code: def get_tiny_position(self, symbol, acc_id=0):
          
          ret, data = self._trade_ctx.position_list_query(code=symbol, trd_env=self._env_type, acc_id=acc_id)
          if 0 != ret:
              return None
  
          for _, row in data.iterrows():
              if row['code'] != symbol:
                  continue
              pos = TinyPosition()
              pos.symbol = symbol
              pos.position = row['qty']
              pos.frozen = pos.position - row['can_sell_qty']
              pos.price = row['cost_price']
              pos.market_value = row['market_val']
              return pos
          return None
docs: 得到股票持仓