def keys(self):
keys = list()<EOL>for attribute_name, type_instance in inspect.getmembers(self):<EOL><INDENT>if attribute_name.startswith('<STR_LIT>') or inspect.ismethod(type_instance):<EOL><INDENT>continue<EOL><DEDENT>keys.append(attribute_name)<EOL><DEDENT>return keys<EOL>
:returns: a list of usable keys
:rtype: list