import unittest
import os
from io import BytesIO
from random import randint
from hashlib import sha256
from telethon import TelegramClient

# Fill in your api_id and api_hash when running the tests
# and REMOVE THEM once you've finished testing them.
api_id = None
api_hash = None

if not api_id or not api_hash:
    raise ValueError('Please fill in both your api_id and api_hash.')

class HigherLevelTests(unittest.TestCase):

    @staticmethod
    def test_cdn_download():
        client = TelegramClient(None, api_id, api_hash)
        client.session.server_address = '149.154.167.40'
        assert client.connect()

        try:
            phone = '+999662' + str(randint(0, 9999)).zfill(4)
            client.send_code_request(phone)
            client.sign_up(phone, '22222', 'Test', 'DC')

            me = client.get_me()
            data = os.urandom(2 ** 17)
            client.send_file(
                me, data,
                progress_callback=lambda c, t:
                    print('test_cdn_download:uploading {:.2%}...'.format(c/t))
            )
            msg = client.get_message_history(me)[1][0]

            out = BytesIO()
            client.download_media(msg, out)
            assert sha256(data).digest() == sha256(out.getvalue()).digest()

            client.log_out()
        finally:
            client.disconnect()

Add a unit test for CDN-downloads