Pointer problem in initializing array of arrays

[mastermarv]

BPO	9982
Nosy	@pitrou

^{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 2010-09-29.10:04:06.510>
created_at = <Date 2010-09-29.08:40:26.651>
labels = ['invalid']
title = 'Pointer problem in initializing array of arrays'
updated_at = <Date 2010-09-29.10:04:06.509>
user = 'https://bugs.python.org/mastermarv'

bugs.python.org fields:

activity = <Date 2010-09-29.10:04:06.509>
actor = 'pitrou'
assignee = 'none'
closed = True
closed_date = <Date 2010-09-29.10:04:06.510>
closer = 'pitrou'
components = ['None']
creation = <Date 2010-09-29.08:40:26.651>
creator = 'mastermarv'
dependencies = []
files = []
hgrepos = []
issue_num = 9982
keywords = []
message_count = 2.0
messages = ['117581', '117589']
nosy_count = 2.0
nosy_names = ['pitrou', 'mastermarv']
pr_nums = []
priority = 'normal'
resolution = 'not a bug'
stage = None
status = 'closed'
superseder = None
type = None
url = 'https://bugs.python.org/issue9982'
versions = ['Python 2.6']

[mastermarv]

>>> m1=[[0,0,0,0]]*4
>>> m1
[[0, 0, 0, 0], [0, 0, 0, 0], [0, 0, 0, 0], [0, 0, 0, 0]]
>>> m1[0][0]+=1
>>> m1
[[1, 0, 0, 0], [1, 0, 0, 0], [1, 0, 0, 0], [1, 0, 0, 0]]

after initializing an array of arrays as done in the first line of the code snippet all elements in the array point to the same object in memory.

[pitrou]

That's expected behaviour, syntactically. Multiplying a sequence doesn't deep-copy its elements.
If you want an array of distinct arrays, just write:

>>> m1 = [[0,0,0,0] for i in range(4)]
>>> m1[1][0] = 6
>>> m1
[[0, 0, 0, 0], [6, 0, 0, 0], [0, 0, 0, 0], [0, 0, 0, 0]]