code: def balance(self, account_id=None):
          
          if not account_id:
              if len(self.accounts()) == 1:
                  account_id = self.accounts()[0].id
              else:
                  raise ValueError("You need to pass account ID")
  
          endpoint = '/balance'
          response = self._get_response(
              method='get', endpoint=endpoint,
              params={
                  'account_id': account_id,
              },
          )
  
          return MonzoBalance(data=response.json())
docs: Returns balance information for a specific account.
  
          Official docs:
              https://monzo.com/docs/#read-balance
  
          :param account_id: Monzo account ID
          :type account_id: str
          :raises: ValueError
          :returns: Monzo balance instance
          :rtype: MonzoBalance