code: def init_db(self, txn=None):
          
          with ExitStack() as stack:
              if txn is None:
                  txn = stack.enter_context(self.engine.begin())
  
              tables_already_exist = self._all_tables_present(txn)
  
              # Create the SQL tables if they do not already exist.
              metadata.create_all(txn, checkfirst=True)
  
              if tables_already_exist:
                  check_version_info(txn, version_info, ASSET_DB_VERSION)
              else:
                  write_version_info(txn, version_info, ASSET_DB_VERSION)
docs: Connect to database and create tables.
  
          Parameters
          ----------
          txn : sa.engine.Connection, optional
              The transaction to execute in. If this is not provided, a new
              transaction will be started with the engine provided.
  
          Returns
          -------
          metadata : sa.MetaData
              The metadata that describes the new assets db.