How to turn all numbers in a list into their negative counterparts?

Question

I am trying to turn a list of positive numbers into a list of negative numbers with the same value in python 3.3.3

For example turning [1,2,3] into

Answer 1

For large list, you will probably better use numpy

import numpy as np

a=np.array([1,2,3,4])

# result as a numpy array
b=-a

# can be casted back to list
c=list(b)