Exclude items from list of lists Python

Question

I have the next list of

testList = []

testList.append([0,-10])
testList.append([-12,122])
testList.append([13,172])
testList.append([17,296])
testList.append([-10,80])
testList.append([-16,230])
testList.append([-18, 296])
testList.append([-2, -8])
testList.append([-5,10])
testList.append([2,-4])

and another lists which contains elements from previous list:

m1 = []
m1.append([0, -10])
m1.append([13, 172])

Then I try to get a subarray from the list testList with the next statement:

[element for i, element in enumerate(testList) if i not in m1]

But I get the same list as testList.

How can I achieve this?

`[element for i, element in enumerate(testList) if element not in m1]` — Kamejoin, Nov 07 '15 at 00:51
Possible duplicate of [Remove all the elements that occur in one list from another](http://stackoverflow.com/questions/4211209/remove-all-the-elements-that-occur-in-one-list-from-another) — skrrgwasme, Nov 07 '15 at 01:10

score 18 · Answer 1 · answered Nov 07 '15 at 01:07

If you don't care about the order in the lists, you can use sets instead:

# result will be a set, not a list
not_in_testlist = set(testlist) - set(m1)

If you want the result to be a list again:

 # result will be a list with a new order
not_in_m1 = list(set(testlist) - set(m1))

Be aware that using sets will lose the order of the original lists because sets are unordered types (they use hashing under the hood).

If you need to preserve the order, then Andrew Allaire's answer is correct:

# result is a list, order is preserved
not_in_testlist = [e for e in testlist if e not in m1]

score 9 · Answer 2 · answered Nov 07 '15 at 00:54

9

The problem is with your use of enumerate. The i is just going to be an integer, and therefor never in a list that only has lists in it. Try this:

[element for element in testList if element not in m1]

answered Nov 07 '15 at 00:54

Andrew Allaire

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Using Sets as skrrgwasme suggested are far more efficient because they use hashing under hood. However you would need your elements to be tuples and not lists, since lists are not hashable (I would comment on his answer, but I lack the reputation to do so). – Andrew Allaire Nov 07 '15 at 01:37

score 4 · Answer 3 · edited Nov 07 '15 at 01:07

4

Try with this:

def clean_list(my_list, exclusion_list):

    new_list = []
    for i in my_list:
        if i in exclusion_list:
            continue
        else:
            new_list.append(i)

    return new_list

edited Nov 07 '15 at 01:07

skrrgwasme

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answered Nov 07 '15 at 01:05

David

41
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This should work, but it's unnecessarily verbose. This type of manipulation is what [list comprehensions](http://www.secnetix.de/olli/Python/list_comprehensions.hawk) are made for. – skrrgwasme Nov 07 '15 at 01:08

Exclude items from list of lists Python

3 Answers3