[issue12951] List behavior is different

[Amaury Forgeot d'Arc]

Amaury Forgeot d'Arc  added the comment:

See also 
http://docs.python.org/faq/programming.html#how-do-i-create-a-multidimensional-list

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nosy: +amaury.forgeotdarc

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[issue12951] List behavior is different

[Ezio Melotti]

Ezio Melotti  added the comment:

I don't see where is the bug.  If you do
>>> lists = [[]] * 3
>>> lists
[[], [], []]

you are creating a list that contains 3 references to the same list, as you can 
see here:
>>> lists[0] is lists[1] is lists[2]
True
>>> [id(l) for l in lists]
[33714832, 33714832, 33714832]

so if you append an element to either of the inner list, it will show up 3 
times, because the 3 lists are really the same object:
>>> lists[0].append(3)
>>> lists
[[3], [3], [3]]

However, if you do
>>> lists[0] = 1
>>> lists
[1, [3], [3]]

you are replacing the first element of 'lists' with the int 1.  This doesn't 
mutate the inner list, and therefore the second and third elements are not 
affected.

See also http://python.net/crew/mwh/hacks/objectthink.html

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nosy: +ezio.melotti
resolution:  -> invalid
stage:  -> committed/rejected
status: open -> closed

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[issue12951] List behavior is different

[शंतनू]

New submission from शंतनू :

URL: http://docs.python.org/library/stdtypes.html

Following example is given.

>>> lists = [[]] * 3
>>> lists
[[], [], []]
>>> lists[0].append(3)
>>> lists
[[3], [3], [3]]


Behavior is as follows.

>>> a = [[]] * 3
>>> a
[[], [], []]
>>> a[0] = 1
>>> a
[1, [], []]
>>> 


Python interpreter details:

$ python
Python 2.7.2 (default, Aug 22 2011, 13:53:27) 
[GCC 4.2.1 (Apple Inc. build 5666) (dot 3)] on darwin
Type "help", "copyright", "credits" or "license" for more information.

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assignee: docs@python
components: Documentation
messages: 143793
nosy: docs@python, शंतनू
priority: normal
severity: normal
status: open
title: List behavior is different
type: behavior
versions: Python 2.7

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