Currently io.IOBase.seek(offset[, whence]) uses magic numbers for the
second argument. Since this is essentially identical the C function
fseek I think adding the same "constants" that C uses (SEEK_SET=0,
SEEK_CUR=1, SEEK_END=2) may be worth considering.

These values could be added to the global scope in the io module:
f = io.open('foo', 'rb')
f.seek(0, io.SEEK_END)

If added to the io.IOBase class then I believe they'd be accessible
without importing io:
f = open('foo', 'rb')
f.seek(0, f.SEEK_END)