python - 在遍历for循环的python中使用ftell()的意外文件指针位置

Question

获取以下代码的意外输出:

sample.txt 包含:

this is the first line
this is the second line
this is the third line
this is the fourth line
this is the fifth line
this is the sixth line

代码:

import sys  

f1=open("sample3.txt",'r')  

print f1.tell()  

for line in f1:  
    print line  
    print "postion of the file pointer",f1.tell()  

输出:

0  
this is the first line  
postion of the file pointer 141  
this is the second line  
postion of the file pointer 141  
this is the third line  
postion of the file pointer 141  
this is the fourth line  
postion of the file pointer 141  
this is the fifth line  
postion of the file pointer 141  
this is the sixth line  
postion of the file pointer 141  

我希望在每行的末尾显示文件指针位置

Answer 1

我在文档中找到了相关部分:

A file object is its own iterator, for example iter(f) returns f (unless f is closed). When a file is used as an iterator, typically in a for loop (for example, for line in f: print line.strip()), the next() method is called repeatedly. This method returns the next input line, or raises StopIteration when EOF is hit when the file is open for reading (behavior is undefined when the file is open for writing). In order to make a for loop the most efficient way of looping over the lines of a file (a very common operation), the next() method uses a hidden read-ahead buffer. As a consequence of using a read-ahead buffer, combining next() with other file methods (like readline()) does not work right. However, using seek() to reposition the file to an absolute position will flush the read-ahead buffer.