[Python-Dev] TextIO seek and tell cookies (original) (raw)

MRAB python at mrabarnett.plus.com
Sun Sep 25 20:21:57 EDT 2016

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 textiowrapperbuildcookie to use PyBytesFromStringAndSize rather than PyLongFromByteArray and equivalent for textiowrapperparsecookie). 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.

More information about the Python-Dev mailing list