<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def dump_begin(self, selector_id):
 """Start dumping a stream. Args: selector_id (int):
 The buffered stream we want to dump. Returns: (int, int, int):
 Error code, second error code, number of available readings """


        if self.dump_walker is not None:
            self.storage.destroy_walker(self.dump_walker)

        selector = DataStreamSelector.FromEncoded(selector_id)
        self.dump_walker = self.storage.create_walker(selector, skip_all=False)

        return Error.NO_ERROR, Error.NO_ERROR, self.dump_walker.count()