在python中读取特定字节的文件

Question

我想指定一个偏移量，然后读取如下文件的字节

offset = 5
read(5) 

然后阅读下面的6-10等。我读到了关于seek的内容，但我不明白它是如何工作的，而且这些例子不够描述性。

seek(offset,1)返回什么？

谢谢

Answer 1

只需使用Python的REPL就可以看到：

[...]:/tmp$ cat hello.txt 
hello world
[...]:/tmp$ python
Python 2.7.6 (default, Mar 22 2014, 22:59:56) 
[GCC 4.8.2] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> f = open('hello.txt', 'rb')
>>> f.seek(6, 1)    # move the file pointer forward 6 bytes (i.e. to the 'w')
>>> f.read()        # read the rest of the file from the current file pointer
'world\n'

Answer 2

seek不返回任何有用的信息。它只是将内部文件指针移动到给定的偏移量。下一次读取将从该指针开始。

Answer 3

seek的第二个参数的值是0、1或2：

0 - offset is relative to start of file
1 - offset is relative to current position
2 - offset is relative to end of file

记住你可以查看帮助-

>>> help(file.seek)
Help on method_descriptor:

seek(...)
    seek(offset[, whence]) -> None.  Move to new file position.

    Argument offset is a byte count.  Optional argument whence defaults to
    0 (offset from start of file, offset should be >= 0); other values are 1
    (move relative to current position, positive or negative), and 2 (move
    relative to end of file, usually negative, although many platforms allow
    seeking beyond the end of a file).  If the file is opened in text mode,
    only offsets returned by tell() are legal.  Use of other offsets causes
    undefined behavior.
    Note that not all file objects are seekable.