from __future__ import division

def setup():
    """ test line_intersect() with visual and textual output """
    (a, b), (c, d) = (4, 0), (6, 10)  # try (4, 0), (6, 4)
    (e, f), (g, h) = (0, 3), (10, 7)  # for non intersecting test
    pt = line_instersect(a, b, c, d, e, f, g, h)
    scale(9)
    line(a, b, c, d)
    line(e, f, g, h)
    if pt:
        x, y = pt
        stroke(255)
        point(x, y)
    println(pt)  # prints x, y coordinates or 'None'

def line_instersect(Ax1, Ay1, Ax2, Ay2, Bx1, By1, Bx2, By2):
    """ returns a (x, y) tuple or None if there is no intersection """
    d = (By2 - By1) * (Ax2 - Ax1) - (Bx2 - Bx1) * (Ay2 - Ay1)
    if d:
        uA = ((Bx2 - Bx1) * (Ay1 - By1) - (By2 - By1) * (Ax1 - Bx1)) / d
        uB = ((Ax2 - Ax1) * (Ay1 - By1) - (Ay2 - Ay1) * (Ax1 - Bx1)) / d
    else:
        return
    if not(0 <= uA <= 1 and 0 <= uB <= 1):
        return
    x = Ax1 + uA * (Ax2 - Ax1)
    y = Ay1 + uA * (Ay2 - Ay1)
    return x, y