import math


def angle_vector_2D(v1, v2):
    """
    Return angle between two 2D vectors (i.e. use only x and y coordinate values)
    Angle (from v1 to v2, positive angle of rotation) is in the range -pi < x < pi.
    """
    dot = v1[0] * v2[0] + v1[1] * v2[1] # dot product
    det = v1[0] * v2[1] - v1[1] * v2[0] # determinant
    angl = math.atan2(det, dot)
    return angl