How to use the africastalking.Airtime function in africastalking

"""
Airtime

send(phoneNumber: String, amount: String): Send airtime to a phone number.

send(recipients: Map): Send airtime to a bunch of phone numbers. The keys in the recipients map are phone
    numbers while the values are airtime amounts.
"""
import africastalking
import unittest
import random
from test import USERNAME, API_KEY

africastalking.initialize(USERNAME, API_KEY)
service = africastalking.Airtime


class TestAirtimeService(unittest.TestCase):

    def test_send_single(self):
        currency_code = "KES"
        amount = str(random.randint(10, 1000))
        phone = '+254718763456'
        idempotency_key = 'req-1234'
        res = service.send(phone_number=phone, amount=amount, currency_code=currency_code, idempotency_key=idempotency_key)
        assert res['numSent'] == 1

    def test_send_multiple(self):
        res = service.send(recipients=[
            {'phoneNumber': '+2348160663047', 'amount': str(random.randint(1, 10)), 'currency_code': 'USD'},
            {'phoneNumber': '+254718769881', 'amount':str(random.randint(138, 13223)), 'currency_code':  'KES'},

import os
from flask import Flask, request, render_template
from flask_restful import Resource, Api

import africastalking

app = Flask(__name__)
api = Api(app)

username = os.getenv('user_name', 'sandbox')
api_key = os.getenv('api_key', 'fake')


africastalking.initialize(username, api_key)
sms = africastalking.SMS
airtime  = africastalking.Airtime
payment = africastalking.Payment

@app.route('/')
def index():
    return render_template('index.html')

class send_sms(Resource):
    def get(self):
      return {'hello': 'world'}
    def post(self):
      number = str(request.form['number'])
      return sms.send("Test message", [number])
api.add_resource(send_sms, '/sms')

class send_airtime(Resource):
    def get(self):