使用协程模仿`tail -f`

Question

我按照David Beazley先生的协程介绍模仿了tail -f

# a python version of tail -f
import time

def follow(thefile):
    thefile.seek(0,2) #go to the end of the file
    while True:
        line = thefile.readline()
        if not line:
            time.sleep(0.1) # sleep briefly
            continue
        yield line

In [195]: !cat access_log                                                                                         
The first line
The second line

测试生成器功能

In [196]: logfile = open("access_log", "a+") #set append and write mode                                           

In [197]: for line in follow(logfile): 
     ...:     print(line) 
     ...:                                                                                                         

然后我打开一个新的terminal2以将内容插入access_log

$ echo "This is the third line" >> access_log
$ echo "This is the fourth line" >> access_log
$ cat access_log
The first line
The second line
This is the third line
This is the fourth line

但是，在上一个终端中，for循环仍在挂起而不是跟随打印行，直到我终止它为止

In [197]: for line in follow(logfile): 
     ...:     print(line) 
     ...:                                                                                                         

^C---------------------------------------------------------------------------
KeyboardInterrupt                         Traceback (most recent call last)
<ipython-input-197-f4a72b70a34f> in <module>
----> 1 for line in follow(logfile):
      2     print(line)

我的问题出了什么问题？