<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def start_session(self):
 """ Starts a Salesforce session and determines which SF instance to use for future requests. """

        if self.has_active_session():
            raise Exception("Session already in progress.")

        response = requests.post(self._get_login_url(),
                                 headers=self._get_login_headers(),
                                 data=self._get_login_xml())
        response.raise_for_status()

        root = ET.fromstring(response.text)
        for e in root.iter("%ssessionId" % self.SOAP_NS):
            if self.session_id:
                raise Exception("Invalid login attempt.  Multiple session ids found.")
            self.session_id = e.text

        for e in root.iter("%sserverUrl" % self.SOAP_NS):
            if self.server_url:
                raise Exception("Invalid login attempt.  Multiple server urls found.")
            self.server_url = e.text

        if not self.has_active_session():
            raise Exception("Invalid login attempt resulted in null sessionId [%s] and/or serverUrl [%s]." %
                            (self.session_id, self.server_url))
        self.hostname = urlsplit(self.server_url).hostname