python how to change element in nested list

Question

I have been using python for a lot and today I get surprised by a simple nested list.

How can I change the value of an element of my list?

>>> l=[[0,0]]*10
>>> l
[[0, 0], [0, 0], [0, 0], [0, 0], [0, 0], [0, 0], [0, 0], [0, 0], [0, 0], [0, 0]]
>>> l[0]
[0, 0]
>>> l[0][0]
0
>>> l[0][0]=1 #here comes the question
>>> l
[[1, 0], [1, 0], [1, 0], [1, 0], [1, 0], [1, 0], [1, 0], [1, 0], [1, 0], [1, 0]]
>>>

I would expect as final result

l=[[1,0],[0,0],[0,0]...]

Why it does not append? (I would like a theoretical explanation ) How can I get the desired result?

Thanks.

EDIT: In starting the list in another way I get the desired result

>> l=[[0,0],[0,0]]
>>> l
[[0, 0], [0, 0]]
>>> l[0][0]=1
>>> l
[[1, 0], [0, 0]]
>>>

but I am still wondering why the first version does not work and how can I initialize a list of a lot of elements?

score 11 · Accepted Answer · answered Feb 19 '14 at 22:45

11

You have a list of the same [0, 0] element 10 times here:

l=[[0,0]]*10

Any time you modify one, it modifies them all, because they're the same list.

One safe way to make them unique would be:

l = [[0, 0] for _ in range(10)]

One easy way to check would be to print the id of each, which is the memory address where it's stored:

>>> for element in l:
...     print id(element)
...
34669128
34669128
34669128
34669128
34669128
34669128
34669128
34669128
34669128
34669128

answered Feb 19 '14 at 22:45

mhlester

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Argh you beat me to it! I suppose we should find one of the thousands of dupes of this, though. – Adam Smith Feb 19 '14 at 22:46
i'm practicing my speed typing. close vote has been cast – mhlester Feb 19 '14 at 22:47
so my previous code was equal to s=[0,0] l=[s*10] and so I have always the same pointer? – Donbeo Feb 19 '14 at 22:48
@Donbeo exactly so, yes. – Adam Smith Feb 19 '14 at 22:48
yes, same as `s=[0,0];l = [s,s,s,s,s,s,s,s,s,s]` – mhlester Feb 19 '14 at 22:49
ok very well . Sometimes python can be dangerous :) – Donbeo Feb 19 '14 at 22:51
It's a surprisingly common paradigm, stemming from how the data is stored in memory. It can be a life saver though when you *want* the feature – mhlester Feb 19 '14 at 22:53

python how to change element in nested list

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