Running an IO task with PTB > 20 while the updates are still handled.

bot.py

# this example is based on https://docs.python.org/3/library/asyncio-eventloop.html#asyncio.loop.run_in_executor

import asyncio
import time
from telegram import Update
from telegram.ext import Application, CommandHandler, ContextTypes, MessageHandler, filters


def blocking_io():
    # File operations (such as logging) can block the
    # event loop: run them in a thread pool.
    time.sleep(10)
    return "wuhu"


# the task will run in the background and start the threading stuff
async def ytb_dl_reply(update: Update, context: ContextTypes.DEFAULT_TYPE):
    loop = asyncio.get_running_loop()
    fut = await loop.run_in_executor(None, blocking_io)
    await update.message.reply_text(fut)


# start will reply with a message and start the background task
async def start(update: Update, context: ContextTypes.DEFAULT_TYPE) -> None:
    """Send a message when the command /start is issued."""
    await update.message.reply_html(
        f"I will reply in 10 seconds!"
    )
    # this will run the task in the background and even await on it on shutdown
    # application is neat
    context.application.create_task(ytb_dl_reply(update, context), update)


# showing that it actually runs in the background, the bot will reply to these instantly even if the thread runs
async def echo(update: Update, context: ContextTypes.DEFAULT_TYPE) -> None:
    """Echo the user message."""
    await update.message.reply_text(update.message.text)


def main() -> None:
    """Start the bot."""
    # Create the Application and pass it your bot's token.
    application = Application.builder().token("TOKEN").build()

    # on different commands - answer in Telegram
    application.add_handler(CommandHandler("start", start))

    # on non command i.e. message - echo the message on Telegram
    application.add_handler(MessageHandler(filters.TEXT & ~filters.COMMAND, echo))

    # Run the bot until the user presses Ctrl-C
    application.run_polling()


if __name__ == "__main__":
    main()