【python】官网例子

地址：https://docs.python.org/3/tutorial/controlflow.html

>>> 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

 为什么range（2，2）不会报错，它返回的是一个空迭代器，for循环作用于空的迭代器上一次也不会执行，而是直接结束