Why am I unable to assign to a particular element in a list of lists

Question

I don't understand why I am assigning to the first element in every list below:

➜  ~  python
Python 2.7.3 (default, Sep 26 2012, 21:53:58) 
[GCC 4.7.2] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> board = [[None]*5]*5
>>> print board
[[None, None, None, None, None], [None, None, None, None, None], [None, None, None, None, None], [None, None, None, None, None], [None, None, None, None, None]]
>>> board[0][0] = 1
>>> print board
[[1, None, None, None, None], [1, None, None, None, None], [1, None, None, None, None], [1, None, None, None, None], [1, None, None, None, None]]

I would expect the final output to be this:

[[1, None, None, None, None], [None, None, None, None, None], [None, None, None, None, None], [None, None, None, None, None], [None, None, None, None, None]]

That's because all lists `[None, None, None, None, None]` are copies of same object, modifying one modifies all. — Ashwini Chaudhary, May 31 '13 at 00:46

score 1 · Accepted Answer · answered May 31 '13 at 00:47

1

Your code is equivalent to this one:

x = [None]*5
board = [x for i in range(5)]

If you want 5 different lists, do it:

board = [[None]*5 for i in range(5)]

answered May 31 '13 at 00:47

iurisilvio

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Why am I unable to assign to a particular element in a list of lists

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