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I have two lists in python list_a and list_b. The list_a have some images links, and the list_b too. 99% of the items are the same, but i have to know this 1%. The all surplus items are in list_a, that means all items in list_b are in list_a. My initial idea is subtract all items:
list_a - list_b = list_c, where the list_c are my surplus items. My code is:

list_a = 

list_b = 

list_c = 



arq_b = open('list_b.txt','r')

for b in arq_b:

    list_b.append(b)



arq_a = open('list_a.txt','r')

for a in arq_a:

    if a not in arq_b:

        list_c.append(a)



arq_c = open('list_c.txt','w')

for c in list_c:

    arq_c.write(c)

I think the logic is right, if i have some items, the code is run fast. But i dont have 10 items, or 1.000, or even 100.000. I have 78.514.022 items in my list_b.txt and 78.616.777 in my list list_a.txt. I dont't know the cost of this expression: if a not in arq_b. But if i execute this code, i think wont finish in this year.

My pc have 8GB, and i allocate 15gb for swap to not explode my RAM.

My question is, there's another way to make this operation more efficiently(Faster)?

• The list_a is ordinate but the list_b not.


• Each item have this size: images/00000cd9fc6ae2fe9ec4bbdb2bf27318f2babc00.png
  


• The order doesnt matter, i want know the surplus.

you can create one set of the first file contents, then just use difference or symmetric_difference depending on what you call a difference

with open("list_a.txt") as f:

    set_a = set(f)



with open("list_b.txt") as f:

    diffs = set_a.difference(f)

if list_b.txt contains more items than list_a.txt you want to swap them or use set_a.symmetric_difference(f) instead, depending on what you need.

difference(f) works but still has to construct a new set internally. Not a great performance gain (see set issubset performance difference depending on the argument type), but it's shorter.

Try using sets:

with open("list_a.txt") as f:

    set_a = set(f)



with open("list_b.txt") as f:

    set_b = set(f)



set_c = set_a - set_b



with open("list_c.txt","w") as f:

    for c in set_c:

        f.write(c)

The complexity of subtracting two sets is O(n) in the size of the set a.

To extend the comment of @L3viathan
If order of element is not important set is the rigth way.
here a dummy example you can adapt:

l1 = [0,1,2,3,4,5]

l2 = [3,4,5]

setL1 = set(l1)  # transform the list into a set

setL2 = set(l2)

setDiff = setl1 - setl2  # make the difference 

listeDiff = list(setDiff)  # if you want to have your element back in a list

as you see is pretty straightforward in python.

In case order matters you can presort the lists together with item indices and then iterate over them together:

list_2 = sorted(list_2)

diff_idx = 

j = 0

for i, x in sorted(enumerate(list_1), key=lambda x: x[1]):

    if x != list_2[j]:

        diff_idx.append(i)

    else:

        j += 1

diff = [list_1[i] for i in sorted(diff_idx)]

This has time complexity of the sorting algorithm, i.e. O(n*log n).