[Python-Dev] TextIO seek and tell cookies

[Guido van Rossum]

Yeah, that should work. The implementation is something like a byte
offset to the start of a line plus a character count, plus some misc
flags. I found this implementation in the 2.6 code (the last version
where it was pure Python code):

    def _pack_cookie(self, position, dec_flags=0,
                           bytes_to_feed=0, need_eof=0, chars_to_skip=0):
        # The meaning of a tell() cookie is: seek to position, set the
        # decoder flags to dec_flags, read bytes_to_feed bytes, feed them
        # into the decoder with need_eof as the EOF flag, then skip
        # chars_to_skip characters of the decoded result.  For most simple
        # decoders, tell() will often just give a byte offset in the file.
        return (position | (dec_flags<<64) | (bytes_to_feed<<128) |
               (chars_to_skip<<192) | bool(need_eof)<<256)

    def _unpack_cookie(self, bigint):
        rest, position = divmod(bigint, 1<<64)
        rest, dec_flags = divmod(rest, 1<<64)
        rest, bytes_to_feed = divmod(rest, 1<<64)
        need_eof, chars_to_skip = divmod(rest, 1<<64)
        return position, dec_flags, bytes_to_feed, need_eof, chars_to_skip

On Mon, Sep 26, 2016 at 3:43 PM, Greg Ewing <greg.ewing at canterbury.ac.nz> wrote:
> Ben Leslie wrote:
>>
>> But the idea of transmitting these offsets outside of a running
>> process is not something that I had anticipated. It got me thinking:
>> is there a guarantee that these opaque values returned from tell() is
>> stable across different versions of Python?
>
>
> Are they even guaranteed to work on a different file
> object in the same process? I.e. if you read some stuff
> from a file, do tell() on it, then close it, open it
> again and seek() with that token, are you guaranteed to
> end up at the same place in the file?
>
> --
> Greg
>
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-- 
--Guido van Rossum (python.org/~guido)