for i, num in enumerate(a)

>>> for i,num in enumerate(a):
...    print(i)
...    print(num)
...    print("i is:{},num is:{}".format(i,num))
...
...
0
sam
i is:0,num is:sam
1
morgan
i is:1,num is:morgan
2
kelon
i is:2,num is:kelon
3
luis
i is:3,num is:luis
4
peter
i is:4,num is:peter

　　

 

>>> d = {}
>>> a = [2,7,2,3,6]
>>> target = 9
>>> for i,num in enumerate(a):
...    if target-num in d:
...       print(d[target-num],i)    # 打印坐标值
...    d[num] = i                       #这一步一直都跑，不管if
...
(0, 1)
(1, 2)
(3, 4)


#打印出对应的坐标
i, num     target-num      print        d[num]
0,2          7                     na           d[2] = 0
1,7          2                     0,1          d[7] = 1
2,2          7                     1,2          d[2] = 2
3,3          6                     na           d[3] = 3
4,6          3                     3,4          d[6] = 4

　　

改成下面，更容易理解

>>> d = {}
>>> target = 9
>>> a = [2,7,2,3,6]
>>> for i,num in enumerate(a):
...    d[num] = i
...    if target-num in d:
...       print(d[target-num],i)
...
(0, 1)
(1, 2)
(3, 4)