<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def startSubscription(self, reqId, subscriber, contract=None):
 """ Register a live subscription. """

        self._reqId2Contract[reqId] = contract
        self.reqId2Subscriber[reqId] = subscriber