Question

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Python list subtraction operation

In Python you can concatenate lists like so:

print([3,4,5]+[4,5])

which gives this output:

[3,4,5,4,5]

But what I’m looking for is an equivalent ‘subtraction’ operation, so that doing something like this:

print([3,4,5]-[4,5])

Will output this:

[3]

However, the subtraction operator isn’t defined for lists. I’ve tried this:

a = [3,4,5]
b = [4,5]
print(list(filter(lambda x : x not in b,a)))

Which works, but I’m uncertain whether or not this is the best way to do this. I would also like to preserve the original item positions

Answer 1

You can easily do this with a list comprehension:

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

Edit

Better to use a set to test against. This will remove duplicates from your list.

bb= set(b)
nl = [elem for elem in a if elem not in bb]