code: def begin_block(self, req_begin_block):
          
          self.abort_if_abci_chain_is_not_synced()
  
          chain_shift = 0 if self.chain is None else self.chain['height']
          logger.debug('BEGIN BLOCK, height:%s, num_txs:%s',
                       req_begin_block.header.height + chain_shift,
                       req_begin_block.header.num_txs)
  
          self.block_txn_ids = []
          self.block_transactions = []
          return ResponseBeginBlock()
docs: Initialize list of transaction.
          Args:
              req_begin_block: block object which contains block header
              and block hash.