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I have a number of lists called: index_1, index_2, index_3, ...., index_n.
What I want is to concatenate them all in a new list.

My code so far:

index_all=[]
for i in range(1,n+1):
    index_all = index_all + globals()["index_"+str(i)]

However, I get an error:

KeyError: index_1

Any ideas as to how to solve this?

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6
  • This means that there is no variable stored in globals with the name index_1... should there be one? Commented Nov 2, 2017 at 9:17
  • Yes, I think there should be one, right? Commented Nov 2, 2017 at 9:19
  • Best test: try yourself to access globals()["index_1"] via python console Commented Nov 2, 2017 at 9:20
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    Please make a full code example, as there seems to be something missing. You say you have lists list_1 to list_n, but I don't see them in your code. Also it is not obvious what index_all represents. Why do you need any indices in a list, if you just want to concatenate the lists? Is that the list where you want to append all the contents of the other lists? Commented Nov 2, 2017 at 9:29
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    Should it be globals()["list_"+str(i)] ? Commented Nov 2, 2017 at 9:30

4 Answers 4

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It seems you may just have a naming error, you try to call "index_" + str(i) but your lists are named "list_" + str(i). The following line should work as long as your lists are named list_1, list_2... list_n with no breaks and your n is correct. 

index_all = [globals()["list_"+str(i)] for i in range(1, n+1)]

If you want the lists to be flat you can 

[item for sublist in index_all for item in sublist]

It might be easier if you can avoid using globals() as it will probably be fragile!

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1 Comment

No, just a typo in the question, sorry my bad. The names of the variables are indeed index_1, index_2, index_3 and I have already tried your suggestion and it won't work
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you may want to use map and filter , and the globals().items() feature:

concat_list  =  map ( lambda list_var : list_var[1] , filter ( lambda list_var  : list_var[0].startswith("list"), globals().items()))

your concat_list is the list of all items from all lists

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8 Comments

still this means that I want to iterrate over my lists, something like: all_lists=[] for i in range(1,n+1): all_lists.append(index_i) but that leads us to the same problem, how do I iterrate over these index_i lists?
you may want to use extend and append these list into one list and then do my solution
Neither do I, but I was hoping there could be some.
use extend like so all_lists.extend((list_1, list_2, list_3)) and then my solution
But that's pretty much the same as the above right? I mean it presupposes that I manually enter my lists. I will work fine if they are 3, but what happens in my case that they are 100? Manually wiritng list_1, list_2,....,list_100 is not a good idea.
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You could try this out:

list1 = [[1,2,3],[4,5,6],[7,8,9]]

def joinAll(mulist):
     index_all=[]
     for i in mulist:
         for u in i:
             index_all.append(u)
     return index_all

print joinAll(list1)
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3 Comments

It looks like I have to manually input the list1, list2, list3 in the function joinAll. Supposing that I have 100 lists: list1 up to list100, this is clearly non functional.
@lmm since index_all is outside of the function, I think there is no use returning it.
also using list.extend(another_list) lets you get rid of the nested loop.
0

use list unpacking

cat_list = []
for i in range(1, n+1):
    cat_list = [ *cat_list, *globals()['index_'+str(i)] ]
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2 Comments

Creating a new list unpacking the elements each time scales really bad both with the number of lists and with the numbers of elements in the lists.
Use cat_list.extend(globals()['index_'+str(i)]) or cat_list += globals()['index_'+str(i)] instead.

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