file.tell() 不一致 [英] file.tell() inconsistency

问题描述

当您以这种方式遍历文件时，有没有人碰巧知道为什么:

Does anybody happen to know why when you iterate over a file this way:

f = open('test.txt', 'r')
for line in f:
    print "f.tell(): ",f.tell()

输出:

f.tell(): 8192
f.tell(): 8192
f.tell(): 8192
f.tell(): 8192

我一直从 tell() 得到错误的文件索引，但是，如果我使用 readline，我会得到适当的 tell() 索引:

I consistently get the wrong file index from tell(), however, if I use readline I get the appropriate index for tell():

f = open('test.txt', 'r')
while True:
    line = f.readline()
    if (line == ''):
        break
    print "f.tell(): ",f.tell()

输出:

f.tell(): 103
f.tell(): 107
f.tell(): 115
f.tell(): 124

我正在运行 python 2.7.1 顺便说一句.

I'm running python 2.7.1 BTW.

推荐答案

使用打开的文件作为迭代器使用预读缓冲区来提高效率.因此，当您遍历各行时，文件指针会在文件中大步前进.

Using open files as an iterator uses a read-ahead buffer to increase efficiency. As a result, the file pointer advances in large steps across the file as you loop over the lines.

来自文件对象文档:

为了使 for 循环成为循环文件行的最有效方式(一种非常常见的操作)，next() 方法使用了一个隐藏的预读缓冲区.使用预读缓冲区的结果是，将 next() 与其他文件方法(如 readline())结合使用无法正常工作.但是，使用 seek() 将文件重新定位到绝对位置将刷新预读缓冲区.

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.

如果需要依赖.tell()，不要使用文件对象作为迭代器.您可以将 .readline() 改为迭代器(以牺牲一些性能为代价):

If you need to rely on .tell(), don't use the file object as an iterator. You can turn .readline() into an iterator instead (at the price of some performance loss):

for line in iter(f.readline, ''):
    print f.tell()

这使用了 iter() 函数 sentinel 参数将任何可调用对象转换为迭代器.

This uses the iter() function sentinel argument to turn any callable into an iterator.

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