<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def read_whole_packet(self):
 """ Reads single packet and returns bytes payload of the packet Can only be called when transport's read pointer is at the beginning of the packet. """

        self._read_packet()
        return readall(self, self._size - _header.size)