Migrating from aiogram

From their GitHub:

from aiogram import Bot, Dispatcher, executor, types

API_TOKEN = 'BOT TOKEN HERE'

# Initialize bot and dispatcher
bot = Bot(token=API_TOKEN)
dp = Dispatcher(bot)

@dp.message_handler(commands=['start'])
async def send_welcome(message: types.Message):
    """
    This handler will be called when client send `/start` command.
    """
    await message.reply("Hi!\nI'm EchoBot!\nPowered by aiogram.")

@dp.message_handler(regexp='(^cat[s]?$|puss)')
async def cats(message: types.Message):
    with open('data/cats.jpg', 'rb') as photo:
        await bot.send_photo(message.chat.id, photo, caption='Cats is here 😺',
                             reply_to_message_id=message.message_id)

@dp.message_handler()
async def echo(message: types.Message):
    await bot.send_message(message.chat.id, message.text)

if __name__ == '__main__':
    executor.start_polling(dp, skip_updates=True)

After rewrite:

from telethon import TelegramClient, events

# Initialize bot and... just the bot!
bot = TelegramClient('bot', 11111, 'a1b2c3d4').start(bot_token='TOKEN')

@bot.on(events.NewMessage(pattern='/start'))
async def send_welcome(event):
    await event.reply('Howdy, how are you doing?')

@bot.on(events.NewMessage(pattern='(^cat[s]?$|puss)'))
async def cats(event):
    await event.reply('Cats is here 😺', file='data/cats.jpg')

@bot.on(events.NewMessage)
async def echo_all(event):
    await event.reply(event.text)

if __name__ == '__main__':
    bot.run_until_disconnected()

Key differences:

• Telethon offers convenience methods to avoid retyping bot.send_photo(message.chat.id, ...) all the time, and instead let you type event.reply.
• Sending files is a lot easier. The methods for sending photos, documents, audios, etc. are all the same!