代码在全局范围内工作,但不在本地范围内?

此函数应返回36,但返回0。如果我在交互模式下一行一行地运行逻辑,我得到36。

代码

from math import *

line = ((2, 5), (4, -1))
point = (6, 11)

def cross(line, point):
    #reference: http://www.topcoder.com/tc?module=Static&d1=tutorials&d2=geometry1
    ab = ac = [None, None]
    ab[0] = line[1][0] - line[0][0]
    ab[1] = line[1][1] - line[0][1]
    print ab
    ac[0] = point[0] - line[0][0]
    ac[1] = point[1] - line[0][1]
    print ac
    step1 = ab[0] * ac[1] 
    print step1
    step2 = ab[1] * ac[0]
    print step2
    step3 = step1 - step2
    print step3
    return float(value)

cross(line, point)

产量

[2, -6] # ab
[4, 6]  #ac
24      #step 1 (Should be 12)
24      #step 2 (Should be -24)
0       #step 3 (Should be 36)

根据交互模式,这应该是步骤1、步骤2和步骤3的结果。

>>> ab = [2, -6]
>>> ac = [4, 6]
>>> step1 = ab[0] * ac[1]
>>> step1
12
>>> step2 = ab[1] * ac[0]
>>> step2
-24
>>> step3 = step1 - step2
>>> step3
36

(如果有人能给这个称谓,那就太好了)