I don't think this is worth fixing; you'll get an I/O error as soon as
I/O is attempted, which is upon the first read() for input, or upon the
firsh (implicit or explicit) flush() on output.  You can't tell whether
a fd is valid or not without attempting I/O on it, so trying to test on
each write() call would defeat the purpose of buffering. Testing on
open() is insufficient as long as anybody could call os.close() any time.