import math


def vector_angle(x, y):
    """
        A method which returns the angle of a vector.

        Parameter(s):
        -------------
        x : Integer/Float
            x-axis value of vector.
        y : Integer/Float
            y-axis value of vector.

        Returns:
        --------
        theta : Integer/Float
            Angle of vector in radians.

        See also:
        ---------
            circle_label_angle
    """
    hypot = math.sqrt(x*x + y*y) # get the hypotenuse
    theta = math.asin(y / hypot) # get the corresponding vector angle
    # apply CAST rule
    if x < 0:
        theta = math.pi - theta
    if theta < 0:
        theta = theta + 2*math.pi
    return theta