Create multiple lists of empty lists in one go

Question

I am trying to do the following in a concise way:

n = 3
lst1 = [[] for _ in range(n)]
lst2 = [[] for _ in range(n)]
lst3 = [[] for _ in range(n)]

Basically generating a number of list of empty lists with the same size.

I am looking for a one-liner on this one. I tried using zip and various itertools but I must admit this set of features is one of my weak links with Python.

What is the expected outcome at the end? Are `lst1`, `lst2` different instances of list? — AKS, Dec 12 '16 at 12:15
Yeah of course that's the whole point, creating different lists. — dabadaba, Dec 12 '16 at 12:16
well, you could write a method which will return you these lists by passing `n`? Or, you could even store these lists in another list? It all depends on how many lists do you need. — AKS, Dec 12 '16 at 12:17

niemmi · Accepted Answer · 2016-12-12T12:24:02.497

3

You can achieve the same with just one additional layer of list comprehension or generator expression and then unpacking it to variables:

>>> n = 3
>>> lst1, lst2, lst3 = ([[] for _ in range(n)] for _ in range(3))
>>> lst1
[[], [], []]

edited Dec 12 '16 at 12:24

answered Dec 12 '16 at 12:20

niemmi

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I don't think the OP has mentioned how many such lists are required? Does it really depend on `n`? – AKS Dec 12 '16 at 12:21
Ah I see what I missed now, I need to wrap my head around these generators, iters and such... But I think the last `range(n)` is wrong. It should be as many as the number of vars to unpack there are, so in this case `3`. – dabadaba Dec 12 '16 at 12:23
@AKS: He hasn't but in the question he has created 3 of them. Doesn't really depend on `n` or hasn't been stated, changed example accordingly. – niemmi Dec 12 '16 at 12:24
@dabadaba: That is exactly what I was asking in the comments above. Exactly how many lists do you need? It pretty sure doesn't depend on `n` like you said. – AKS Dec 12 '16 at 12:25
@dabadaba: My bad, thought `n` had something to do with list size and the number of them, changed the answer accordingly. – niemmi Dec 12 '16 at 12:25

Moinuddin Quadri · Answer 2 · 2016-12-12T12:32:47.130

3

Instead of storing the lists in different variables, I will suggest to wrap all the list within single list. Your code should be like:

>>> num_of_list, size_of_list = 5, 3
>>> my_list = [[[] for _ in range(size_of_list)] for _ in range(num_of_list)]

The value hold by my_list will be:

[
   [[], [], []], 
   [[], [], []], 
   [[], [], []], 
   [[], [], []], 
   [[], [], []]
]

For accessing each sub-list, you may simply use the index as:

>>> my_list[0]
[[], [], []]

>>> my_list[1]
[[], [], []]

If it is MUST to store the values in separate variables, you may unwrap the content of main list to individual variables as:

list1, list2, list3, list4, list5 = my_list

edited Dec 12 '16 at 12:32

answered Dec 12 '16 at 12:25

Moinuddin Quadri

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Not really. Even though here I named them `lst1`, `lst2`, etc... in practice they're going to be entirely separate (conceptually) ones, so it would not be a good idea to throw them in the same structure. – dabadaba Dec 12 '16 at 12:28
Updated the answer. I will suggest to initialize them as one variable say `raw_list` and then unwrap the values to variables for better clarity. My this comment is opinion based. – Moinuddin Quadri Dec 12 '16 at 12:30
Why would I want a temporary useless variable when I what those concise variables I know I am going to use? – dabadaba Dec 12 '16 at 12:32
Why is the variable useless? It acts as a wrapper object to all the lists you initialized together at the first place. By not declaring this variable are you saving your memory space? The answer is NO. Remaining sub-list variables will still point to existing objects. This is the pattern I follow and have seen advantages of it with time. As I mentioned, I **suggest** that. To follow or not is your wish :) – Moinuddin Quadri Dec 12 '16 at 12:37
Relax, I am just trying to understand your point of view, I think you took it way too bad. And well, sorry but I still don't see your point. And I never implied that would save memory, what I meant is that it seems a meaningless variable. Say that you want to have lists of `teams`, `scores` and `dates`. That's what you want. Not a `raw_list` with 3 sublists. What are you even going to do with `raw_list`? In this case, it serves no purpose, so why not just do what the other answer suggested? – dabadaba Dec 12 '16 at 12:43

score -1 · Answer 3 · edited May 23 '17 at 11:45

-1

n = 3
for i in range(1, n+1):
    exec("lst%s = [[] for _ in range(%s)]" % (i, n))

Based on To convert string to variable name

edited May 23 '17 at 11:45

Community

1
1

answered Dec 12 '16 at 15:23

JCA

167
4

Create multiple lists of empty lists in one go

3 Answers3