Can't generate different random numbers in Python

I'm having this problem in python where I can't generate different random numbers in a loop. Every loop generates same numbers. My code looks like this:

import random

class Dna :
    genes = []
    lifespan = 0

    def __init__(self, lifespan) :
        self.lifespan = lifespan

        sum = 0
        for i in range(lifespan) :
            self.genes.append(PVector(random.randrange(-10, 10), random.randrange(-10, 10)))
            sum += self.genes[i].mag()

        print(sum)

Here, I'm trying to generate random vectors in the range -10 and 10 but every different DNA object prints the same sum. Here is the main file:

import dna

def setup() :

    size(600, 600)

    d = dna.Dna(200)
    d2 = dna.Dna(200)

And I'm using processing.py for this.