net> - 2014-02-15 22:20 +0200
Re: Explanation of list reference Ian Kelly <ian.g.kelly@gmail.com> - 2014-02-15 14:20 -0700
Re: Explanation of list reference Rustom Mody <rustompmody@gmail.com> - 2014-02-15 09:47 -0800
Re: Explanation of list reference Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2014-02-15 19:17 +0000
Re: Explanation of list reference Marko Rauhamaa <marko@pacujo.net> - 2014-02-15 23:01 +0200
Re: Explanation of list reference Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2014-02-16 02:25 +0000
Re: Explanation of list reference Marko Rauhamaa <marko@pacujo.net> - 2014-02-16 13:18 +0200
Re: Explanation of list reference Chris Angelico <rosuav@gmail.com> - 2014-02-15 22:31 +1100
Re: Explanation of list reference Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2014-02-15 10:40 +0000
Re: Explanation of list reference Chris Angelico <rosuav@gmail.com> - 2014-02-15 22:17 +1100
Re: Explanation of list reference Marko Rauhamaa <marko@pacujo.net> - 2014-02-14 23:20 +0200
Re: Explanation of list reference Marko Rauhamaa <marko@pacujo.net> - 2014-02-16 10:08 +0200
Re: Explanation of list reference Ian Kelly <ian.g.kelly@gmail.com> - 2014-02-16 01:28 -0700
Re: Explanation of list reference Marko Rauhamaa <marko@pacujo.net> - 2014-02-16 12:52 +0200
Re: Explanation of list reference Chris Angelico <rosuav@gmail.com> - 2014-02-16 22:24 +1100
Re: Explanation of list reference Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2014-02-16 12:04 +0000
Re: Explanation of list reference Marko Rauhamaa <marko@pacujo.net> - 2014-02-16 18:17 +0200
Re: Explanation of list reference Jussi Piitulainen <jpiitula@ling.helsinki.fi> - 2014-02-16 10:35 +0200
Re: Explanation of list reference Ian Kelly <ian.g.kelly@gmail.com> - 2014-02-16 01:35 -0700
Re: Explanation of list reference Alan Bawden <alan@scooby-doo.csail.mit.edu> - 2014-02-16 03:38 -0500
Re: Explanation of list reference Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2014-02-16 08:40 +0000
Re: Explanation of list reference Chris Angelico <rosuav@gmail.com> - 2014-02-16 21:18 +1100
Re: Explanation of list reference Dennis Lee Bieber <wlfraed@ix.netcom.com> - 2014-02-16 12:10 -0500
Re: Explanation of list reference Ned Batchelder <ned@nedbatchelder.com> - 2014-02-14 15:45 -0500
Re: Explanation of list reference Ian Kelly <ian.g.kelly@gmail.com> - 2014-02-14 13:12 -0700
Re: Explanation of list reference Jussi Piitulainen <jpiitula@ling.helsinki.fi> - 2014-02-14 22:36 +0200
Re: Explanation of list reference Gregory Ewing <greg.ewing@canterbury.ac.nz> - 2014-02-16 01:03 +1300
Re: Explanation of list reference Ben Finney <ben+python@benfinney.id.au> - 2014-02-15 23:21 +1100
Re: Explanation of list reference Ned Batchelder <ned@nedbatchelder.com> - 2014-02-14 13:42 -0500
Re: Explanation of list reference Ryan Gonzalez <rymg19@gmail.com> - 2014-02-14 12:31 -0600
Re: Explanation of list reference Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2014-02-15 05:36 +0000
Re: Explanation of list reference Chris Angelico <rosuav@gmail.com> - 2014-02-15 17:07 +1100
Re: Explanation of list reference Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2014-02-15 06:48 +0000
Re: Explanation of list reference Chris Angelico <rosuav@gmail.com> - 2014-02-15 17:57 +1100
Re: Explanation of list reference Gregory Ewing <greg.ewing@canterbury.ac.nz> - 2014-02-16 01:18 +1300
Re: Explanation of list reference Chris Angelico <rosuav@gmail.com> - 2014-02-15 23:24 +1100
Re: Explanation of list reference Marko Rauhamaa <marko@pacujo.net> - 2014-02-15 15:25 +0200
Re: Explanation of list reference Ben Finney <ben+python@benfinney.id.au> - 2014-02-16 00:59 +1100
Re: Explanation of list reference Gregory Ewing <greg.ewing@canterbury.ac.nz> - 2014-02-17 11:54 +1300
Re: Explanation of list reference Chris Angelico <rosuav@gmail.com> - 2014-02-17 10:02 +1100
Re: Explanation of list reference Roy Smith <roy@panix.com> - 2014-02-16 18:46 -0500
Re: Explanation of list reference Chris Angelico <rosuav@gmail.com> - 2014-02-17 12:26 +1100
Re: Explanation of list reference Ben Finney <ben+python@benfinney.id.au> - 2014-02-17 10:05 +1100
Re: Explanation of list reference Roy Smith <roy@panix.com> - 2014-02-16 18:43 -0500
Re: Explanation of list reference Ben Finney <ben+python@benfinney.id.au> - 2014-02-17 11:04 +1100
Re: Explanation of list reference Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2014-02-17 06:31 +0000
Re: Explanation of list reference Rustom Mody <rustompmody@gmail.com> - 2014-02-17 05:43 -0800
Re: Explanation of list reference Ned Batchelder <ned@nedbatchelder.com> - 2014-02-16 20:43 -0500
Re: Explanation of list reference Chris Angelico <rosuav@gmail.com> - 2014-02-17 12:59 +1100
Re: Explanation of list reference Roy Smith <roy@panix.com> - 2014-02-16 22:28 -0500
Re: Explanation of list reference Ben Finney <ben+python@benfinney.id.au> - 2014-02-17 14:52 +1100
Re: Explanation of list reference Rustom Mody <rustompmody@gmail.com> - 2014-02-16 20:35 -0800
Re: Explanation of list reference Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2014-02-17 08:03 +0000
Re: Explanation of list reference Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2014-02-17 06:21 +0000
Re: Explanation of list reference Chris Angelico <rosuav@gmail.com> - 2014-02-17 18:11 +1100
Re: Explanation of list reference Rotwang <sg552@hotmail.co.uk> - 2014-02-17 18:24 +0000
Re: Explanation of list reference Dennis Lee Bieber <wlfraed@ix.netcom.com> - 2014-02-15 14:30 -0500
Re: Explanation of list reference Terry Reedy <tjreedy@udel.edu> - 2014-02-14 16:37 -0500
Re: Explanation of list reference pecore@pascolo.net - 2014-02-14 23:34 +0100
Re: Explanation of list reference Christian Gollwitzer <auriocus@gmx.de> - 2014-02-15 10:38 +0100
Re: Explanation of list reference Ben Finney <ben+python@benfinney.id.au> - 2014-02-16 01:20 +1100
Re: Explanation of list reference Chris Angelico <rosuav@gmail.com> - 2014-02-16 01:26 +1100
Re: Explanation of list reference Joshua Landau <joshua@landau.ws> - 2014-02-15 20:44 +0000
Re: Explanation of list reference Chris Angelico <rosuav@gmail.com> - 2014-02-16 09:27 +1100
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