file.seek是将文件游标移动到文件的任意位置，然后对文件的当前位置进行操作（增加、删除内容等）

python3.*

>>> help(f.seek)
Help on built-in function seek:

seek(cookie, whence=0, /) method of _io.TextIOWrapper instance Change stream position. Change the stream position to the given byte offset. The offset is interpreted relative to the position indicated by whence. Values for whence are:

    * 0 -- start of stream (the default); offset should be zero or positive
    * 1 -- current stream position; offset may be negative
    * 2 -- end of stream; offset is usually negative

    Return the new absolute position.

python2.*

Help on built-in function seek:

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.

offset：开始的偏移量，也就是代表需要移动偏移的字节数
whence：给offset参数一个定义，表示要从哪个位置开始偏移；0代表从文件开头开始算起，1代表从当前位置开始算起，2代表从文件末尾算起。

file.tell()方法的返回值就告诉大家当前文件游标的位置。