def tar(self):
            """
                return a data stream of the files in uncompressed tar format.
                Returns None if the list is empty. 
            """
            import cStringIO, tarfile
            fobj = cStringIO.StringIO()
            # check the list contains something
            if len(self) >= 0:
                try:
                    tar = tarfile.open('', "w", fobj)
                except:
                    sys.stderr.write("Unable to open tar file.")
                    raise
                try:
                    for name in self:
                        # only add files
                        if os.path.isfile(name):
                            try:
                                tar.add(name)
                            except ValueError:
                                # it can't store extremely long names
                                # so tell which ones and continue
                                sys.stderr.write('Warning: Unable to store: ')
                                sys.stderr.write(name)
                                sys.stderr.write('\n')
                    tar.close()
                    return fobj.getvalue()
                finally:
                    fobj.close()

            else:
                # return None if the list is empty
                return None

                return a data stream of the files in uncompressed tar format.
                Returns None if the list is empty. 
            
recipes/Python/259147_FSList/recipe-259147.py