.remove() method in Python for list of lists

Question

I was trying to remove an element from multiple lists within a list using .remove() method, which I know works for lists. I also used list comprehension. As an example, consider:

abc = [["a","b"],["a","c"]]
bc = [apple.remove("a") for apple in abc]
print(bc)

It will return the output [None, None]. I am curious why does happen and also what is the method I can use to achieve my desired output of [["b"], ["c"]].

Oh you are right... How to then remove a single element and yet output the desired the list? — Tan Yong Boon, Jun 17 '22 at 06:55
Notice that with remove you are altering the original list, so just do what @PicxyB said — Dani Mesejo, Jun 17 '22 at 06:58

Pranav Hosangadi · Accepted Answer · 2022-06-17T07:22:09.927

You get the output [None, None] because list.remove() operates in-place and modifies the original list. You have two options:

Just operate on the original list. Note that it is not a good practice to use a list-comprehension for its side-effects ^{(see here)}, so we should just write a regular loop here:

>>> for apple in abc:
...    apple.remove("a")

Then, you can abc to see that you have the desired result:

>>> print(abc)
[['b'], ['c']]

Create a copy of the original lists, excluding the items to be removed:

>>> bc = [[item for item in apple if item != "a"] for apple in abc]

which gives you the desired output in bc, but leaves the original list unchanged:

>>> print(abc)
[['a', 'b'], ['a', 'c']]
>>> print(bc) 
[['b'], ['c']]

score 0 · Answer 2 · answered Jun 17 '22 at 06:58

0

Remove is an in-place operation

you are changing the original abc data structure. to see the changes you should print(abc)

answered Jun 17 '22 at 06:58

Glauco

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.remove() method in Python for list of lists

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