[Python-Dev] TextIO seek and tell cookies

[MRAB]

On 2016-09-26 00:21, Ben Leslie wrote:
> Hi all,
>
> I recently shot myself in the foot by assuming that TextIO.tell
> returned integers rather than opaque cookies. Specifically I was
> adding an offset to the value returned by TextIO.tell. In retrospect
> this doesn't make sense/
>
> Now, I don't want to drive change simply because I failed to read the
> documentation carefully, but I think the current API is very easy to
> misuse. Most of the time TextIO.tell returns a cookie that is actually
> an integer and adding an offset to it and seek-ing works fine.
>
> The only indication you get that you are mis-using the API is that
> sometimes tell returns a cookie that when you add an integer offset to
> it will cause seek() to fail with an OverflowError.
>
> Would it be possible to change the API to return something more
> opaque? E.g.: rather than converting the C cookie structure to a long,
> could it instead be converted to  a bytes() object.
>
> (I.e.: Change textiowrapper_build_cookie to use
> PyBytes_FromStringAndSize rather than _PyLong_FromByteArray and
> equivalent for textiowrapper_parse_cookie).
>
> This would ensure the return value is never mis-used and is probably
> also faster using bytes objects than converting to/from an integer.
>
why would it be faster? It's an integer internally.

> Are there any downsides to this? I've made some progress developing a
> patch to change this functionality. Is it worth polishing and
> submitting?
>
An alternative might be a subclass of int.