class Lexer:
    def tokenize_by_style(self, buffer, call_back = None):
        if call_back is not None:
            return self._lexer.tokenize_by_style(
                    buffer,
                    self._keyword_lists,
                    self._properties,
                    call_back
                )
        else:        
            return self._lexer.tokenize_by_style(
                    buffer,
                    self._keyword_lists,
                    self._properties
                )