for循环错误1

>>> sequence=(1,2,None,4,None,5)
>>> total=0
>>> def sum(ss):
    for value in sequence:
        if value is None:
            continue
        total +=total
    return(total)

>>> 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
>>> def sum(ss):
    for value in sequence:
        if value is None:
            continue
        total +=value
    return(total)

>>> 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
>>> def sum(ss):
    total=0
    for value in sequence:
        if value is None:
            continue
        total +=value
    return(total)

>>> sum(sequence)
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。