code: def order_book(self, selling_asset_code, buying_asset_code, selling_asset_issuer=None, buying_asset_issuer=None,
                     limit=10):
          
          selling_asset = Asset(selling_asset_code, selling_asset_issuer)
          buying_asset = Asset(buying_asset_code, buying_asset_issuer)
          asset_params = {
              'selling_asset_type': selling_asset.type,
              'selling_asset_code': None if selling_asset.is_native() else selling_asset.code,
              'selling_asset_issuer': selling_asset.issuer,
              'buying_asset_type': buying_asset.type,
              'buying_asset_code': None if buying_asset.is_native() else buying_asset.code,
              'buying_asset_issuer': buying_asset.issuer,
          }
          endpoint = '/order_book'
          params = self.__query_params(limit=limit, **asset_params)
          return self.query(endpoint, params)
docs: Return, for each orderbook, a summary of the orderbook and the bids
          and asks associated with that orderbook.
  
          See the external docs below for information on the arguments required.
  
          `GET /order_book
          <https://www.stellar.org/developers/horizon/reference/endpoints/orderbook-details.html>`_
  
          :param str selling_asset_code: Code of the Asset being sold.
          :param str buying_asset_code: Type of the Asset being bought.
          :param str selling_asset_issuer: Account ID of the issuer of the Asset being sold,
              if it is a native asset, let it be `None`.
          :param str buying_asset_issuer: Account ID of the issuer of the Asset being bought,
              if it is a native asset, let it be `None`.
          :param int limit: Limit the number of items returned.
          :return: A list of orderbook summaries as a JSON object.
          :rtype: dict