code: def tickerId(self, contract_identifier):
          
          # contract passed instead of symbol?
          symbol = contract_identifier
          if isinstance(symbol, Contract):
              symbol = self.contractString(symbol)
  
          for tickerId in self.tickerIds:
              if symbol == self.tickerIds[tickerId]:
                  return tickerId
          else:
              tickerId = len(self.tickerIds)
              self.tickerIds[tickerId] = symbol
              return tickerId
docs: returns the tickerId for the symbol or
          sets one if it doesn't exits