> I use the type "long" to store the lseek() result, because I don't 
> know if off_t is available on all OS. Py_off_t may be used, but it's 
> defined above (after fileio_init).

Instead of checking the return type, you can first set errno to 0, and
then check errno after the function returns.

> fileio_seekable() uses the 
> type "int" for lseek() result, which looks worse than long :-)

Nice catch! http://bugs.python.org/issue5016

PS : about the patch, "0 < f.tell()" is really strange coding style...
"f.tell() > 0" looks much more consistent with the rest of the Python
code base