making assignments to an empty two dimensional list

[drathlian]

BPO	13925
Nosy	@mdickinson
Files	twoDimensionalList.py

^{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 2012-02-02.00:47:08.345>
created_at = <Date 2012-02-02.00:40:48.436>
labels = ['type-bug', 'invalid']
title = 'making assignments to an empty two dimensional list'
updated_at = <Date 2012-02-02.00:47:08.343>
user = 'https://bugs.python.org/drathlian'

bugs.python.org fields:

activity = <Date 2012-02-02.00:47:08.343>
actor = 'mark.dickinson'
assignee = 'none'
closed = True
closed_date = <Date 2012-02-02.00:47:08.345>
closer = 'mark.dickinson'
components = []
creation = <Date 2012-02-02.00:40:48.436>
creator = 'drathlian'
dependencies = []
files = ['24390']
hgrepos = []
issue_num = 13925
keywords = []
message_count = 2.0
messages = ['152450', '152451']
nosy_count = 2.0
nosy_names = ['mark.dickinson', 'drathlian']
pr_nums = []
priority = 'normal'
resolution = 'not a bug'
stage = None
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue13925'
versions = ['Python 2.7']

[drathlian]

I might be missing something, but I am expecting the following code to print out a list of lists with each internal list holding one number[0-4], but instead the internal lists are a copy of the list "count".

#!/usr/bin/python
count = range(4)
twoDimensionList = [[]] * len(count)
for i in range(len(count)):
   twoDimensionList[i].append(count[i])
print twoDimensionList

Should print:
[[0], [1], [2], [3]]

but erroneously prints:
[[0, 1, 2, 3], [0, 1, 2, 3], [0, 1, 2, 3], [0, 1, 2, 3]]

[mdickinson]

This isn't a bug, but a known gotcha: see.

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