def _open(self, f):
if None in self.files:<EOL><INDENT>fd = self.files.index(None)<EOL>self.files[fd] = f<EOL><DEDENT>else:<EOL><INDENT>fd = len(self.files)<EOL>self.files.append(f)<EOL><DEDENT>return fd<EOL>
Adds a file descriptor to the current file descriptor list

:rtype: int
:param f: the file descriptor to add.
:return: the index of the file descriptor in the file descr. list