code: def file_size(self):
          
          if self._file_size is None:
              if isinstance(self._path_or_fd, string_types()):
                  self._file_size = os.stat(self._path_or_fd).st_size
              else:
                  self._file_size = os.fstat(self._path_or_fd).st_size
              # END handle path type
          # END update file size
          return self._file_size
docs: :return: size of file we manager