creating my own file-like object

[Skaperen]

how much must i implement to create a file-like object that can have a reference to it passed along as argument one in open() as well as the file object in other functions like bz2.open() (an arbitrary example). i am making an I/O-like data filter but suspect i cannot thoroughly make test code for it. i will be making objects for reading and objects for writing but no objects for both reading and writing although that might also be interesting.