handle positions changes

def handlePosition(self, msg):
        """ handle positions changes """

        # log handler msg
        self.log_msg("position", msg)

        # contract identifier
        contract_tuple = self.contract_to_tuple(msg.contract)
        contractString = self.contractString(contract_tuple)

        # try creating the contract
        self.registerContract(msg.contract)

        # new account?
        if msg.account not in self._positions.keys():
            self._positions[msg.account] = {}

        # if msg.pos != 0 or contractString in self.contracts.keys():
        self._positions[msg.account][contractString] = {
            "symbol":        contractString,
            "position":      int(msg.pos),
            "avgCost":       float(msg.avgCost),
            "account":       msg.account
        }

        # fire callback
        self.ibCallback(caller="handlePosition", msg=msg)