Wrong behavior for list of lists

[dechamps]

BPO	24589
Nosy	@vadmium

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = <Date 2015-07-08.11:08:12.246>
created_at = <Date 2015-07-08.08:01:16.474>
labels = ['interpreter-core', 'type-bug', 'invalid']
title = 'Wrong behavior for list of lists'
updated_at = <Date 2015-07-08.11:08:12.246>
user = 'https://bugs.python.org/dechamps'

bugs.python.org fields:

activity = <Date 2015-07-08.11:08:12.246>
actor = 'martin.panter'
assignee = 'none'
closed = True
closed_date = <Date 2015-07-08.11:08:12.246>
closer = 'martin.panter'
components = ['Interpreter Core']
creation = <Date 2015-07-08.08:01:16.474>
creator = 'dechamps'
dependencies = []
files = []
hgrepos = []
issue_num = 24589
keywords = []
message_count = 4.0
messages = ['246450', '246451', '246454', '246455']
nosy_count = 4.0
nosy_names = ['martin.panter', 'zorceta', 'dechamps', 'Bastiaan Albarda']
pr_nums = []
priority = 'normal'
resolution = 'not a bug'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue24589'
versions = ['Python 2.7']

[dechamps]

After creating a list of lists, changing one element, leads to changes of all the elements:

>>> v=[[]]*10
>>> v
[[], [], [], [], [], [], [], [], [], []]
>>> v[3].append(3)
>>> v
[[3], [3], [3], [3], [3], [3], [3], [3], [3], [3]]
>>> 
>>> v=[[]]*10
>>> v
[[], [], [], [], [], [], [], [], [], []]
>>> v[3] += [3]
>>> v
[[3], [3], [3], [3], [3], [3], [3], [3], [3], [3]]
>>>

[zorceta]

FYI:

>>> ll = [[]]*10
>>> [id(l) for l in ll]
[67940296, 67940296, 67940296, 67940296, 67940296, 67940296, 67940296, 67940296, 67940296, 67940296]

[BastiaanAlbarda]

The * operator lets you use the same object multiple times, thereby saving resources.

If you want unique objects use comprehension:
>>> v = [[] for x in range(10)]
>>> v[3].append(3)
>>> v
[[], [], [], [3], [], [], [], [], [], []]
>>> v[3] += [3]
>>> v
[[], [], [], [3, 3], [], [], [], [], [], []]

[vadmium]

This is how Python is meant to work; see <https://docs.python.org/2.7/faq/programming.html#how-do-i-create-a-multidimensional-list\>. Unless there is something in the documentation that gave you the wrong impression, I suggest we close this.