from math import sqrt

K = 9.0e9

def potential(x, y, q, xq, yq):
    phi = 0.
    for i in range(len(q)):

        dx = x - xq[i]
        dy = y - yq[i]
        dr = sqrt(dx*dx + dy*dy)

        phi += K*q[i]/dr

    return phi

if __name__ == "__main__" :

    q = []
    xq = []
    yq = []

    # ... set up nq, xq, yq ...

    pot = potential(0, 0, q, xq, yq)
    print('pot =', pot)