In this article I will quickly create a showcase of using Telethon Python library in order to make a simple Telegram bot. Our bot will be a member of a group/channel and once someone changes group’s description bot will post that about information as a separate message.

Preparation

We need a couple of things in order to get started

  1. Register a new bot with a help of BotFather
  2. Create new API application to obtain APP_ID and APP_HASH variables. Follow the steps here
    registering an app
  3. Install Telethon library. Both regular pip install telethon and modern poetry add telethon will work.
  4. Make sure you are familiar with asyncio and how asynchronous programming works in Python.

Handling the right event

We can listen to any event that happens in a chat. For example when a title/description has been changed we will receive ChatAction event. In the example below we will listen to any incoming message in the group responding with an updated description. First we need to create a bot instance

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from telethon.sync import TelegramClient

# make sure you have defined api_id, api_has, bot_token somewhere in the code
bot = TelegramClient('bot', API_ID,  API_HASH).start(bot_token=BOT_TOKEN)

Once done we need to create a handler responsible of dealing with the specific kind of the events

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from telethon.sync import events

@bot.on(events.NewMessage)
async def any_message_arrived_handler(event):
    print('We are handling message events')

Now let’s run execution loop and add the bot to any test group we already have

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bot.run_until_disconnected()

You will see logging in your console after any new message arrives into the channel.

Implementing the logic

The one most important thing left is to implement the main application logic

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from telethon import functions

DESC = ''

async def any_message_arrived_handler(event):
    global DESC
    chat = await event.get_chat()
    result = await bot(functions.messages.GetFullChatRequest(
        chat_id=chat.id
    ))
    description = result.full_chat.about
    if description != DESC:
        DESC = description
        await bot.send_message(
            chat.id, f'Description has been changed to **{description}**')

As you can see we store current topic in a global variable, so once process restarts we would loose latest topic. You can use Sqlite to have a simple persistent storage or just write it to the file. The only trick here is to obtain ChatFull instance otherwise we will not be able to get the description.

That’s it, now you are able to enhance it with any custom features you want. Happy coding!

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