How to use the africastalking.Airtime.AirtimeService function in africastalking

def _init_service(self):
        super(AirtimeService, self)._init_service()
        self._baseUrl = self._baseUrl + '/version1/airtime'

def initialize(username, api_key):

    if username is None or api_key is None:
        raise RuntimeError('Invalid username and/or api_key')

    globals()['SMS'] = SMSService(username, api_key)
    globals()['Airtime'] = AirtimeService(username, api_key)
    globals()['Payment'] = PaymentService(username, api_key)
    globals()['Voice'] = VoiceService(username, api_key)
    globals()['Application'] = ApplicationService(username, api_key)
    globals()['Token'] = TokenService(username, api_key)
    # globals()['USSD'] = USSDService(username, api_key)