python的字典数组问题。

对于一个已经存有一定数据的字典数组，比如：
dict = {'a':10, 'b':12, 'c':3, 'd':4}
假设我现在又有了个新数据b:5
请问如何可以将原来的字典中的b的值变成两个相减的结果(12-5=7)?

for a,b in result.items():
if line[7] == a:
b = b-line[4]
我写的这段代码好像没起作用

 for a,b in result.items():
    if line[7] == a:
        result[a] = b-line[4]

import csv,sys

reader= csv.reader(file("F:\testwiki2.csv",'rb'))

result = {}

for line in reader:
if reader.line_num == 1:
continue
if line[4] == "added":

result[line[6]] = line[5]

for line in reader:
if reader.line_num == 1:
continue
if line[4] == "deleted":
for a,b in result.items():
if line[7] == a:
result[a] = b-line[4]

for c,d in result.items():
print c+" "+d

这是完整代码，然后这个csv文件是这样的：![图片说明](https://img-ask.csdn.net/upload/201603/25/1458889731_393006.png)
我要做的是 如果操作为deleted的话，就将它的被操作者的对应值减去这个deleted的数值。。

import csv,sys
reader= csv.reader(file("F:\testwiki2.csv",'rb'))

result = {}

for line in reader:
if reader.line_num == 1:
continue
if line[4] == "added":
result[line[6]] = line[5]

for line in reader:
if reader.line_num == 1:
continue
if line[4] == "deleted":
for a,b in result.items():
if line[7] == a:
result[a] = b-line[4]

for c,d in result.items():
print c+" "+d