code: def load(self, filename=None):
          
          assert not self.__flag_loaded, "File can be loaded only once"
          if filename is None:
              filename = self.default_filename
          assert filename is not None, \
              "{0!s} class has no default filename".format(self.__class__.__name__)
  
          # Convention: trying to open empty file is an error,
          # because it could be of (almost) any type.
  
          size = os.path.getsize(filename)
          if size == 0:
              raise RuntimeError("Empty file: '{0!s}'".format(filename))
  
          self._test_magic(filename)
          self._do_load(filename)
          self.filename = filename
          self.__flag_loaded = True
docs: Loads file and registers filename as attribute.