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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def getService(self, serviceIdentifier):
 """ Return the requested service instance. :param serviceIdentifier: <str> service identifier :return: <object> service instance """

        if serviceIdentifier in self._services:
            return self._services[serviceIdentifier]
        else:
            message = "Application - getService() - " \
                      "Service with identifier {} does not exist." \
                      .format(serviceIdentifier)
            raise Exception(message)