Python list of lists assignment

Question

One thing first: No, this is not a question about why

someThing = [[]] * 10
someThing[0] = 1

gives [1,1,1,1,1,1,1,1,1,1].

My question is a bit more tricky: I do the folling:

x, y, z = ([[]] for i in range(3))

Now I assign a value:

x[0] = 1   # This works fine

And now another one:

x[1] = 2   # This does not work anymore

It gives me an IndexError: list assignment index out of range.

How can I make this work? As you can see, I want an empty list that contains 3 other lists to which i can append values.

Thanks in advance!

Did you look at what x[0] = 1 actually does? It replaces one of your nested lists with the number 1. If you want to append values, you should use append, not item assignment. — BrenBarn
– BrenBarn, Commented Apr 29, 2014 at 8:19
yes you are right. but this is only an example. in my implementation, i do use append ;) — PKlumpp
– PKlumpp, Commented Apr 29, 2014 at 8:31

Burhan Khalid · Accepted Answer · 2014-04-29 09:44:47Z

3

I want an empty list that contains 3 other lists to which i can append values.

Your problem correctly stated is: "I want a list of three empty lists" (something cannot be empty and contain things at the same time).

If you want that, why are you doing it in such a convoluted way? What's wrong with the straightforward approach:

empty_list = [[], [], []]

If you have such a list, when you do empty_list[0] = 1, what you end up with is:

empty_list = [1, [], []]

Instead, you should do empty_list[0].append(1), which will give you:

empty_list = [[1], [], []]

If you want to create an arbitrary number of nested lists:

empty_list = []
for i in range(0, 15):
   empty_list.append([])

edited Apr 29, 2014 at 9:44

answered Apr 29, 2014 at 8:24

Burhan Khalid

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1 Comment

PKlumpp Over a year ago

good question! the answer is simple: my 3 lists in one list were only an example. originally, i wanted to put 15 lists into a list. of course, your way is working, but it looks somewhat weird if i implement it that way. i just wanted to learn if there is an advanced way of doing this. but thanks ;)

freakish · Accepted Answer · 2014-04-29 08:19:56Z

3

Since when

someThing = [] * 10
someThing[0] = 1

gives [1,1,1,1,1,1,1,1,1,1]? It's an index error (you can't assign at a nonexisting index), because actually []*10 == []. Your code does exactly the same (I mean an index error). This line

x, y, z = ([[]] for i in range(3))

is short (or not) for

x, y, z = [[]], [[]], [[]]

so x[1] is an index error.

answered Apr 29, 2014 at 8:19

freakish

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2 Comments

PKlumpp Over a year ago

sry, you are right, i forgot brackets there. Yes i know the index does not exist, that is what the error message already told me. but how do i solve my problem?

freakish Over a year ago

@ZerO I want an empty list that contains 3 other lists I don't understand - it can be empty and contain something at the same time. If you want to append something to a list, then use .append. See Burhan's answer.

user3233949 · Accepted Answer · 2014-04-29 10:47:23Z

2

Maybe something like this could work.

lists = [[] for i in range(3)]

The output would be:

[[],[],[]]

If you would like to access/add elements, you could:

lists[0].append(1)
lists[0].append(2)
lists[1].append('a')
lists[1].append('b')

The output would be:

[[1, 2], ['a', 'b'], []]

answered Apr 29, 2014 at 10:47

user3233949

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1 Comment

PKlumpp Over a year ago

yes, this does work. but i need a multi-assignment ;)

vapace · Accepted Answer · 2014-04-29 19:31:31Z

0

Comparing the output of:

for i in [[]] * 3:
    print id(i)

with:

for i in ([[]] for j in range(3)):
    print id(i)

may help answer your finding.

answered Apr 29, 2014 at 19:31

vapace

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2 Comments

PKlumpp Over a year ago

are you kidding me? Did you even read my question? I think i clearly showed that this is NOT the problem -.-

vapace Over a year ago

@ZerO: Did you even try the above code? I see no bearing on your question apart from the subtle but significant difference shown by the above. You have a 10 in your first example, and a 3 in your second. Why? If you already knew the difference between multiplication of lists versus generators, then print i and print i[0] and print i[1] as well and see why you are getting an IndexError. I am on SO thinking I like to learn and help, and I may be wrong in my answer.

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