for循环错误1

 1 >>> sequence=(1,2,None,4,None,5)
 2 >>> total=0
 3 >>> def sum(ss):
 4     for value in sequence:
 5         if value is None:
 6             continue
 7         total +=total
 8     return(total)
 9 
10 >>> sum(sequence)
11 Traceback (most recent call last):
12   File "<pyshell#326>", line 1, in <module>
13     sum(sequence)
14   File "<pyshell#325>", line 5, in sum
15     total +=total
16 UnboundLocalError: local variable 'total' referenced before assignment
17 >>> def sum(ss):
18     for value in sequence:
19         if value is None:
20             continue
21         total +=value
22     return(total)
23 
24 >>> sum(sequence)
25 Traceback (most recent call last):
26   File "<pyshell#329>", line 1, in <module>
27     sum(sequence)
28   File "<pyshell#328>", line 5, in sum
29     total +=value
30 
31 UnboundLocalError: local variable 'total' referenced before assignment
32 >>> def sum(ss):
33     total=0
34     for value in sequence:
35         if value is None:
36             continue
37         total +=value
38     return(total)
39 
40 >>> sum(sequence)
41 12

local variable 'total' referenced before assignment的中文意思是：局部变量在引用前要先赋值。也就是说，def定义的函数中的局部变量total要在函数内进行预定义，
定义为全局变量不可用。