通过看源码可以知道：

    def _bind(self, what, sequence, func, add, needcleanup=1):
        """Internal function."""
        if isinstance(func, str):
            self.tk.call(what + (sequence, func))
        elif func:
            funcid = self._register(func, self._substitute,
                        needcleanup)
            cmd = ('%sif {"[%s %s]" == "break"} break\n'
                   %
                   (add and '+' or '',
                funcid, self._subst_format_str))
            self.tk.call(what + (sequence, cmd))
            return funcid
        elif sequence:
            return self.tk.call(what + (sequence,))
        else:
            return self.tk.splitlist(self.tk.call(what))