Python file.tell() 给出错误的值

Question

我在 Windows 7 64 字节中使用 Spyder 2.3.7 和 Python 2.7.10。

我想读取一个文本文件，读取一行后想读取文件中的位置；主要原因是稍后能够在完成一些搜索后移回文件中。不管怎样，当我使用以下代码时:

FileOrig = "testtext{Number}";
for index_file in range(1, 2):
    fileName = FileOrig.format(Number = str(index_file))

    ff = open(fileName, "rb")
    numline = 1;
    for line in ff: 
        numline = numline + 1;
        position = ff.tell();
        print(position);

    ff.close()

文件testtext1的内容在哪里(这只是一个例子):

Overall accuracy
Overall accuracy
2.2603e+03
2.3179e+03
2.5265e+03
4.8463e+03
1.7547e+03
3.0143e+03
3.1387e+03


Overall accuracy
Overall accuracy
2.2414e+03
3.9409e+03
1.8902e+03
4.1157e+03


Overall accuracy
Overall accuracy
2.2275e+03
1.3579e+03
2.3712e+03
6.4970e+03
5.8891e+03



    SPLITBIB.STY     -- N. MARKEY <markey@lsv.ens-cachan.fr>
                    v1.17 -- 2005/12/22

This package allows you to split a bibliography into several categories
and subcategories. It does not depend on BibTeX, and any bibliography
may be split and reordered.

split­bib – Split and re­order your bib­li­og­ra­phy

This pack­age en­ables you to split a bib­li­og­ra­phy into sev­eral cat­e­gories and sub­cat­e­gories. It does not de­pend on BibTeX: any bib­li­og­ra­phy may be split and re­ordered.

这会产生非常奇怪的输出:

916
916
916
916
916
916
916
916
916
916
916
916
916
916
916
916
916
916
916
916
916
916
916
916
916
916
916
916
916
916
916
916
916
916
916
916
916
916
916
916
916
916
916

上面的代码有什么问题，无论是否使用二进制读取都没有什么不同。

Answer 1

混合迭代文件和使用文件方法将不起作用，因为迭代涉及使用预读缓冲区。使用 file.readline而不是使用 file.tell:

...
while True:
    line = ff.readline()
    if not line:
        break
    numline = numline + 1
    position = ff.tell()
    print(position)
...

根据 file.next documentation :

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.