Python list of lists

Question

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Unexpected feature in a Python list of lists

I want to make a list of lists in Python, but apparently this doesn't work, as changing one number changes several. Why is this, and how can I fix it?

>>> a = [[0]*3]*4
>>> a[0][0] = 1
>>> a
[[1, 0, 0], [1, 0, 0], [1, 0, 0], [1, 0, 0]]

Dont use `*`. It makes copies. Use list comprehension instead — mshsayem, Oct 21 '12 at 02:06

score 8 · Accepted Answer · edited May 23 '17 at 12:27

8

What you've discovered is a classic Python pitfall.

x = [0]*3 is a list. No problem there, but [x]*4 creates a list with 4 references to the exact same list x. So modifying the first element, x, also modifies the other elements as well.

Instead, if you do this:

In [193]: a = [[0]*3 for i in range(4)]

then you get 4 distinct items in the list:

In [194]: a[0][0] = 1

In [195]: a
Out[195]: [[1, 0, 0], [0, 0, 0], [0, 0, 0], [0, 0, 0]]

edited May 23 '17 at 12:27

Community

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answered Oct 21 '12 at 02:07

unutbu

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StackOverflow needs the ability to bookmark an answer. This is a prime example for why. – Jonathan Vanasco Oct 21 '12 at 02:23
@JonathanVanasco: Star the question. – nneonneo Oct 21 '12 at 02:34
I starred the question. I want to star the answer. – Jonathan Vanasco Oct 21 '12 at 02:45

score 4 · Answer 2 · answered Oct 21 '12 at 02:12

In Python, [x]*n creates a new list with n references to x. This is perfectly good to have if x is immutable (string, tuple, etc.), but it gets rather confusing if x is mutable since changes to one element affect x, and thus appear to affect all elements of the list.

So, the solution is to use a list comprehension to create n new things:

[x for _ in xrange(n)] # we use _ here as a "dummy variable"

In your case, it would look like

a = [[0]*3 for _ in xrange(4)]

mshsayem · Answer 3 · 2012-10-21T02:19:16.350

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Dont use *. Use list comprehension and create like:

>>> a = [[0 for i in range(3)] for j in range(4)]
>>> a[0][0] = 1
>>> a
[[1, 0, 0], [0, 0, 0], [0, 0, 0], [0, 0, 0]]

See unutbu and nneonneo's answer for better explanation.

edited Oct 21 '12 at 02:19

answered Oct 21 '12 at 02:08

mshsayem

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Actually, it would be nice if `*` made copies. The correct thing to say is that it does *not* make copies, only extra references. – nneonneo Oct 21 '12 at 02:17
hmm, I have read your answer; nice. Removed the statement. – mshsayem Oct 21 '12 at 02:19

score -1 · Answer 4 · answered Oct 21 '12 at 02:07

-1

When using * in list's it creates copies, so when you do a[0][0]=1 all other sub-list's copy this.

In order to fix this, use a generator expression to create your list.

answered Oct 21 '12 at 02:07

IT Ninja

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Actually, the problem is that it *doesn't* make copies, not that it does. – kindall Oct 21 '12 at 04:23

Python list of lists

4 Answers4