遍历循环

1 for <循环变量> in <便利结构>：
语句块
for i in range(m, n, k):设置步长
2 字符串便利循环
for c in s:
语句
for c in ‘python’:
print(c, end = ‘,’)
3 文件：
for line in f：
4 while 条件：
语句
5 break continue
break指打破当前循环
continue跳过这次循环
s = s[:-5]