原文链接: Python for-else 语句

当循环正常退出时，包括循环结束和continue时，才会调用else中的语句 

当使用break结束循环时，不会执行else中的语句

for i in range(5):
    print(i)
else:
    print('else')
0
1
2
3
4
else


for i in range(5):
    print(i)
    if i == 3:
        break
else:
    print('else')
0
1
2
3

for i in range(5):
    print(i)
    if i == 3:
        continue
else:
    print('else')
0
1
2
3
4
else

一种使用场景，当没有找到需要处理的items时，会执行not_found

for item in container:
    if search_something(item):
        # Found it!
        process(item)
        break
else:
    # Didn't find anything..
    not_found_in_container()

素数的寻找，内循环中，如果没有执行break，则表示没有不是2--n-1的因子，也就是说该数是素数

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 * 2.0
5 is a prime number
6 equals 2 * 3.0
7 is a prime number
8 equals 2 * 4.0
9 equals 3 * 3.0