python setting value to list elements

Question

Basically what I'm trying to do is, create a nestled list and set a value of one of its element as a function of other elements in the list.

>>> a = [[1]*5]*5
>>> a
[[1, 1, 1, 1, 1], [1, 1, 1, 1, 1], [1, 1, 1, 1, 1], [1, 1, 1, 1, 1], [1, 1, 1, 1, 1]]
>>> a[2][2] = a[0][2] + a[2][1]
>>> a
[[1, 1, 2, 1, 1], [1, 1, 2, 1, 1], [1, 1, 2, 1, 1], [1, 1, 2, 1, 1], [1, 1, 2, 1, 1]]
>>> a[3][2]   
2
>>> a[4][2]
2
>>> a[4][4]
1

I just set the value of a[2][2] but the same value got set to every element in the 3rd column. What is going on exactly and how can I get the desired behavior?

NPE · Accepted Answer · 2013-03-27 07:03:50Z

2

What happens is that a ends up containing five references to the same sublist. When you change one sublist, they all change.

To see this, apply id() to each of the sublists:

>>> map(id, a)
[8189352, 8189352, 8189352, 8189352, 8189352]

As you can see, they all have the same ID, meaning they are the same object.

To fix, replace

a = [[1]*5]*5

with

a = [[1]*5 for _ in range(5)]

Now the sublists are independent objects:

>>> map(id, a)
[21086256, 18525680, 18524720, 19331112, 18431472]

edited Mar 27, 2013 at 7:03

answered Mar 27, 2013 at 6:58

NPE

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recursive · Accepted Answer · 2013-03-27 06:58:47Z

1

The problem is your list a contains five references to the same list. You need to do something like this:

a = []
for _ in range(5):
    a += [[1] * 5]

answered Mar 27, 2013 at 6:58

recursive

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