def load_from_file(cls, filename, format=None):
        """ Return an instance of the class that is saved in the file with the
            given filename in the specified format.
        """
        if format is None:
            # try to derive protocol from file extension
            format = format_from_extension(filename)
        with file(filename,'rbU') as fp:
            obj = cls.load_from_file_like(fp, format)
            obj.filename = filename
            return obj
Return an instance of the class that is saved in the file with the
            given filename in the specified format.