def look_at(u, v):
    u.normalize()
    v.normalize()
    dot = u.dot(v)
    cross = u.cross(v)
    q = mathutils.Quaternion()
    q.x = cross.x
    q.y = cross.y
    q.z = cross.z
    q.w = math.sqrt(u.length_squared*v.length_squared) + dot
    q.normalize()
    print(q)
    return q

def look_at(u, v):
    u.normalize()
    v.normalize()
    dot = u.dot(v)
    cross = u.cross(v)
    q = mathutils.Quaternion()

    if dot >= 1.0:
        print("Case 1")
        return q.identity()
    elif dot < 0.000001 - 1.0:
        print("Case 2")
    else:
        print("Case 3")
        s = math.sqrt((1+dot)*2)
        invis = 1/s
        q.x = cross.x * invis
        q.y = cross.x * invis
        q.z = cross.x * invis
        q.w = s * 0.5
        q.normalize()