python中的生成器yield

def  countdow(n):
print("Start!");
while n>0:
yield n;
n -= 1;
c = countdow(5);
print(c.__next__())
print(c.__next__())

for i in  countdow(5):
print(i);

def tail(f):
for line in f:
if not line : #如果 非真 则暂时休眠并且再次尝试
time.sleep(0.1);
continue;
yield line; # 将获取到的文件中的值 生成一个序列的值
filecount = tail(open('E:/work.txt'));

#grep方法 用于在上面方法生成器中查找特定的子字符串
def grep(lines,searchtext):
for line in lines:
if searchtext in line :
yield  line;
lines  = grep(filecount,'tom');  #查找带有tom 的子字符串
for line in lines:
print(line);