python的循环中else子句的必要的应用场景是什么？

Question

例如python官方文档里举得这个例子

for n in range(2, 10):
...     for x in range(2, n):
...         if n % x == 0:
...             print n, 'equals', x, '*', n/x
...             break
...     else:
...         # loop fell through without finding a factor
...         print n, 'is a prime number'
...
2 is a prime number
3 is a prime number
4 equals 2 * 2
5 is a prime number
6 equals 2 * 3
7 is a prime number
8 equals 2 * 4
9 equals 3 * 3

直接这样写不是也能实现同样的功能吗？



  

    

      
for n in range(2, 10):
      ...     for x in range(2, n):
      ...         if n % x == 0:
      ...             print n, 'equals', x, '*', n/x
      ...             break
      ...         else:
      ...             # loop fell through without finding a factor
      ...             print n, 'is a prime number'
      ...
      2 is a prime number
      3 is a prime number
      4 equals 2 * 2
      5 is a prime number
      6 equals 2 * 3
      7 is a prime number
      8 equals 2 * 4
      9 equals 3 * 3
      

    

  

Answer 1

如果一个循环里面有多个地方需要break的话，用while..else..就不用在每个break后面写同样的代码。（记错了）

举个具体的例子，在C时代，可以这么写：

for (i = 0; i < 10; i++)
     if (some condition)
         break;
if i == 10:
     printf("no break\n");

但是在python里面用 for x in xx 的话就不行了，如果没有这样的else的话，就得设置一个额外的flag：

flag = False
for x in xx:
    if some condition:
        flag = True
        break
if flag:
    print 'no break'

看起来很丑；而有了else。

for x in xx:
    if some condition:
        break
else:
    print 'no break'

Answer 2

当前的for被break掉以后，其对应的else不会执行