@property<EOL><INDENT>def _files_preserve(self):<DEDENT>
result = set()<EOL>files = [] if not self.files_preserve else self.files_preserve<EOL>files.extend([self.stdin, self.stdout, self.stderr])<EOL>for item in files:<EOL><INDENT>if hasattr(item, '<STR_LIT>'):<EOL><INDENT>result.add(item.fileno())<EOL><DEDENT>if isinstance(item, int):<EOL><INDENT>result.add(item)<EOL><DEDENT><DEDENT>return result<EOL>
create a set of protected files

        create a set of files, based on self.files_preserve and
        self.stdin, self,stdout and self.stderr, that should not get
        closed while daemonizing.

        :return: set