Actually the open() method will create an io.BufferedWriter which inherits from IOBase and FileIO also inherits from IOBase. Though not exactly the same classes, they support basically identical functionality. I suppose you could use FileIO if you know you don't need buffering. There's a lot of info on the python io docs page The most important sentence there is:

Raw binary I/O typically provides low-level access to an underlying OS device or API, and does not try to encapsulate it in high-level primitives (this is left to Buffered I/O and Text I/O, described later in this page).

Raw binary I/O relates to io.FileIO and Buffered I/O to the io.BufferedWriter (open() method)