python: removing multiple item in list

Question

I want to remove every item in b that’s in a, the out would be [7,8,9,0], how can I do it, this doesn’t seem to work

In [21]:
a=[1,2,3,4,5]
b=[1,2,3,5,5,5,7,8,9,0]
for i in b:
    if i in a:
        print i
        b.remove(i)
print b

#

Out[21]:
1
3
5
[2, 5, 5, 7, 8, 9, 0]

Asked By: Hon

||

Source

Answer 1

Use list comprehension and the in operator.

b = [ elem for elem in b if elem not in a ]

For speed, you can first change a into a set, to make lookup faster:

a = set(a)

EDIT: as pointed out by @Ignacio, this does not modify the original list inplace, but creates a new list and assigns it to b. If you must change the original list, you can assign to b[:] (read: replace all elements in b with the elements in RHS), instead of b, like:

b[:] = [ elem for ... ]

Answered By: shx2

Answer 2

This will remove the items from list ‘b’ that are already in list ‘a’

[b.remove(item) for item in a if item in b]

Updated as per @shx2:

 for item in a:
     while item in b:
         b.remove(item)

Also, you could make it faster by making list ‘a’ a set

 for item in set(a):
     while item in b:
         b.remove(item)

Answered By: amehta

Answer 3

a=[1,2,3,4,5]
b=[1,2,3,5,5,5,7,8,9,0]

c = set(a+b)
print(c)

Answered By: falconx

python: removing multiple item in list

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