[Python-Dev] TextIO seek and tell cookies

[Ben Leslie]

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.

Are there any downsides to this? I've made some progress developing a
patch to change this functionality. Is it worth polishing and
submitting?

Cheers,

Ben