python - return values in list that also have a negated value

Question

def negated(a):
    a = set(a)
    for i in a:
        a.add(-i)

    return list(a)

if a = [ 3, 4, 5, 6, 7, 8, -3, -4]. I only want to print the values that have a negated counterpart ex: 3, -3, 4, -4

I don't know what's wrong with my code.

Answer 1

>>> s = set(a)
>>> [item for item in a if -item in s]
[3, 4, -3, -4]

In your code you've reassigned the original list to a set, better assign it to different variable.

def negated(a):
    s = set(a)
    for item in a:
        if -item not in s:
            s.remove(item)
    return list(s)
... 
>>> negated(a)
[3, 4, -4, -3]